int j remote sensing 2001 vol 22 no 10 1847ndash1859

NDVI anomaly patterns over Africa during the 1997 98 ENSO warmevent

A ANYAMBA C J TUCKERNASAGoddard Space Flight Center Biospheric Sciences Branch Code 9230Greenbelt MD 20771 USA e-mail assafcerradogsfcnasagov

and J R EASTMANClark University Graduate School of Geography 950 Main 3 WorcesterMA 01610 USA

(Received 26 May 1999 in nal form 28 June 2000)

Abstract Normalized DiVerence Vegetation Index (NDVI) departure patternsfor Africa during the 199798 El NinoSouthern Oscillation (ENSO) warm eventshow two dominant patterns Over equatorial Eastern Africa above normalNDVI anomalies persisted from October 1997 through the normal dry season(DecemberndashFebruary) and into the long rains season in MarchndashMay OverSouthern Africa the spatial NDVI anomaly shows a dry western half and arelatively greener than normal eastern half Correlations between the temporalNDVI anomalies with ENSO indices shows that the anomalous conditions overEastern Africa were a direct result of anomalous warming of sea surface temper-atures (~+3degC) in the western equatorial Indian Ocean (WIO) and a laggedresponse to the warming in the eastern Paci c Ocean (+4degC) We suggest thatthis anomalous warming of the WIO and the equatorial eastern Atlantic Oceanbasin dampened the normal severe drought response pattern over Southern Africawhere mild drought conditions were experiencedThe overall continental responsepattern shows a meridional dipole pattern with above normal NDVI straddlingthe equator between 10deg N and 10deg S and normal to slightly below normal NDVIsouth of 15deg S predominantly over south-western Africa

1 IntroductionDuring the period June 1997 to May 1998 the global climate system was

perturbed by the largest El NinoSouthern Oscillation (ENSO) phenomenonobserved this century (McPhaden 1999) Anomalous warming in the eastern Paci cOcean reached a record +4degC as seen in gure 1(a) The eVect of this warming wasillustrated by the shift in the belt of maximum tropical deep convection from thewestern Paci c region to the central and eastern Paci c regions as inferred fromoutgoing longwave radiation (OLR) data ( gure 1(b)) This shift resulted in abovenormal rainfall over the eastern Paci c islands Peru extending into the extra tropicsto aVect California and the US Gulf States On the western side of the Indian Oceanextremely elevated levels of rainfall fell over much of Eastern Africa On the otherhand Indonesia north-east Brazil Hawaii and Papua New Guinea experienced

International Journal of Remote SensingISSN 0143-1161 printISSN 1366-5901 online copy 2001 Taylor amp Francis Ltd

httpwwwtandfcoukjournalsDOI 10108001431160010029156

A Anyamba et al1848

(a)

(b)

Figure 1 (a) Composite SST anomalies (degC) for DecemberndashFebruary 199798 The pro-nounced dipole pattern between the western Paci c (cold negative SSTs) and thecentral-eastern Paci c (warm positive SSTs) is a typical occurrence during ENSOwarm events Note also the warmer than normal SSTs in the WIO (gt+3degC) and inthe equatorial South Atlantic Ocean (~+2degC) Departures in SST are calculated withrespect to 1982ndash1998 means (b) Composite Outgoing Longwave Radiation (OLR)anomalies (W m Otilde 2) for DecemberndashFebruary 199798 Positive anomalies indicate dryconditions while negative departures indicate areas of deep cold clouds where precip-itation is likely to occur Note the eastndashwest dipole pattern of negative (east wet) vspositive (west dry) across the Paci c basin Over Africa there is a marked meridionaldiVerence in the departures with negative anomalies over East Africa indicative ofintense convectiveactivity and positive anomalies covering the western halfof SouthernAfrica indicative of dry conditions

unprecedented drought that precipitated outbreaks of forest res especially inIndonesia and Brazil This event was in many ways a classic illustration of theteleconnection patterns resulting from the interannual reversal in the Tropical WalkerCirculation system The rainfall anomalies observed during this event closely followthe results from a study by Ropelewski and Halpert (1987 1996)

In this study we analyse and describe the biosphere response to these anomalous

NDVI anomalies during the ENSO event 1849

climatic conditions using normalized diVerence vegetation index (NDVI) data forAfrica for the period July 1997 to May 1998 The index is derived from broad bandmeasurements in the visible and infrared channels made by the Advanced Very High-Resolution Radiometer (AVHRR) instrument onboard the National Oceanographicand Atmospheric Administration (NOAA) series of polar orbiting meteorologicalsatellites The index is calculated as NDVI=(Ch2 shy Ch1)(Ch2+Ch1) where Ch1and Ch2 are upwelling land surface radiances in the red and near-infrared wave-lengths respectively It has been found to provide a strong vegetation signal andgood spectral contrast from most background materials (Tucker and Sellers 1986)with restrictions in some arid and semi-arid areas where bare soil re ectance maycause large NDVI variations (Huete and Tucker 1991 Farrar et al 1994) It hasalso been shown to be a good indicator of various vegetation parameters includinggreen leaf area index (LAI) biomass percentage green cover green biomass produc-tion and the fraction of absorbed photosynthetically active radiation (Tucker 1979Sellers 1985Asrar 1989) Taken as time-series measurements NDVI is well correlatedwith climate variables including rainfall and evapotranspiration in a wide range ofenvironmental conditions (Gray and Tapley 1985 Justice et al 1986 Nicholson et al1990 Cihlar et al 1991) NDVI may therefore be considered to represent theintegration of the land surface responses to climate variability at various time andspace scales Employing various data decomposition techniques it has been illus-trated recently that seasonal scale signals (Eastman and Fulk 1993 Andres et al1994) and interannual signals are related to ENSO (Eastman and Fulk 1993 Myneniet al 1995 Anyamba and Eastman 1996) Furthermore examination of monthlyNDVI anomaly patterns shows that there are unique land surface response patternsfrom one ENSO warm event to another (Eastman and Anyamba 1996) In totalityNDVI can be used as an indicator of the biosphere response to climate variabilityat a range of time scales

2 Data and analysis methodsThe Global Inventory Monitoring and Modeling Systems (GIMMS) group at

NASAGSFC processed the NDVI data used in this analysis Near-real time dataprocessing for Africa is carried out to support the activities of the United StatesAgency for International Development (USAID) Famine Early Warning System(FEWS) project The data at 8 km spatial resolution are rst processed as 10-daycomposites using the maximum value compositing procedure to minimize eVects ofcloud contamination (Holben 1986 Tucker and Newcomb 1994) For the purposeof this research we created monthly composites from three 10-day composites inany given month to further minimize the eVects of clouds on the vegetation signaland to match the monthly temporal resolution of the associated ENSO indices Inaddition calibration based on invariant desert targets has been applied to the datato minimize the eVects of sensor degradation (Los 1993) The time series of thesemeasurements began in July 1981 and continues to date So far it constitutes thelongest available representation of global biosphere dynamics for studies of climatendashvegetation interactions (Los et al 1994) The ENSO indices used in the analysisinclude the following

1 Sea surface temperatures (SST) anomaly data drawn from the NINO34 ENSOmonitoring region in the eastern Paci c Ocean (5deg Nndash5deg S 170ndash120deg W)

A Anyamba et al1850

2 The Southern Oscillation Index (SOI) which represents the basin-widesee-saw in atmospheric pressure patterns between the eastern and westernPaci c measured as normalized diVerence in sea level pressure betweenPapeete Tahiti and Darwin Australia

3 The Eastern Paci c Wind Index (EPWI) (5deg Nndash5deg S 135ndash120deg W) whichrepresents the interannual variability in the wind ow patterns in the easternPaci c region

4 The equatorial Western Indian Ocean (WIO) SST index (10deg Nndash10deg S40ndash64deg E) which was extracted from the gridded global SST time series datasetThe region selected for computation of this index (excluding land areas) showedthe largest magnitude of positive SST departures in the WIO during the199798 ENSO event It is more or less similar to the domain used by Sajiet al (1999) in the computation of an eastndashwest dipole index for the IndianOcean

Other indicators of tropical interannual climate dynamics used in this analysisinclude variations in the North Atlantic SST [ NAT] (5ndash20deg N 60ndash30deg W) SouthAtlantic SST [ SAT] (0ndash20deg S 30deg Wndash10deg E) Global Tropics SST [ TROP](10deg Sndash10deg N 0ndash360deg) and the Quasi-Biennial Oscillation Index (QBO30) of zonallyaveraged winds at 30 MB over the equator Variations in patterns of SSTs in theAtlantic have an in uence on the location and intensity of the IntertropicalConvergence Zone and hence on precipitation over Africa (Lamb et al 1986Hastenrath 1990) All the ENSO and tropical SST indices were obtained from theactive archive of NOAArsquos Climate Prediction Center (CPC)

NDVI anomaly images were computed as diVerences from monthly long-termmeans using the 17-year record (1982ndash1998 used as the base period) excluding cloudpixels Though this period of anomaly analysis is relatively short and characterizedby some discontinuities in the mapping instruments (Kaufman and Holben 1993)the results obtained agree well with the departure patterns in both OLR ( gure 1(b))and in rainfall as illustrated in Bell and Halpert (1998) The NDVI anomaly patternsfor the period September 1997 to April 1998 are shown in gure 2 The nature ofthe general teleconnections between ENSO and patterns of variability in NDVI overAfrica is explored using correlation and cross-correlation analysis techniques (Katz1988) between NDVI anomalies at selected sites and ENSO indices Three sites weresampled for use in the correlation analysis For each site box averages of the long-term means monthly values and monthly anomalies were computed from a100kmtimes100 km area centred on each of these locations For East Africa the samplesite is located in eastern Kenya centred at 40deg E 0deg S with two sites in SouthernAfrica in Namibia (18deg E 22deg S) and South Africa (29deg E 29deg S) These sites wereselected taking into account the areas that show anomalous precipitation responseto ENSO (Ropelewski and Halpert 1987 1989 1996 Nicholson and Kim 1997) andthe observed anomaly patterns in NDVI during this event and during previousENSO warm events (Myneni et al 1995 Eastman and Anyamba 1996)

3 Results31 Spatial evolution of NDVI anomaly patterns

Monthly NDVI anomalies for the 199798 ENSO warm event from September1997 to April 1998 are shown in gure 2 Anomalies are computed in NDVI units(NDVI range shy 1 to +1) September 1997 shows a discontinuous band of negative

NDVI anomalies during the ENSO event 1851

Figure 2 Monthly AVHRR NDVI anomaly images of continental Africa during the 199798ENSO warm event (September 1997ndashApril 1998) Departures are calculated withrespect to 17-year means (1982ndash1998) Green areas show positive NDVI anomaliesyellow neutral and brown-red show negative NDVI anomalies Positive anomaliesin East Africa during the entire period correspond well with the location of negativeanomalies in OLR shown in gure 1(b) Negative departures are observed in thewestern half of Southern Africa in December 1997 and from February to April 1998

NDVI anomalies across the Sahel into parts of East Africa and positive anomaliesalong the West African coastal belt the Congo Basin region and south-easternAfrica September is for the most part a transitional month when the rains recedefrom the Sahel with the movement of the Intertropical Convergence Zone (ITCZ)from the Northern hemisphere to the Southern hemisphere over the continent The rst strong indicator of anomalous conditions occurs in October Positive anomaliesin NDVI (~02) occur in East Africa predominantly in areas east of the Rift Valleycovering the coastal plain lands and semi-arid and arid areas of eastern and north-eastern Kenya and Somalia The western half of East Africa shows negative NDVIanomalies resulting in a zonal east (positive)ndashwest (negative) anomaly in vegetationconditions Isolated clusters of positive NDVI anomalies occur in south-easternAfrica The positive anomaly pattern over Kenya and Somalia is enhanced inNovemberndashDecember (gt+020) as the short rains season progresses to cover mostof East Africa extending westwards into Uganda the Congo Basin region Cameroonand northwards into Ethiopia an area approximately bounded by 10deg N and 10deg S

The above normal NDVI across the equatorial belt of Africa was a result ofenhanced convective activity within the ITCZ ( gure 1(b)) due to elevated SSTs inboth the equatorial WIO and the equatorial southern Atlantic Ocean ( gure 1(a))Strong positive anomalies occur along the West Africa coast southwards to Angola

A Anyamba et al1852

and in North Africa over Morocco and Algeria Lamb and Peppler (1991) haveshown that variability in Moroccan rainfall between November and April is relatedto the state of the North Atlantic Oscillation (NAO) and weakly related to ENSOThe NAO is an interannual climate variability signal measured by the pressurediVerence between the region surrounding Iceland and the region around the AzoresIslands north-west of Morocco in the Atlantic Ocean The observed NDVI anomaliesover north-western Africa during this period may thus be a result of the interactionof these two phenomena

At a continental scale December 1997 shows a meridional dipole in NDVIpatterns A band of positive NDVI anomalies stretches across Africa betweenapproximately 12deg N and 10deg S The area south of 10deg S shows negative NDVIanomalies (shy 01 to shy 02) primarily in Botswana north-eastern Namibia southernAngola and southern Zambia The positive NDVI anomaly patterns over the entireeastern Africa region persist through February 1998 The area of high positiveanomalies is reduced by March and April primarily covering Kenya and central tosouthern Somalia while negative NDVI anomalies emerge along the west Africancoastal strip and over the western half of East Africa Over southern Africa thenegative NDVI anomalies become more marked in MarchndashApril The spatial struc-ture shows a north-west (negative) and south-east (positive) dipole pattern in Marchand April This pattern is somewhat similar to that observed during the 198687 and199495 ENSO warm events over the region and represents one of the two types ofENSO spatial manifestation identi ed in Southern Africa using NDVI data (Eastmanand Anyamba 1996) One of the most signi cant manifestations of the 199798ENSO warm event is the persistence and extension of positive NDVI anomaliesfrom the short rains growing season (OctoberndashDecember) into the dry season(JanuaryndashFebruary) and into the long rains season (MarchndashJune) over eastern AfricaAs such there was no dry period experienced early in 1998 which was quite adramatic departure from the normal seasonal cycle In general the continental-scalepatterns of NDVI anomalies correspond well with rainfall teleconnections patternsas shown in Ogallo (1988) and Ropelweski and Halpert (1987) with a somewhatlimited drought extent over Southern Africa The classic ENSO teleconnectionpattern over Africa is the tendency for equatorial East Africa to receive above normalrainfall and for drought to persist over Southern Africa during ENSO warm events(Lindsey 1988)

32 Temporal evolution of NDVI means and anomaliesThree sites were selected for analysis of the temporal evolution of NDVI means

and anomalies Plots of the long-term means monthly means and anomalies forthese sites are shown in gure 3(andashf ) The East Africa site ( gure 3(a)) shows adramatic increase in NDVI from September to October 1997 reaching a maximumin February 1998 and decreasing to near previous October 1997 levels by AprilndashMay 1998 The evolution of the anomalies ( gure 3(d)) shows a normal curve patterncentred on DecemberJanuary and is very typical of the phase locking character ofENSO to the SST annual cycle in the eastern Paci c Ocean (Rasmusson andCarpenter 1982) The NDVI levels for the period October to March are much higherthan the long-term average ( gure 3(a)) As can be noted the typical low NDVIlevels during the dry season over the region during DecemberndashFebruary were notobserved As a result the region experienced continuous above normal NDVI levelsfor a period of 8 months from October 1997 to May 1998 ( gure 1) This was a

NDVI anomalies during the ENSO event 1853

Figure 3 Temporal pro les of monthly NDVI values and long-term means (top panel) for(a) East Africa (40deg E 0deg S) (b) Namibia (18deg E 22deg S) and (c) south-eastern SouthAfrica (29deg E 29deg S) The corresponding NDVI anomalies for these sites are shown inthe bottom panel (d e f) Black line plots are long-term mean values grey bars aremonthly values and anomalies The NDVI values are averages extracted from a100kmtimes100 km box centred on each of these locations For East Africa during theperiod July 1997ndashMay 1998 the NDVI anomalies are well above the long-term means(d) indicative of greener than normal conditions For the Southern Africa sites (e f)the anomalies are above their respective long-term means during the early part of thegrowing season (SeptemberndashNovember) and near normal or slightly below normalduring the peak growing season (DecemberndashMarch)

A Anyamba et al1854

result of the record breaking above normal rainfall received during this period someareas in coastal central and eastern Kenya and in Somalia averaged 5ndash10 times thenormal rainfall (Bell and Halpert 1998)

The temporal pro le drawn from Namibia shows a slight decrease in NDVI fromJuly to September then an increase to values above the long-term mean in November1997 during the Southern hemisphere spring season ( gure 3(b)) This is followed bya dramatic decrease in December 1997 below the long-term mean then an increaseto just above the long-term mean values in January 1998 The later part of thegrowing season (February to May) shows slightly below normal NDVI with themaximum negative anomaly in March 1998 ( gure 3(e)) One noticeable feature ofthe anomalies is the increase in NDVI prior to the beginning of the growing season(SeptemberndashNovember) and a decrease in December This character has been notedas a precursor during warm ENSO years (Eastman and Anyamba 1996) This patternis consistent with the tendency for above normal precipitation over the region duringthe early phase of a developing warm event and lower than normal precipitationduring the mature phase of a warm event (Diaz and Kiladis 1993 Nicholson andEntekhabi 1986) Long-term means and monthly NDVI values from the second sitein south-eastern South Africa are shown gure 3(c) The annual cycle de ned by thelong-term average shows a minimum in NDVI during September and a maximumin February The corresponding monthly NDVI values during the period of analysisfollow a similar pattern The earlier part of the growing season (SeptemberndashNovember) shows signi cantly above normal NDVI decreasing to slightly belownormal conditions in February 1998 and a recovery to slightly above normal con-ditions from March to May ( gure 3( f )) Overall the drought conditions were notas severe over the Southern Africa region as in previous ENSO warm events Thewestern portions of the region were slightly drier than normal especially in Namibiaand northern Botswana in December FebruaryndashApril ( gure 2) as compared withthe south-eastern region covering the Drakensburg Mountains and parts of Transvaalwhich were greener than normal The dominant regional pattern over SouthernAfrica shows a north-west (negative)ndashsouth-east (positive) dipole pattern in NDVIanomalies This pattern supports ndings by Eastman and Anyamba (1996) of thediVerent manifestations of ENSO warm event teleconnection patterns over SouthernAfrica This north-westndashsouth-east spatial pattern is also well marked in the OLRanomalies ( gure 1(b))

33 Correlation patterns between NDVI anomalies and ENSO indicesIn the context of ENSO teleconnections in global weather patterns are based

on the notion that there is a tendency for anomalous warming in SSTs in the easternPaci c Ocean ( gure 1(a)) to have widespread impacts over distant or remote loca-tions Speci cally this is manifested in the ampli cationdampening of the precipita-tion signal (and hence vegetation greenness patterns in our case) over widespreadlocations over the global tropics The Paci c signal is transmitted via the AtmosphericWalker Circulation System (Southern Oscillation) causing dramatic changes in thenormal patterns of precipitation in the major centres of tropical convection includingthe Amazon Basin Indonesia and Congo Basin and the surrounding areas( gure 1(b)) These changes result in above normal precipitation in some regions andbelow normal precipitation in others Typically anomalous conditions associatedwith ENSO are observed during the rainfallgrowing season for speci c tropicallocations To test whether there is any correlation between NDVI departure patterns

NDVI anomalies during the ENSO event 1855

over Africa and ENSO during the 199798 period we computed correlations betweenENSO indices and NDVI anomalies for selected sites (in sect31 above) in East andSouthern Africa

The evolution of NDVI anomalies over eastern Kenya compared with the SSTanomalies in the adjacent region in the WIO is shown in gure 4(a) for the periodJuly 1997 to May 1998 As can be seen the patterns of evolution of the two variablesco-evolve well with a peak in January (WIO-SST ) and February (NDVI) and a dropin the anomalies towards the end of the long rains growing season in May For theentire time period both the NDVI and SST show positive anomalies There is noapparent lag relationship except for the peak period in the series where the SSTreaches a maximum in January and the NDVI reaches a maximum in FebruarySimultaneous variations between NDVI and SST are observed nevertheless NDVIattests to a time response of 1ndash2 months in the semi-arid lands of Africa (Nicholsonet al 1990 Richard and Poccard 1998) The coeYcient of correlation between thetwo time series is 083 (plt0005) This shows the close coupling between WIOwarming precipitation and vegetation greenness patterns over large areas of EastAfrica during this period Signi cant correlations are also found with the globaltropical SSTs (087) and the Southern Atlantic SSTs (083) (table 1) This in partshows the in uence of an enhanced ITCZ due to above normal SST across theglobal tropics and its eVects on precipitation ( gure 1(a)) and thus vegetation overEast Africa ( gure 2)

Lag correlation with the WIO leads to a signi cant drop in correlation coeYcientsfrom 080 (lag 0) to 058 at lag 3 (see gure 4(b)) showing that at the monthly timescale the NDVI response to WIO SST is near simultaneous Correlations with otherENSO indicators at zero-lag are rather low (see table 1) However at the 1ndash4 monthlag there is a signi cant increase in correlations for example to shy 080 with NINO34with NDVI lagging NINO34 SST by about three months ( gure 4(c d)) Althoughconvective activity was already enhanced in the eastern Paci c by August 1997 thecorresponding pattern does not show up in the WIO until OctoberndashDecember duringthe short rains growing season in East Africa (Bell and Halpert 1998) This is notunexpected as the Indian Ocean response to El Nino involves basin scale warmingwith a lag of about one season (Cadet and Diehl 1984) This in part accounts forthe lagged response between NDVI over East Africa and NINO34 SST anomalies

The Namibia sample site shows a positive relationship with NINO34 SST fromSeptember to November ( gure 4(e)) but this gives way to an inverse relationshipin December and February to May during the late phase of eastern Paci c warming( gure 4(e)) Correlations with most of the ENSO and other interannual variabilityindicators are rather weak (see table 1) The highest coeYcient of correlation is withNINO34 at 050 The temporal evolution pattern with NINO34 SST for south-eastern South Africa is shown in gure 4( f ) As in the previous case the minimumin NDVI is from December to March corresponding to the period of peak warmingin the eastern Paci c The highest correlations are with SOI (043) and WIO-SST(043) At the two-month lag the coeYcient of correlation with NINO34 increasessigni cantly to shy 075 (plt0005) Although correlations with the QBO and AtlanticSSTs were not signi cant for the two Southern Africa sites previous work by Masonand Tyson (1992) and Jury and Pathack (1993) show that the phase of QBO andAtlantic Ocean SSTs can modulate the eVects of ENSO over the Southern Africaregion When the QBO is in an anti-SOI phase as was the case during this periodit has a tendency to dampen the normal ENSO drought impacts over Southern

A Anyamba et al1856

(a) (d )

(b) (e)

(c) ( f )

Figure 4 Comparison and correlations between NDVI anomalies and ENSO indices EastAfrica (andashd) Southern Africa (endashf) NDVI anomalies over East Africa closely followthe pattern of SST departure patterns in the WIO region (a) with a signi cantcoeYcient of correlation at lag 0 (r=083) (b) There is an apparent lag of about threemonths between East Africa NDVI and NINO34 SST with a peak in NINO34 SSTin December 1997 followed by a peak in East Africa NDVI in February 1998 (c) Thelag correlation plot is shown in (d) Shown in (e) and ( f) are NDVI anomalies forNamibia and south-eastern South Africa respectively plotted against NINO34 SSTanomalies In both cases in the early phase of the event there are slightly positiveNDVI anomalies followed by near normal to negative NDVI anomalies at themaximum of Paci c warming in DecemberndashMarch This is a recognized feature ofrainfall patterns in this region during ENSO warm events with tendency for precipita-tion to be above normal during the early phase and below normal during the maturephase (Nicholson and Entekhabi 1986)

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

NDVI anomalies during the ENSO event 1849

climatic conditions using normalized diVerence vegetation index (NDVI) data forAfrica for the period July 1997 to May 1998 The index is derived from broad bandmeasurements in the visible and infrared channels made by the Advanced Very High-Resolution Radiometer (AVHRR) instrument onboard the National Oceanographicand Atmospheric Administration (NOAA) series of polar orbiting meteorologicalsatellites The index is calculated as NDVI=(Ch2 shy Ch1)(Ch2+Ch1) where Ch1and Ch2 are upwelling land surface radiances in the red and near-infrared wave-lengths respectively It has been found to provide a strong vegetation signal andgood spectral contrast from most background materials (Tucker and Sellers 1986)with restrictions in some arid and semi-arid areas where bare soil re ectance maycause large NDVI variations (Huete and Tucker 1991 Farrar et al 1994) It hasalso been shown to be a good indicator of various vegetation parameters includinggreen leaf area index (LAI) biomass percentage green cover green biomass produc-tion and the fraction of absorbed photosynthetically active radiation (Tucker 1979Sellers 1985Asrar 1989) Taken as time-series measurements NDVI is well correlatedwith climate variables including rainfall and evapotranspiration in a wide range ofenvironmental conditions (Gray and Tapley 1985 Justice et al 1986 Nicholson et al1990 Cihlar et al 1991) NDVI may therefore be considered to represent theintegration of the land surface responses to climate variability at various time andspace scales Employing various data decomposition techniques it has been illus-trated recently that seasonal scale signals (Eastman and Fulk 1993 Andres et al1994) and interannual signals are related to ENSO (Eastman and Fulk 1993 Myneniet al 1995 Anyamba and Eastman 1996) Furthermore examination of monthlyNDVI anomaly patterns shows that there are unique land surface response patternsfrom one ENSO warm event to another (Eastman and Anyamba 1996) In totalityNDVI can be used as an indicator of the biosphere response to climate variabilityat a range of time scales

2 Data and analysis methodsThe Global Inventory Monitoring and Modeling Systems (GIMMS) group at

NASAGSFC processed the NDVI data used in this analysis Near-real time dataprocessing for Africa is carried out to support the activities of the United StatesAgency for International Development (USAID) Famine Early Warning System(FEWS) project The data at 8 km spatial resolution are rst processed as 10-daycomposites using the maximum value compositing procedure to minimize eVects ofcloud contamination (Holben 1986 Tucker and Newcomb 1994) For the purposeof this research we created monthly composites from three 10-day composites inany given month to further minimize the eVects of clouds on the vegetation signaland to match the monthly temporal resolution of the associated ENSO indices Inaddition calibration based on invariant desert targets has been applied to the datato minimize the eVects of sensor degradation (Los 1993) The time series of thesemeasurements began in July 1981 and continues to date So far it constitutes thelongest available representation of global biosphere dynamics for studies of climatendashvegetation interactions (Los et al 1994) The ENSO indices used in the analysisinclude the following

1 Sea surface temperatures (SST) anomaly data drawn from the NINO34 ENSOmonitoring region in the eastern Paci c Ocean (5deg Nndash5deg S 170ndash120deg W)

A Anyamba et al1850

2 The Southern Oscillation Index (SOI) which represents the basin-widesee-saw in atmospheric pressure patterns between the eastern and westernPaci c measured as normalized diVerence in sea level pressure betweenPapeete Tahiti and Darwin Australia

3 The Eastern Paci c Wind Index (EPWI) (5deg Nndash5deg S 135ndash120deg W) whichrepresents the interannual variability in the wind ow patterns in the easternPaci c region

4 The equatorial Western Indian Ocean (WIO) SST index (10deg Nndash10deg S40ndash64deg E) which was extracted from the gridded global SST time series datasetThe region selected for computation of this index (excluding land areas) showedthe largest magnitude of positive SST departures in the WIO during the199798 ENSO event It is more or less similar to the domain used by Sajiet al (1999) in the computation of an eastndashwest dipole index for the IndianOcean

Other indicators of tropical interannual climate dynamics used in this analysisinclude variations in the North Atlantic SST [ NAT] (5ndash20deg N 60ndash30deg W) SouthAtlantic SST [ SAT] (0ndash20deg S 30deg Wndash10deg E) Global Tropics SST [ TROP](10deg Sndash10deg N 0ndash360deg) and the Quasi-Biennial Oscillation Index (QBO30) of zonallyaveraged winds at 30 MB over the equator Variations in patterns of SSTs in theAtlantic have an in uence on the location and intensity of the IntertropicalConvergence Zone and hence on precipitation over Africa (Lamb et al 1986Hastenrath 1990) All the ENSO and tropical SST indices were obtained from theactive archive of NOAArsquos Climate Prediction Center (CPC)

NDVI anomaly images were computed as diVerences from monthly long-termmeans using the 17-year record (1982ndash1998 used as the base period) excluding cloudpixels Though this period of anomaly analysis is relatively short and characterizedby some discontinuities in the mapping instruments (Kaufman and Holben 1993)the results obtained agree well with the departure patterns in both OLR ( gure 1(b))and in rainfall as illustrated in Bell and Halpert (1998) The NDVI anomaly patternsfor the period September 1997 to April 1998 are shown in gure 2 The nature ofthe general teleconnections between ENSO and patterns of variability in NDVI overAfrica is explored using correlation and cross-correlation analysis techniques (Katz1988) between NDVI anomalies at selected sites and ENSO indices Three sites weresampled for use in the correlation analysis For each site box averages of the long-term means monthly values and monthly anomalies were computed from a100kmtimes100 km area centred on each of these locations For East Africa the samplesite is located in eastern Kenya centred at 40deg E 0deg S with two sites in SouthernAfrica in Namibia (18deg E 22deg S) and South Africa (29deg E 29deg S) These sites wereselected taking into account the areas that show anomalous precipitation responseto ENSO (Ropelewski and Halpert 1987 1989 1996 Nicholson and Kim 1997) andthe observed anomaly patterns in NDVI during this event and during previousENSO warm events (Myneni et al 1995 Eastman and Anyamba 1996)

3 Results31 Spatial evolution of NDVI anomaly patterns

Monthly NDVI anomalies for the 199798 ENSO warm event from September1997 to April 1998 are shown in gure 2 Anomalies are computed in NDVI units(NDVI range shy 1 to +1) September 1997 shows a discontinuous band of negative

NDVI anomalies during the ENSO event 1851

Figure 2 Monthly AVHRR NDVI anomaly images of continental Africa during the 199798ENSO warm event (September 1997ndashApril 1998) Departures are calculated withrespect to 17-year means (1982ndash1998) Green areas show positive NDVI anomaliesyellow neutral and brown-red show negative NDVI anomalies Positive anomaliesin East Africa during the entire period correspond well with the location of negativeanomalies in OLR shown in gure 1(b) Negative departures are observed in thewestern half of Southern Africa in December 1997 and from February to April 1998

NDVI anomalies across the Sahel into parts of East Africa and positive anomaliesalong the West African coastal belt the Congo Basin region and south-easternAfrica September is for the most part a transitional month when the rains recedefrom the Sahel with the movement of the Intertropical Convergence Zone (ITCZ)from the Northern hemisphere to the Southern hemisphere over the continent The rst strong indicator of anomalous conditions occurs in October Positive anomaliesin NDVI (~02) occur in East Africa predominantly in areas east of the Rift Valleycovering the coastal plain lands and semi-arid and arid areas of eastern and north-eastern Kenya and Somalia The western half of East Africa shows negative NDVIanomalies resulting in a zonal east (positive)ndashwest (negative) anomaly in vegetationconditions Isolated clusters of positive NDVI anomalies occur in south-easternAfrica The positive anomaly pattern over Kenya and Somalia is enhanced inNovemberndashDecember (gt+020) as the short rains season progresses to cover mostof East Africa extending westwards into Uganda the Congo Basin region Cameroonand northwards into Ethiopia an area approximately bounded by 10deg N and 10deg S

The above normal NDVI across the equatorial belt of Africa was a result ofenhanced convective activity within the ITCZ ( gure 1(b)) due to elevated SSTs inboth the equatorial WIO and the equatorial southern Atlantic Ocean ( gure 1(a))Strong positive anomalies occur along the West Africa coast southwards to Angola

A Anyamba et al1852

and in North Africa over Morocco and Algeria Lamb and Peppler (1991) haveshown that variability in Moroccan rainfall between November and April is relatedto the state of the North Atlantic Oscillation (NAO) and weakly related to ENSOThe NAO is an interannual climate variability signal measured by the pressurediVerence between the region surrounding Iceland and the region around the AzoresIslands north-west of Morocco in the Atlantic Ocean The observed NDVI anomaliesover north-western Africa during this period may thus be a result of the interactionof these two phenomena

At a continental scale December 1997 shows a meridional dipole in NDVIpatterns A band of positive NDVI anomalies stretches across Africa betweenapproximately 12deg N and 10deg S The area south of 10deg S shows negative NDVIanomalies (shy 01 to shy 02) primarily in Botswana north-eastern Namibia southernAngola and southern Zambia The positive NDVI anomaly patterns over the entireeastern Africa region persist through February 1998 The area of high positiveanomalies is reduced by March and April primarily covering Kenya and central tosouthern Somalia while negative NDVI anomalies emerge along the west Africancoastal strip and over the western half of East Africa Over southern Africa thenegative NDVI anomalies become more marked in MarchndashApril The spatial struc-ture shows a north-west (negative) and south-east (positive) dipole pattern in Marchand April This pattern is somewhat similar to that observed during the 198687 and199495 ENSO warm events over the region and represents one of the two types ofENSO spatial manifestation identi ed in Southern Africa using NDVI data (Eastmanand Anyamba 1996) One of the most signi cant manifestations of the 199798ENSO warm event is the persistence and extension of positive NDVI anomaliesfrom the short rains growing season (OctoberndashDecember) into the dry season(JanuaryndashFebruary) and into the long rains season (MarchndashJune) over eastern AfricaAs such there was no dry period experienced early in 1998 which was quite adramatic departure from the normal seasonal cycle In general the continental-scalepatterns of NDVI anomalies correspond well with rainfall teleconnections patternsas shown in Ogallo (1988) and Ropelweski and Halpert (1987) with a somewhatlimited drought extent over Southern Africa The classic ENSO teleconnectionpattern over Africa is the tendency for equatorial East Africa to receive above normalrainfall and for drought to persist over Southern Africa during ENSO warm events(Lindsey 1988)

32 Temporal evolution of NDVI means and anomaliesThree sites were selected for analysis of the temporal evolution of NDVI means

and anomalies Plots of the long-term means monthly means and anomalies forthese sites are shown in gure 3(andashf ) The East Africa site ( gure 3(a)) shows adramatic increase in NDVI from September to October 1997 reaching a maximumin February 1998 and decreasing to near previous October 1997 levels by AprilndashMay 1998 The evolution of the anomalies ( gure 3(d)) shows a normal curve patterncentred on DecemberJanuary and is very typical of the phase locking character ofENSO to the SST annual cycle in the eastern Paci c Ocean (Rasmusson andCarpenter 1982) The NDVI levels for the period October to March are much higherthan the long-term average ( gure 3(a)) As can be noted the typical low NDVIlevels during the dry season over the region during DecemberndashFebruary were notobserved As a result the region experienced continuous above normal NDVI levelsfor a period of 8 months from October 1997 to May 1998 ( gure 1) This was a

NDVI anomalies during the ENSO event 1853

Figure 3 Temporal pro les of monthly NDVI values and long-term means (top panel) for(a) East Africa (40deg E 0deg S) (b) Namibia (18deg E 22deg S) and (c) south-eastern SouthAfrica (29deg E 29deg S) The corresponding NDVI anomalies for these sites are shown inthe bottom panel (d e f) Black line plots are long-term mean values grey bars aremonthly values and anomalies The NDVI values are averages extracted from a100kmtimes100 km box centred on each of these locations For East Africa during theperiod July 1997ndashMay 1998 the NDVI anomalies are well above the long-term means(d) indicative of greener than normal conditions For the Southern Africa sites (e f)the anomalies are above their respective long-term means during the early part of thegrowing season (SeptemberndashNovember) and near normal or slightly below normalduring the peak growing season (DecemberndashMarch)

A Anyamba et al1854

result of the record breaking above normal rainfall received during this period someareas in coastal central and eastern Kenya and in Somalia averaged 5ndash10 times thenormal rainfall (Bell and Halpert 1998)

The temporal pro le drawn from Namibia shows a slight decrease in NDVI fromJuly to September then an increase to values above the long-term mean in November1997 during the Southern hemisphere spring season ( gure 3(b)) This is followed bya dramatic decrease in December 1997 below the long-term mean then an increaseto just above the long-term mean values in January 1998 The later part of thegrowing season (February to May) shows slightly below normal NDVI with themaximum negative anomaly in March 1998 ( gure 3(e)) One noticeable feature ofthe anomalies is the increase in NDVI prior to the beginning of the growing season(SeptemberndashNovember) and a decrease in December This character has been notedas a precursor during warm ENSO years (Eastman and Anyamba 1996) This patternis consistent with the tendency for above normal precipitation over the region duringthe early phase of a developing warm event and lower than normal precipitationduring the mature phase of a warm event (Diaz and Kiladis 1993 Nicholson andEntekhabi 1986) Long-term means and monthly NDVI values from the second sitein south-eastern South Africa are shown gure 3(c) The annual cycle de ned by thelong-term average shows a minimum in NDVI during September and a maximumin February The corresponding monthly NDVI values during the period of analysisfollow a similar pattern The earlier part of the growing season (SeptemberndashNovember) shows signi cantly above normal NDVI decreasing to slightly belownormal conditions in February 1998 and a recovery to slightly above normal con-ditions from March to May ( gure 3( f )) Overall the drought conditions were notas severe over the Southern Africa region as in previous ENSO warm events Thewestern portions of the region were slightly drier than normal especially in Namibiaand northern Botswana in December FebruaryndashApril ( gure 2) as compared withthe south-eastern region covering the Drakensburg Mountains and parts of Transvaalwhich were greener than normal The dominant regional pattern over SouthernAfrica shows a north-west (negative)ndashsouth-east (positive) dipole pattern in NDVIanomalies This pattern supports ndings by Eastman and Anyamba (1996) of thediVerent manifestations of ENSO warm event teleconnection patterns over SouthernAfrica This north-westndashsouth-east spatial pattern is also well marked in the OLRanomalies ( gure 1(b))

33 Correlation patterns between NDVI anomalies and ENSO indicesIn the context of ENSO teleconnections in global weather patterns are based

on the notion that there is a tendency for anomalous warming in SSTs in the easternPaci c Ocean ( gure 1(a)) to have widespread impacts over distant or remote loca-tions Speci cally this is manifested in the ampli cationdampening of the precipita-tion signal (and hence vegetation greenness patterns in our case) over widespreadlocations over the global tropics The Paci c signal is transmitted via the AtmosphericWalker Circulation System (Southern Oscillation) causing dramatic changes in thenormal patterns of precipitation in the major centres of tropical convection includingthe Amazon Basin Indonesia and Congo Basin and the surrounding areas( gure 1(b)) These changes result in above normal precipitation in some regions andbelow normal precipitation in others Typically anomalous conditions associatedwith ENSO are observed during the rainfallgrowing season for speci c tropicallocations To test whether there is any correlation between NDVI departure patterns

NDVI anomalies during the ENSO event 1855

over Africa and ENSO during the 199798 period we computed correlations betweenENSO indices and NDVI anomalies for selected sites (in sect31 above) in East andSouthern Africa

The evolution of NDVI anomalies over eastern Kenya compared with the SSTanomalies in the adjacent region in the WIO is shown in gure 4(a) for the periodJuly 1997 to May 1998 As can be seen the patterns of evolution of the two variablesco-evolve well with a peak in January (WIO-SST ) and February (NDVI) and a dropin the anomalies towards the end of the long rains growing season in May For theentire time period both the NDVI and SST show positive anomalies There is noapparent lag relationship except for the peak period in the series where the SSTreaches a maximum in January and the NDVI reaches a maximum in FebruarySimultaneous variations between NDVI and SST are observed nevertheless NDVIattests to a time response of 1ndash2 months in the semi-arid lands of Africa (Nicholsonet al 1990 Richard and Poccard 1998) The coeYcient of correlation between thetwo time series is 083 (plt0005) This shows the close coupling between WIOwarming precipitation and vegetation greenness patterns over large areas of EastAfrica during this period Signi cant correlations are also found with the globaltropical SSTs (087) and the Southern Atlantic SSTs (083) (table 1) This in partshows the in uence of an enhanced ITCZ due to above normal SST across theglobal tropics and its eVects on precipitation ( gure 1(a)) and thus vegetation overEast Africa ( gure 2)

Lag correlation with the WIO leads to a signi cant drop in correlation coeYcientsfrom 080 (lag 0) to 058 at lag 3 (see gure 4(b)) showing that at the monthly timescale the NDVI response to WIO SST is near simultaneous Correlations with otherENSO indicators at zero-lag are rather low (see table 1) However at the 1ndash4 monthlag there is a signi cant increase in correlations for example to shy 080 with NINO34with NDVI lagging NINO34 SST by about three months ( gure 4(c d)) Althoughconvective activity was already enhanced in the eastern Paci c by August 1997 thecorresponding pattern does not show up in the WIO until OctoberndashDecember duringthe short rains growing season in East Africa (Bell and Halpert 1998) This is notunexpected as the Indian Ocean response to El Nino involves basin scale warmingwith a lag of about one season (Cadet and Diehl 1984) This in part accounts forthe lagged response between NDVI over East Africa and NINO34 SST anomalies

The Namibia sample site shows a positive relationship with NINO34 SST fromSeptember to November ( gure 4(e)) but this gives way to an inverse relationshipin December and February to May during the late phase of eastern Paci c warming( gure 4(e)) Correlations with most of the ENSO and other interannual variabilityindicators are rather weak (see table 1) The highest coeYcient of correlation is withNINO34 at 050 The temporal evolution pattern with NINO34 SST for south-eastern South Africa is shown in gure 4( f ) As in the previous case the minimumin NDVI is from December to March corresponding to the period of peak warmingin the eastern Paci c The highest correlations are with SOI (043) and WIO-SST(043) At the two-month lag the coeYcient of correlation with NINO34 increasessigni cantly to shy 075 (plt0005) Although correlations with the QBO and AtlanticSSTs were not signi cant for the two Southern Africa sites previous work by Masonand Tyson (1992) and Jury and Pathack (1993) show that the phase of QBO andAtlantic Ocean SSTs can modulate the eVects of ENSO over the Southern Africaregion When the QBO is in an anti-SOI phase as was the case during this periodit has a tendency to dampen the normal ENSO drought impacts over Southern

A Anyamba et al1856

(a) (d )

(b) (e)

(c) ( f )

Figure 4 Comparison and correlations between NDVI anomalies and ENSO indices EastAfrica (andashd) Southern Africa (endashf) NDVI anomalies over East Africa closely followthe pattern of SST departure patterns in the WIO region (a) with a signi cantcoeYcient of correlation at lag 0 (r=083) (b) There is an apparent lag of about threemonths between East Africa NDVI and NINO34 SST with a peak in NINO34 SSTin December 1997 followed by a peak in East Africa NDVI in February 1998 (c) Thelag correlation plot is shown in (d) Shown in (e) and ( f) are NDVI anomalies forNamibia and south-eastern South Africa respectively plotted against NINO34 SSTanomalies In both cases in the early phase of the event there are slightly positiveNDVI anomalies followed by near normal to negative NDVI anomalies at themaximum of Paci c warming in DecemberndashMarch This is a recognized feature ofrainfall patterns in this region during ENSO warm events with tendency for precipita-tion to be above normal during the early phase and below normal during the maturephase (Nicholson and Entekhabi 1986)

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

A Anyamba et al1850

2 The Southern Oscillation Index (SOI) which represents the basin-widesee-saw in atmospheric pressure patterns between the eastern and westernPaci c measured as normalized diVerence in sea level pressure betweenPapeete Tahiti and Darwin Australia

3 The Eastern Paci c Wind Index (EPWI) (5deg Nndash5deg S 135ndash120deg W) whichrepresents the interannual variability in the wind ow patterns in the easternPaci c region

4 The equatorial Western Indian Ocean (WIO) SST index (10deg Nndash10deg S40ndash64deg E) which was extracted from the gridded global SST time series datasetThe region selected for computation of this index (excluding land areas) showedthe largest magnitude of positive SST departures in the WIO during the199798 ENSO event It is more or less similar to the domain used by Sajiet al (1999) in the computation of an eastndashwest dipole index for the IndianOcean

Other indicators of tropical interannual climate dynamics used in this analysisinclude variations in the North Atlantic SST [ NAT] (5ndash20deg N 60ndash30deg W) SouthAtlantic SST [ SAT] (0ndash20deg S 30deg Wndash10deg E) Global Tropics SST [ TROP](10deg Sndash10deg N 0ndash360deg) and the Quasi-Biennial Oscillation Index (QBO30) of zonallyaveraged winds at 30 MB over the equator Variations in patterns of SSTs in theAtlantic have an in uence on the location and intensity of the IntertropicalConvergence Zone and hence on precipitation over Africa (Lamb et al 1986Hastenrath 1990) All the ENSO and tropical SST indices were obtained from theactive archive of NOAArsquos Climate Prediction Center (CPC)

NDVI anomaly images were computed as diVerences from monthly long-termmeans using the 17-year record (1982ndash1998 used as the base period) excluding cloudpixels Though this period of anomaly analysis is relatively short and characterizedby some discontinuities in the mapping instruments (Kaufman and Holben 1993)the results obtained agree well with the departure patterns in both OLR ( gure 1(b))and in rainfall as illustrated in Bell and Halpert (1998) The NDVI anomaly patternsfor the period September 1997 to April 1998 are shown in gure 2 The nature ofthe general teleconnections between ENSO and patterns of variability in NDVI overAfrica is explored using correlation and cross-correlation analysis techniques (Katz1988) between NDVI anomalies at selected sites and ENSO indices Three sites weresampled for use in the correlation analysis For each site box averages of the long-term means monthly values and monthly anomalies were computed from a100kmtimes100 km area centred on each of these locations For East Africa the samplesite is located in eastern Kenya centred at 40deg E 0deg S with two sites in SouthernAfrica in Namibia (18deg E 22deg S) and South Africa (29deg E 29deg S) These sites wereselected taking into account the areas that show anomalous precipitation responseto ENSO (Ropelewski and Halpert 1987 1989 1996 Nicholson and Kim 1997) andthe observed anomaly patterns in NDVI during this event and during previousENSO warm events (Myneni et al 1995 Eastman and Anyamba 1996)

3 Results31 Spatial evolution of NDVI anomaly patterns

Monthly NDVI anomalies for the 199798 ENSO warm event from September1997 to April 1998 are shown in gure 2 Anomalies are computed in NDVI units(NDVI range shy 1 to +1) September 1997 shows a discontinuous band of negative

NDVI anomalies during the ENSO event 1851

Figure 2 Monthly AVHRR NDVI anomaly images of continental Africa during the 199798ENSO warm event (September 1997ndashApril 1998) Departures are calculated withrespect to 17-year means (1982ndash1998) Green areas show positive NDVI anomaliesyellow neutral and brown-red show negative NDVI anomalies Positive anomaliesin East Africa during the entire period correspond well with the location of negativeanomalies in OLR shown in gure 1(b) Negative departures are observed in thewestern half of Southern Africa in December 1997 and from February to April 1998

NDVI anomalies across the Sahel into parts of East Africa and positive anomaliesalong the West African coastal belt the Congo Basin region and south-easternAfrica September is for the most part a transitional month when the rains recedefrom the Sahel with the movement of the Intertropical Convergence Zone (ITCZ)from the Northern hemisphere to the Southern hemisphere over the continent The rst strong indicator of anomalous conditions occurs in October Positive anomaliesin NDVI (~02) occur in East Africa predominantly in areas east of the Rift Valleycovering the coastal plain lands and semi-arid and arid areas of eastern and north-eastern Kenya and Somalia The western half of East Africa shows negative NDVIanomalies resulting in a zonal east (positive)ndashwest (negative) anomaly in vegetationconditions Isolated clusters of positive NDVI anomalies occur in south-easternAfrica The positive anomaly pattern over Kenya and Somalia is enhanced inNovemberndashDecember (gt+020) as the short rains season progresses to cover mostof East Africa extending westwards into Uganda the Congo Basin region Cameroonand northwards into Ethiopia an area approximately bounded by 10deg N and 10deg S

The above normal NDVI across the equatorial belt of Africa was a result ofenhanced convective activity within the ITCZ ( gure 1(b)) due to elevated SSTs inboth the equatorial WIO and the equatorial southern Atlantic Ocean ( gure 1(a))Strong positive anomalies occur along the West Africa coast southwards to Angola

A Anyamba et al1852

and in North Africa over Morocco and Algeria Lamb and Peppler (1991) haveshown that variability in Moroccan rainfall between November and April is relatedto the state of the North Atlantic Oscillation (NAO) and weakly related to ENSOThe NAO is an interannual climate variability signal measured by the pressurediVerence between the region surrounding Iceland and the region around the AzoresIslands north-west of Morocco in the Atlantic Ocean The observed NDVI anomaliesover north-western Africa during this period may thus be a result of the interactionof these two phenomena

At a continental scale December 1997 shows a meridional dipole in NDVIpatterns A band of positive NDVI anomalies stretches across Africa betweenapproximately 12deg N and 10deg S The area south of 10deg S shows negative NDVIanomalies (shy 01 to shy 02) primarily in Botswana north-eastern Namibia southernAngola and southern Zambia The positive NDVI anomaly patterns over the entireeastern Africa region persist through February 1998 The area of high positiveanomalies is reduced by March and April primarily covering Kenya and central tosouthern Somalia while negative NDVI anomalies emerge along the west Africancoastal strip and over the western half of East Africa Over southern Africa thenegative NDVI anomalies become more marked in MarchndashApril The spatial struc-ture shows a north-west (negative) and south-east (positive) dipole pattern in Marchand April This pattern is somewhat similar to that observed during the 198687 and199495 ENSO warm events over the region and represents one of the two types ofENSO spatial manifestation identi ed in Southern Africa using NDVI data (Eastmanand Anyamba 1996) One of the most signi cant manifestations of the 199798ENSO warm event is the persistence and extension of positive NDVI anomaliesfrom the short rains growing season (OctoberndashDecember) into the dry season(JanuaryndashFebruary) and into the long rains season (MarchndashJune) over eastern AfricaAs such there was no dry period experienced early in 1998 which was quite adramatic departure from the normal seasonal cycle In general the continental-scalepatterns of NDVI anomalies correspond well with rainfall teleconnections patternsas shown in Ogallo (1988) and Ropelweski and Halpert (1987) with a somewhatlimited drought extent over Southern Africa The classic ENSO teleconnectionpattern over Africa is the tendency for equatorial East Africa to receive above normalrainfall and for drought to persist over Southern Africa during ENSO warm events(Lindsey 1988)

32 Temporal evolution of NDVI means and anomaliesThree sites were selected for analysis of the temporal evolution of NDVI means

and anomalies Plots of the long-term means monthly means and anomalies forthese sites are shown in gure 3(andashf ) The East Africa site ( gure 3(a)) shows adramatic increase in NDVI from September to October 1997 reaching a maximumin February 1998 and decreasing to near previous October 1997 levels by AprilndashMay 1998 The evolution of the anomalies ( gure 3(d)) shows a normal curve patterncentred on DecemberJanuary and is very typical of the phase locking character ofENSO to the SST annual cycle in the eastern Paci c Ocean (Rasmusson andCarpenter 1982) The NDVI levels for the period October to March are much higherthan the long-term average ( gure 3(a)) As can be noted the typical low NDVIlevels during the dry season over the region during DecemberndashFebruary were notobserved As a result the region experienced continuous above normal NDVI levelsfor a period of 8 months from October 1997 to May 1998 ( gure 1) This was a

NDVI anomalies during the ENSO event 1853

Figure 3 Temporal pro les of monthly NDVI values and long-term means (top panel) for(a) East Africa (40deg E 0deg S) (b) Namibia (18deg E 22deg S) and (c) south-eastern SouthAfrica (29deg E 29deg S) The corresponding NDVI anomalies for these sites are shown inthe bottom panel (d e f) Black line plots are long-term mean values grey bars aremonthly values and anomalies The NDVI values are averages extracted from a100kmtimes100 km box centred on each of these locations For East Africa during theperiod July 1997ndashMay 1998 the NDVI anomalies are well above the long-term means(d) indicative of greener than normal conditions For the Southern Africa sites (e f)the anomalies are above their respective long-term means during the early part of thegrowing season (SeptemberndashNovember) and near normal or slightly below normalduring the peak growing season (DecemberndashMarch)

A Anyamba et al1854

result of the record breaking above normal rainfall received during this period someareas in coastal central and eastern Kenya and in Somalia averaged 5ndash10 times thenormal rainfall (Bell and Halpert 1998)

The temporal pro le drawn from Namibia shows a slight decrease in NDVI fromJuly to September then an increase to values above the long-term mean in November1997 during the Southern hemisphere spring season ( gure 3(b)) This is followed bya dramatic decrease in December 1997 below the long-term mean then an increaseto just above the long-term mean values in January 1998 The later part of thegrowing season (February to May) shows slightly below normal NDVI with themaximum negative anomaly in March 1998 ( gure 3(e)) One noticeable feature ofthe anomalies is the increase in NDVI prior to the beginning of the growing season(SeptemberndashNovember) and a decrease in December This character has been notedas a precursor during warm ENSO years (Eastman and Anyamba 1996) This patternis consistent with the tendency for above normal precipitation over the region duringthe early phase of a developing warm event and lower than normal precipitationduring the mature phase of a warm event (Diaz and Kiladis 1993 Nicholson andEntekhabi 1986) Long-term means and monthly NDVI values from the second sitein south-eastern South Africa are shown gure 3(c) The annual cycle de ned by thelong-term average shows a minimum in NDVI during September and a maximumin February The corresponding monthly NDVI values during the period of analysisfollow a similar pattern The earlier part of the growing season (SeptemberndashNovember) shows signi cantly above normal NDVI decreasing to slightly belownormal conditions in February 1998 and a recovery to slightly above normal con-ditions from March to May ( gure 3( f )) Overall the drought conditions were notas severe over the Southern Africa region as in previous ENSO warm events Thewestern portions of the region were slightly drier than normal especially in Namibiaand northern Botswana in December FebruaryndashApril ( gure 2) as compared withthe south-eastern region covering the Drakensburg Mountains and parts of Transvaalwhich were greener than normal The dominant regional pattern over SouthernAfrica shows a north-west (negative)ndashsouth-east (positive) dipole pattern in NDVIanomalies This pattern supports ndings by Eastman and Anyamba (1996) of thediVerent manifestations of ENSO warm event teleconnection patterns over SouthernAfrica This north-westndashsouth-east spatial pattern is also well marked in the OLRanomalies ( gure 1(b))

33 Correlation patterns between NDVI anomalies and ENSO indicesIn the context of ENSO teleconnections in global weather patterns are based

on the notion that there is a tendency for anomalous warming in SSTs in the easternPaci c Ocean ( gure 1(a)) to have widespread impacts over distant or remote loca-tions Speci cally this is manifested in the ampli cationdampening of the precipita-tion signal (and hence vegetation greenness patterns in our case) over widespreadlocations over the global tropics The Paci c signal is transmitted via the AtmosphericWalker Circulation System (Southern Oscillation) causing dramatic changes in thenormal patterns of precipitation in the major centres of tropical convection includingthe Amazon Basin Indonesia and Congo Basin and the surrounding areas( gure 1(b)) These changes result in above normal precipitation in some regions andbelow normal precipitation in others Typically anomalous conditions associatedwith ENSO are observed during the rainfallgrowing season for speci c tropicallocations To test whether there is any correlation between NDVI departure patterns

NDVI anomalies during the ENSO event 1855

over Africa and ENSO during the 199798 period we computed correlations betweenENSO indices and NDVI anomalies for selected sites (in sect31 above) in East andSouthern Africa

The evolution of NDVI anomalies over eastern Kenya compared with the SSTanomalies in the adjacent region in the WIO is shown in gure 4(a) for the periodJuly 1997 to May 1998 As can be seen the patterns of evolution of the two variablesco-evolve well with a peak in January (WIO-SST ) and February (NDVI) and a dropin the anomalies towards the end of the long rains growing season in May For theentire time period both the NDVI and SST show positive anomalies There is noapparent lag relationship except for the peak period in the series where the SSTreaches a maximum in January and the NDVI reaches a maximum in FebruarySimultaneous variations between NDVI and SST are observed nevertheless NDVIattests to a time response of 1ndash2 months in the semi-arid lands of Africa (Nicholsonet al 1990 Richard and Poccard 1998) The coeYcient of correlation between thetwo time series is 083 (plt0005) This shows the close coupling between WIOwarming precipitation and vegetation greenness patterns over large areas of EastAfrica during this period Signi cant correlations are also found with the globaltropical SSTs (087) and the Southern Atlantic SSTs (083) (table 1) This in partshows the in uence of an enhanced ITCZ due to above normal SST across theglobal tropics and its eVects on precipitation ( gure 1(a)) and thus vegetation overEast Africa ( gure 2)

Lag correlation with the WIO leads to a signi cant drop in correlation coeYcientsfrom 080 (lag 0) to 058 at lag 3 (see gure 4(b)) showing that at the monthly timescale the NDVI response to WIO SST is near simultaneous Correlations with otherENSO indicators at zero-lag are rather low (see table 1) However at the 1ndash4 monthlag there is a signi cant increase in correlations for example to shy 080 with NINO34with NDVI lagging NINO34 SST by about three months ( gure 4(c d)) Althoughconvective activity was already enhanced in the eastern Paci c by August 1997 thecorresponding pattern does not show up in the WIO until OctoberndashDecember duringthe short rains growing season in East Africa (Bell and Halpert 1998) This is notunexpected as the Indian Ocean response to El Nino involves basin scale warmingwith a lag of about one season (Cadet and Diehl 1984) This in part accounts forthe lagged response between NDVI over East Africa and NINO34 SST anomalies

The Namibia sample site shows a positive relationship with NINO34 SST fromSeptember to November ( gure 4(e)) but this gives way to an inverse relationshipin December and February to May during the late phase of eastern Paci c warming( gure 4(e)) Correlations with most of the ENSO and other interannual variabilityindicators are rather weak (see table 1) The highest coeYcient of correlation is withNINO34 at 050 The temporal evolution pattern with NINO34 SST for south-eastern South Africa is shown in gure 4( f ) As in the previous case the minimumin NDVI is from December to March corresponding to the period of peak warmingin the eastern Paci c The highest correlations are with SOI (043) and WIO-SST(043) At the two-month lag the coeYcient of correlation with NINO34 increasessigni cantly to shy 075 (plt0005) Although correlations with the QBO and AtlanticSSTs were not signi cant for the two Southern Africa sites previous work by Masonand Tyson (1992) and Jury and Pathack (1993) show that the phase of QBO andAtlantic Ocean SSTs can modulate the eVects of ENSO over the Southern Africaregion When the QBO is in an anti-SOI phase as was the case during this periodit has a tendency to dampen the normal ENSO drought impacts over Southern

A Anyamba et al1856

(a) (d )

(b) (e)

(c) ( f )

Figure 4 Comparison and correlations between NDVI anomalies and ENSO indices EastAfrica (andashd) Southern Africa (endashf) NDVI anomalies over East Africa closely followthe pattern of SST departure patterns in the WIO region (a) with a signi cantcoeYcient of correlation at lag 0 (r=083) (b) There is an apparent lag of about threemonths between East Africa NDVI and NINO34 SST with a peak in NINO34 SSTin December 1997 followed by a peak in East Africa NDVI in February 1998 (c) Thelag correlation plot is shown in (d) Shown in (e) and ( f) are NDVI anomalies forNamibia and south-eastern South Africa respectively plotted against NINO34 SSTanomalies In both cases in the early phase of the event there are slightly positiveNDVI anomalies followed by near normal to negative NDVI anomalies at themaximum of Paci c warming in DecemberndashMarch This is a recognized feature ofrainfall patterns in this region during ENSO warm events with tendency for precipita-tion to be above normal during the early phase and below normal during the maturephase (Nicholson and Entekhabi 1986)

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

NDVI anomalies during the ENSO event 1851

Figure 2 Monthly AVHRR NDVI anomaly images of continental Africa during the 199798ENSO warm event (September 1997ndashApril 1998) Departures are calculated withrespect to 17-year means (1982ndash1998) Green areas show positive NDVI anomaliesyellow neutral and brown-red show negative NDVI anomalies Positive anomaliesin East Africa during the entire period correspond well with the location of negativeanomalies in OLR shown in gure 1(b) Negative departures are observed in thewestern half of Southern Africa in December 1997 and from February to April 1998

NDVI anomalies across the Sahel into parts of East Africa and positive anomaliesalong the West African coastal belt the Congo Basin region and south-easternAfrica September is for the most part a transitional month when the rains recedefrom the Sahel with the movement of the Intertropical Convergence Zone (ITCZ)from the Northern hemisphere to the Southern hemisphere over the continent The rst strong indicator of anomalous conditions occurs in October Positive anomaliesin NDVI (~02) occur in East Africa predominantly in areas east of the Rift Valleycovering the coastal plain lands and semi-arid and arid areas of eastern and north-eastern Kenya and Somalia The western half of East Africa shows negative NDVIanomalies resulting in a zonal east (positive)ndashwest (negative) anomaly in vegetationconditions Isolated clusters of positive NDVI anomalies occur in south-easternAfrica The positive anomaly pattern over Kenya and Somalia is enhanced inNovemberndashDecember (gt+020) as the short rains season progresses to cover mostof East Africa extending westwards into Uganda the Congo Basin region Cameroonand northwards into Ethiopia an area approximately bounded by 10deg N and 10deg S

The above normal NDVI across the equatorial belt of Africa was a result ofenhanced convective activity within the ITCZ ( gure 1(b)) due to elevated SSTs inboth the equatorial WIO and the equatorial southern Atlantic Ocean ( gure 1(a))Strong positive anomalies occur along the West Africa coast southwards to Angola

A Anyamba et al1852

and in North Africa over Morocco and Algeria Lamb and Peppler (1991) haveshown that variability in Moroccan rainfall between November and April is relatedto the state of the North Atlantic Oscillation (NAO) and weakly related to ENSOThe NAO is an interannual climate variability signal measured by the pressurediVerence between the region surrounding Iceland and the region around the AzoresIslands north-west of Morocco in the Atlantic Ocean The observed NDVI anomaliesover north-western Africa during this period may thus be a result of the interactionof these two phenomena

At a continental scale December 1997 shows a meridional dipole in NDVIpatterns A band of positive NDVI anomalies stretches across Africa betweenapproximately 12deg N and 10deg S The area south of 10deg S shows negative NDVIanomalies (shy 01 to shy 02) primarily in Botswana north-eastern Namibia southernAngola and southern Zambia The positive NDVI anomaly patterns over the entireeastern Africa region persist through February 1998 The area of high positiveanomalies is reduced by March and April primarily covering Kenya and central tosouthern Somalia while negative NDVI anomalies emerge along the west Africancoastal strip and over the western half of East Africa Over southern Africa thenegative NDVI anomalies become more marked in MarchndashApril The spatial struc-ture shows a north-west (negative) and south-east (positive) dipole pattern in Marchand April This pattern is somewhat similar to that observed during the 198687 and199495 ENSO warm events over the region and represents one of the two types ofENSO spatial manifestation identi ed in Southern Africa using NDVI data (Eastmanand Anyamba 1996) One of the most signi cant manifestations of the 199798ENSO warm event is the persistence and extension of positive NDVI anomaliesfrom the short rains growing season (OctoberndashDecember) into the dry season(JanuaryndashFebruary) and into the long rains season (MarchndashJune) over eastern AfricaAs such there was no dry period experienced early in 1998 which was quite adramatic departure from the normal seasonal cycle In general the continental-scalepatterns of NDVI anomalies correspond well with rainfall teleconnections patternsas shown in Ogallo (1988) and Ropelweski and Halpert (1987) with a somewhatlimited drought extent over Southern Africa The classic ENSO teleconnectionpattern over Africa is the tendency for equatorial East Africa to receive above normalrainfall and for drought to persist over Southern Africa during ENSO warm events(Lindsey 1988)

32 Temporal evolution of NDVI means and anomaliesThree sites were selected for analysis of the temporal evolution of NDVI means

and anomalies Plots of the long-term means monthly means and anomalies forthese sites are shown in gure 3(andashf ) The East Africa site ( gure 3(a)) shows adramatic increase in NDVI from September to October 1997 reaching a maximumin February 1998 and decreasing to near previous October 1997 levels by AprilndashMay 1998 The evolution of the anomalies ( gure 3(d)) shows a normal curve patterncentred on DecemberJanuary and is very typical of the phase locking character ofENSO to the SST annual cycle in the eastern Paci c Ocean (Rasmusson andCarpenter 1982) The NDVI levels for the period October to March are much higherthan the long-term average ( gure 3(a)) As can be noted the typical low NDVIlevels during the dry season over the region during DecemberndashFebruary were notobserved As a result the region experienced continuous above normal NDVI levelsfor a period of 8 months from October 1997 to May 1998 ( gure 1) This was a

NDVI anomalies during the ENSO event 1853

Figure 3 Temporal pro les of monthly NDVI values and long-term means (top panel) for(a) East Africa (40deg E 0deg S) (b) Namibia (18deg E 22deg S) and (c) south-eastern SouthAfrica (29deg E 29deg S) The corresponding NDVI anomalies for these sites are shown inthe bottom panel (d e f) Black line plots are long-term mean values grey bars aremonthly values and anomalies The NDVI values are averages extracted from a100kmtimes100 km box centred on each of these locations For East Africa during theperiod July 1997ndashMay 1998 the NDVI anomalies are well above the long-term means(d) indicative of greener than normal conditions For the Southern Africa sites (e f)the anomalies are above their respective long-term means during the early part of thegrowing season (SeptemberndashNovember) and near normal or slightly below normalduring the peak growing season (DecemberndashMarch)

A Anyamba et al1854

result of the record breaking above normal rainfall received during this period someareas in coastal central and eastern Kenya and in Somalia averaged 5ndash10 times thenormal rainfall (Bell and Halpert 1998)

The temporal pro le drawn from Namibia shows a slight decrease in NDVI fromJuly to September then an increase to values above the long-term mean in November1997 during the Southern hemisphere spring season ( gure 3(b)) This is followed bya dramatic decrease in December 1997 below the long-term mean then an increaseto just above the long-term mean values in January 1998 The later part of thegrowing season (February to May) shows slightly below normal NDVI with themaximum negative anomaly in March 1998 ( gure 3(e)) One noticeable feature ofthe anomalies is the increase in NDVI prior to the beginning of the growing season(SeptemberndashNovember) and a decrease in December This character has been notedas a precursor during warm ENSO years (Eastman and Anyamba 1996) This patternis consistent with the tendency for above normal precipitation over the region duringthe early phase of a developing warm event and lower than normal precipitationduring the mature phase of a warm event (Diaz and Kiladis 1993 Nicholson andEntekhabi 1986) Long-term means and monthly NDVI values from the second sitein south-eastern South Africa are shown gure 3(c) The annual cycle de ned by thelong-term average shows a minimum in NDVI during September and a maximumin February The corresponding monthly NDVI values during the period of analysisfollow a similar pattern The earlier part of the growing season (SeptemberndashNovember) shows signi cantly above normal NDVI decreasing to slightly belownormal conditions in February 1998 and a recovery to slightly above normal con-ditions from March to May ( gure 3( f )) Overall the drought conditions were notas severe over the Southern Africa region as in previous ENSO warm events Thewestern portions of the region were slightly drier than normal especially in Namibiaand northern Botswana in December FebruaryndashApril ( gure 2) as compared withthe south-eastern region covering the Drakensburg Mountains and parts of Transvaalwhich were greener than normal The dominant regional pattern over SouthernAfrica shows a north-west (negative)ndashsouth-east (positive) dipole pattern in NDVIanomalies This pattern supports ndings by Eastman and Anyamba (1996) of thediVerent manifestations of ENSO warm event teleconnection patterns over SouthernAfrica This north-westndashsouth-east spatial pattern is also well marked in the OLRanomalies ( gure 1(b))

33 Correlation patterns between NDVI anomalies and ENSO indicesIn the context of ENSO teleconnections in global weather patterns are based

on the notion that there is a tendency for anomalous warming in SSTs in the easternPaci c Ocean ( gure 1(a)) to have widespread impacts over distant or remote loca-tions Speci cally this is manifested in the ampli cationdampening of the precipita-tion signal (and hence vegetation greenness patterns in our case) over widespreadlocations over the global tropics The Paci c signal is transmitted via the AtmosphericWalker Circulation System (Southern Oscillation) causing dramatic changes in thenormal patterns of precipitation in the major centres of tropical convection includingthe Amazon Basin Indonesia and Congo Basin and the surrounding areas( gure 1(b)) These changes result in above normal precipitation in some regions andbelow normal precipitation in others Typically anomalous conditions associatedwith ENSO are observed during the rainfallgrowing season for speci c tropicallocations To test whether there is any correlation between NDVI departure patterns

NDVI anomalies during the ENSO event 1855

over Africa and ENSO during the 199798 period we computed correlations betweenENSO indices and NDVI anomalies for selected sites (in sect31 above) in East andSouthern Africa

The evolution of NDVI anomalies over eastern Kenya compared with the SSTanomalies in the adjacent region in the WIO is shown in gure 4(a) for the periodJuly 1997 to May 1998 As can be seen the patterns of evolution of the two variablesco-evolve well with a peak in January (WIO-SST ) and February (NDVI) and a dropin the anomalies towards the end of the long rains growing season in May For theentire time period both the NDVI and SST show positive anomalies There is noapparent lag relationship except for the peak period in the series where the SSTreaches a maximum in January and the NDVI reaches a maximum in FebruarySimultaneous variations between NDVI and SST are observed nevertheless NDVIattests to a time response of 1ndash2 months in the semi-arid lands of Africa (Nicholsonet al 1990 Richard and Poccard 1998) The coeYcient of correlation between thetwo time series is 083 (plt0005) This shows the close coupling between WIOwarming precipitation and vegetation greenness patterns over large areas of EastAfrica during this period Signi cant correlations are also found with the globaltropical SSTs (087) and the Southern Atlantic SSTs (083) (table 1) This in partshows the in uence of an enhanced ITCZ due to above normal SST across theglobal tropics and its eVects on precipitation ( gure 1(a)) and thus vegetation overEast Africa ( gure 2)

Lag correlation with the WIO leads to a signi cant drop in correlation coeYcientsfrom 080 (lag 0) to 058 at lag 3 (see gure 4(b)) showing that at the monthly timescale the NDVI response to WIO SST is near simultaneous Correlations with otherENSO indicators at zero-lag are rather low (see table 1) However at the 1ndash4 monthlag there is a signi cant increase in correlations for example to shy 080 with NINO34with NDVI lagging NINO34 SST by about three months ( gure 4(c d)) Althoughconvective activity was already enhanced in the eastern Paci c by August 1997 thecorresponding pattern does not show up in the WIO until OctoberndashDecember duringthe short rains growing season in East Africa (Bell and Halpert 1998) This is notunexpected as the Indian Ocean response to El Nino involves basin scale warmingwith a lag of about one season (Cadet and Diehl 1984) This in part accounts forthe lagged response between NDVI over East Africa and NINO34 SST anomalies

The Namibia sample site shows a positive relationship with NINO34 SST fromSeptember to November ( gure 4(e)) but this gives way to an inverse relationshipin December and February to May during the late phase of eastern Paci c warming( gure 4(e)) Correlations with most of the ENSO and other interannual variabilityindicators are rather weak (see table 1) The highest coeYcient of correlation is withNINO34 at 050 The temporal evolution pattern with NINO34 SST for south-eastern South Africa is shown in gure 4( f ) As in the previous case the minimumin NDVI is from December to March corresponding to the period of peak warmingin the eastern Paci c The highest correlations are with SOI (043) and WIO-SST(043) At the two-month lag the coeYcient of correlation with NINO34 increasessigni cantly to shy 075 (plt0005) Although correlations with the QBO and AtlanticSSTs were not signi cant for the two Southern Africa sites previous work by Masonand Tyson (1992) and Jury and Pathack (1993) show that the phase of QBO andAtlantic Ocean SSTs can modulate the eVects of ENSO over the Southern Africaregion When the QBO is in an anti-SOI phase as was the case during this periodit has a tendency to dampen the normal ENSO drought impacts over Southern

A Anyamba et al1856

(a) (d )

(b) (e)

(c) ( f )

Figure 4 Comparison and correlations between NDVI anomalies and ENSO indices EastAfrica (andashd) Southern Africa (endashf) NDVI anomalies over East Africa closely followthe pattern of SST departure patterns in the WIO region (a) with a signi cantcoeYcient of correlation at lag 0 (r=083) (b) There is an apparent lag of about threemonths between East Africa NDVI and NINO34 SST with a peak in NINO34 SSTin December 1997 followed by a peak in East Africa NDVI in February 1998 (c) Thelag correlation plot is shown in (d) Shown in (e) and ( f) are NDVI anomalies forNamibia and south-eastern South Africa respectively plotted against NINO34 SSTanomalies In both cases in the early phase of the event there are slightly positiveNDVI anomalies followed by near normal to negative NDVI anomalies at themaximum of Paci c warming in DecemberndashMarch This is a recognized feature ofrainfall patterns in this region during ENSO warm events with tendency for precipita-tion to be above normal during the early phase and below normal during the maturephase (Nicholson and Entekhabi 1986)

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

A Anyamba et al1852

and in North Africa over Morocco and Algeria Lamb and Peppler (1991) haveshown that variability in Moroccan rainfall between November and April is relatedto the state of the North Atlantic Oscillation (NAO) and weakly related to ENSOThe NAO is an interannual climate variability signal measured by the pressurediVerence between the region surrounding Iceland and the region around the AzoresIslands north-west of Morocco in the Atlantic Ocean The observed NDVI anomaliesover north-western Africa during this period may thus be a result of the interactionof these two phenomena

At a continental scale December 1997 shows a meridional dipole in NDVIpatterns A band of positive NDVI anomalies stretches across Africa betweenapproximately 12deg N and 10deg S The area south of 10deg S shows negative NDVIanomalies (shy 01 to shy 02) primarily in Botswana north-eastern Namibia southernAngola and southern Zambia The positive NDVI anomaly patterns over the entireeastern Africa region persist through February 1998 The area of high positiveanomalies is reduced by March and April primarily covering Kenya and central tosouthern Somalia while negative NDVI anomalies emerge along the west Africancoastal strip and over the western half of East Africa Over southern Africa thenegative NDVI anomalies become more marked in MarchndashApril The spatial struc-ture shows a north-west (negative) and south-east (positive) dipole pattern in Marchand April This pattern is somewhat similar to that observed during the 198687 and199495 ENSO warm events over the region and represents one of the two types ofENSO spatial manifestation identi ed in Southern Africa using NDVI data (Eastmanand Anyamba 1996) One of the most signi cant manifestations of the 199798ENSO warm event is the persistence and extension of positive NDVI anomaliesfrom the short rains growing season (OctoberndashDecember) into the dry season(JanuaryndashFebruary) and into the long rains season (MarchndashJune) over eastern AfricaAs such there was no dry period experienced early in 1998 which was quite adramatic departure from the normal seasonal cycle In general the continental-scalepatterns of NDVI anomalies correspond well with rainfall teleconnections patternsas shown in Ogallo (1988) and Ropelweski and Halpert (1987) with a somewhatlimited drought extent over Southern Africa The classic ENSO teleconnectionpattern over Africa is the tendency for equatorial East Africa to receive above normalrainfall and for drought to persist over Southern Africa during ENSO warm events(Lindsey 1988)

32 Temporal evolution of NDVI means and anomaliesThree sites were selected for analysis of the temporal evolution of NDVI means

and anomalies Plots of the long-term means monthly means and anomalies forthese sites are shown in gure 3(andashf ) The East Africa site ( gure 3(a)) shows adramatic increase in NDVI from September to October 1997 reaching a maximumin February 1998 and decreasing to near previous October 1997 levels by AprilndashMay 1998 The evolution of the anomalies ( gure 3(d)) shows a normal curve patterncentred on DecemberJanuary and is very typical of the phase locking character ofENSO to the SST annual cycle in the eastern Paci c Ocean (Rasmusson andCarpenter 1982) The NDVI levels for the period October to March are much higherthan the long-term average ( gure 3(a)) As can be noted the typical low NDVIlevels during the dry season over the region during DecemberndashFebruary were notobserved As a result the region experienced continuous above normal NDVI levelsfor a period of 8 months from October 1997 to May 1998 ( gure 1) This was a

NDVI anomalies during the ENSO event 1853

Figure 3 Temporal pro les of monthly NDVI values and long-term means (top panel) for(a) East Africa (40deg E 0deg S) (b) Namibia (18deg E 22deg S) and (c) south-eastern SouthAfrica (29deg E 29deg S) The corresponding NDVI anomalies for these sites are shown inthe bottom panel (d e f) Black line plots are long-term mean values grey bars aremonthly values and anomalies The NDVI values are averages extracted from a100kmtimes100 km box centred on each of these locations For East Africa during theperiod July 1997ndashMay 1998 the NDVI anomalies are well above the long-term means(d) indicative of greener than normal conditions For the Southern Africa sites (e f)the anomalies are above their respective long-term means during the early part of thegrowing season (SeptemberndashNovember) and near normal or slightly below normalduring the peak growing season (DecemberndashMarch)

A Anyamba et al1854

result of the record breaking above normal rainfall received during this period someareas in coastal central and eastern Kenya and in Somalia averaged 5ndash10 times thenormal rainfall (Bell and Halpert 1998)

The temporal pro le drawn from Namibia shows a slight decrease in NDVI fromJuly to September then an increase to values above the long-term mean in November1997 during the Southern hemisphere spring season ( gure 3(b)) This is followed bya dramatic decrease in December 1997 below the long-term mean then an increaseto just above the long-term mean values in January 1998 The later part of thegrowing season (February to May) shows slightly below normal NDVI with themaximum negative anomaly in March 1998 ( gure 3(e)) One noticeable feature ofthe anomalies is the increase in NDVI prior to the beginning of the growing season(SeptemberndashNovember) and a decrease in December This character has been notedas a precursor during warm ENSO years (Eastman and Anyamba 1996) This patternis consistent with the tendency for above normal precipitation over the region duringthe early phase of a developing warm event and lower than normal precipitationduring the mature phase of a warm event (Diaz and Kiladis 1993 Nicholson andEntekhabi 1986) Long-term means and monthly NDVI values from the second sitein south-eastern South Africa are shown gure 3(c) The annual cycle de ned by thelong-term average shows a minimum in NDVI during September and a maximumin February The corresponding monthly NDVI values during the period of analysisfollow a similar pattern The earlier part of the growing season (SeptemberndashNovember) shows signi cantly above normal NDVI decreasing to slightly belownormal conditions in February 1998 and a recovery to slightly above normal con-ditions from March to May ( gure 3( f )) Overall the drought conditions were notas severe over the Southern Africa region as in previous ENSO warm events Thewestern portions of the region were slightly drier than normal especially in Namibiaand northern Botswana in December FebruaryndashApril ( gure 2) as compared withthe south-eastern region covering the Drakensburg Mountains and parts of Transvaalwhich were greener than normal The dominant regional pattern over SouthernAfrica shows a north-west (negative)ndashsouth-east (positive) dipole pattern in NDVIanomalies This pattern supports ndings by Eastman and Anyamba (1996) of thediVerent manifestations of ENSO warm event teleconnection patterns over SouthernAfrica This north-westndashsouth-east spatial pattern is also well marked in the OLRanomalies ( gure 1(b))

33 Correlation patterns between NDVI anomalies and ENSO indicesIn the context of ENSO teleconnections in global weather patterns are based

on the notion that there is a tendency for anomalous warming in SSTs in the easternPaci c Ocean ( gure 1(a)) to have widespread impacts over distant or remote loca-tions Speci cally this is manifested in the ampli cationdampening of the precipita-tion signal (and hence vegetation greenness patterns in our case) over widespreadlocations over the global tropics The Paci c signal is transmitted via the AtmosphericWalker Circulation System (Southern Oscillation) causing dramatic changes in thenormal patterns of precipitation in the major centres of tropical convection includingthe Amazon Basin Indonesia and Congo Basin and the surrounding areas( gure 1(b)) These changes result in above normal precipitation in some regions andbelow normal precipitation in others Typically anomalous conditions associatedwith ENSO are observed during the rainfallgrowing season for speci c tropicallocations To test whether there is any correlation between NDVI departure patterns

NDVI anomalies during the ENSO event 1855

over Africa and ENSO during the 199798 period we computed correlations betweenENSO indices and NDVI anomalies for selected sites (in sect31 above) in East andSouthern Africa

The evolution of NDVI anomalies over eastern Kenya compared with the SSTanomalies in the adjacent region in the WIO is shown in gure 4(a) for the periodJuly 1997 to May 1998 As can be seen the patterns of evolution of the two variablesco-evolve well with a peak in January (WIO-SST ) and February (NDVI) and a dropin the anomalies towards the end of the long rains growing season in May For theentire time period both the NDVI and SST show positive anomalies There is noapparent lag relationship except for the peak period in the series where the SSTreaches a maximum in January and the NDVI reaches a maximum in FebruarySimultaneous variations between NDVI and SST are observed nevertheless NDVIattests to a time response of 1ndash2 months in the semi-arid lands of Africa (Nicholsonet al 1990 Richard and Poccard 1998) The coeYcient of correlation between thetwo time series is 083 (plt0005) This shows the close coupling between WIOwarming precipitation and vegetation greenness patterns over large areas of EastAfrica during this period Signi cant correlations are also found with the globaltropical SSTs (087) and the Southern Atlantic SSTs (083) (table 1) This in partshows the in uence of an enhanced ITCZ due to above normal SST across theglobal tropics and its eVects on precipitation ( gure 1(a)) and thus vegetation overEast Africa ( gure 2)

Lag correlation with the WIO leads to a signi cant drop in correlation coeYcientsfrom 080 (lag 0) to 058 at lag 3 (see gure 4(b)) showing that at the monthly timescale the NDVI response to WIO SST is near simultaneous Correlations with otherENSO indicators at zero-lag are rather low (see table 1) However at the 1ndash4 monthlag there is a signi cant increase in correlations for example to shy 080 with NINO34with NDVI lagging NINO34 SST by about three months ( gure 4(c d)) Althoughconvective activity was already enhanced in the eastern Paci c by August 1997 thecorresponding pattern does not show up in the WIO until OctoberndashDecember duringthe short rains growing season in East Africa (Bell and Halpert 1998) This is notunexpected as the Indian Ocean response to El Nino involves basin scale warmingwith a lag of about one season (Cadet and Diehl 1984) This in part accounts forthe lagged response between NDVI over East Africa and NINO34 SST anomalies

The Namibia sample site shows a positive relationship with NINO34 SST fromSeptember to November ( gure 4(e)) but this gives way to an inverse relationshipin December and February to May during the late phase of eastern Paci c warming( gure 4(e)) Correlations with most of the ENSO and other interannual variabilityindicators are rather weak (see table 1) The highest coeYcient of correlation is withNINO34 at 050 The temporal evolution pattern with NINO34 SST for south-eastern South Africa is shown in gure 4( f ) As in the previous case the minimumin NDVI is from December to March corresponding to the period of peak warmingin the eastern Paci c The highest correlations are with SOI (043) and WIO-SST(043) At the two-month lag the coeYcient of correlation with NINO34 increasessigni cantly to shy 075 (plt0005) Although correlations with the QBO and AtlanticSSTs were not signi cant for the two Southern Africa sites previous work by Masonand Tyson (1992) and Jury and Pathack (1993) show that the phase of QBO andAtlantic Ocean SSTs can modulate the eVects of ENSO over the Southern Africaregion When the QBO is in an anti-SOI phase as was the case during this periodit has a tendency to dampen the normal ENSO drought impacts over Southern

A Anyamba et al1856

(a) (d )

(b) (e)

(c) ( f )

Figure 4 Comparison and correlations between NDVI anomalies and ENSO indices EastAfrica (andashd) Southern Africa (endashf) NDVI anomalies over East Africa closely followthe pattern of SST departure patterns in the WIO region (a) with a signi cantcoeYcient of correlation at lag 0 (r=083) (b) There is an apparent lag of about threemonths between East Africa NDVI and NINO34 SST with a peak in NINO34 SSTin December 1997 followed by a peak in East Africa NDVI in February 1998 (c) Thelag correlation plot is shown in (d) Shown in (e) and ( f) are NDVI anomalies forNamibia and south-eastern South Africa respectively plotted against NINO34 SSTanomalies In both cases in the early phase of the event there are slightly positiveNDVI anomalies followed by near normal to negative NDVI anomalies at themaximum of Paci c warming in DecemberndashMarch This is a recognized feature ofrainfall patterns in this region during ENSO warm events with tendency for precipita-tion to be above normal during the early phase and below normal during the maturephase (Nicholson and Entekhabi 1986)

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

NDVI anomalies during the ENSO event 1853

Figure 3 Temporal pro les of monthly NDVI values and long-term means (top panel) for(a) East Africa (40deg E 0deg S) (b) Namibia (18deg E 22deg S) and (c) south-eastern SouthAfrica (29deg E 29deg S) The corresponding NDVI anomalies for these sites are shown inthe bottom panel (d e f) Black line plots are long-term mean values grey bars aremonthly values and anomalies The NDVI values are averages extracted from a100kmtimes100 km box centred on each of these locations For East Africa during theperiod July 1997ndashMay 1998 the NDVI anomalies are well above the long-term means(d) indicative of greener than normal conditions For the Southern Africa sites (e f)the anomalies are above their respective long-term means during the early part of thegrowing season (SeptemberndashNovember) and near normal or slightly below normalduring the peak growing season (DecemberndashMarch)

A Anyamba et al1854

result of the record breaking above normal rainfall received during this period someareas in coastal central and eastern Kenya and in Somalia averaged 5ndash10 times thenormal rainfall (Bell and Halpert 1998)

The temporal pro le drawn from Namibia shows a slight decrease in NDVI fromJuly to September then an increase to values above the long-term mean in November1997 during the Southern hemisphere spring season ( gure 3(b)) This is followed bya dramatic decrease in December 1997 below the long-term mean then an increaseto just above the long-term mean values in January 1998 The later part of thegrowing season (February to May) shows slightly below normal NDVI with themaximum negative anomaly in March 1998 ( gure 3(e)) One noticeable feature ofthe anomalies is the increase in NDVI prior to the beginning of the growing season(SeptemberndashNovember) and a decrease in December This character has been notedas a precursor during warm ENSO years (Eastman and Anyamba 1996) This patternis consistent with the tendency for above normal precipitation over the region duringthe early phase of a developing warm event and lower than normal precipitationduring the mature phase of a warm event (Diaz and Kiladis 1993 Nicholson andEntekhabi 1986) Long-term means and monthly NDVI values from the second sitein south-eastern South Africa are shown gure 3(c) The annual cycle de ned by thelong-term average shows a minimum in NDVI during September and a maximumin February The corresponding monthly NDVI values during the period of analysisfollow a similar pattern The earlier part of the growing season (SeptemberndashNovember) shows signi cantly above normal NDVI decreasing to slightly belownormal conditions in February 1998 and a recovery to slightly above normal con-ditions from March to May ( gure 3( f )) Overall the drought conditions were notas severe over the Southern Africa region as in previous ENSO warm events Thewestern portions of the region were slightly drier than normal especially in Namibiaand northern Botswana in December FebruaryndashApril ( gure 2) as compared withthe south-eastern region covering the Drakensburg Mountains and parts of Transvaalwhich were greener than normal The dominant regional pattern over SouthernAfrica shows a north-west (negative)ndashsouth-east (positive) dipole pattern in NDVIanomalies This pattern supports ndings by Eastman and Anyamba (1996) of thediVerent manifestations of ENSO warm event teleconnection patterns over SouthernAfrica This north-westndashsouth-east spatial pattern is also well marked in the OLRanomalies ( gure 1(b))

33 Correlation patterns between NDVI anomalies and ENSO indicesIn the context of ENSO teleconnections in global weather patterns are based

on the notion that there is a tendency for anomalous warming in SSTs in the easternPaci c Ocean ( gure 1(a)) to have widespread impacts over distant or remote loca-tions Speci cally this is manifested in the ampli cationdampening of the precipita-tion signal (and hence vegetation greenness patterns in our case) over widespreadlocations over the global tropics The Paci c signal is transmitted via the AtmosphericWalker Circulation System (Southern Oscillation) causing dramatic changes in thenormal patterns of precipitation in the major centres of tropical convection includingthe Amazon Basin Indonesia and Congo Basin and the surrounding areas( gure 1(b)) These changes result in above normal precipitation in some regions andbelow normal precipitation in others Typically anomalous conditions associatedwith ENSO are observed during the rainfallgrowing season for speci c tropicallocations To test whether there is any correlation between NDVI departure patterns

NDVI anomalies during the ENSO event 1855

over Africa and ENSO during the 199798 period we computed correlations betweenENSO indices and NDVI anomalies for selected sites (in sect31 above) in East andSouthern Africa

The evolution of NDVI anomalies over eastern Kenya compared with the SSTanomalies in the adjacent region in the WIO is shown in gure 4(a) for the periodJuly 1997 to May 1998 As can be seen the patterns of evolution of the two variablesco-evolve well with a peak in January (WIO-SST ) and February (NDVI) and a dropin the anomalies towards the end of the long rains growing season in May For theentire time period both the NDVI and SST show positive anomalies There is noapparent lag relationship except for the peak period in the series where the SSTreaches a maximum in January and the NDVI reaches a maximum in FebruarySimultaneous variations between NDVI and SST are observed nevertheless NDVIattests to a time response of 1ndash2 months in the semi-arid lands of Africa (Nicholsonet al 1990 Richard and Poccard 1998) The coeYcient of correlation between thetwo time series is 083 (plt0005) This shows the close coupling between WIOwarming precipitation and vegetation greenness patterns over large areas of EastAfrica during this period Signi cant correlations are also found with the globaltropical SSTs (087) and the Southern Atlantic SSTs (083) (table 1) This in partshows the in uence of an enhanced ITCZ due to above normal SST across theglobal tropics and its eVects on precipitation ( gure 1(a)) and thus vegetation overEast Africa ( gure 2)

Lag correlation with the WIO leads to a signi cant drop in correlation coeYcientsfrom 080 (lag 0) to 058 at lag 3 (see gure 4(b)) showing that at the monthly timescale the NDVI response to WIO SST is near simultaneous Correlations with otherENSO indicators at zero-lag are rather low (see table 1) However at the 1ndash4 monthlag there is a signi cant increase in correlations for example to shy 080 with NINO34with NDVI lagging NINO34 SST by about three months ( gure 4(c d)) Althoughconvective activity was already enhanced in the eastern Paci c by August 1997 thecorresponding pattern does not show up in the WIO until OctoberndashDecember duringthe short rains growing season in East Africa (Bell and Halpert 1998) This is notunexpected as the Indian Ocean response to El Nino involves basin scale warmingwith a lag of about one season (Cadet and Diehl 1984) This in part accounts forthe lagged response between NDVI over East Africa and NINO34 SST anomalies

The Namibia sample site shows a positive relationship with NINO34 SST fromSeptember to November ( gure 4(e)) but this gives way to an inverse relationshipin December and February to May during the late phase of eastern Paci c warming( gure 4(e)) Correlations with most of the ENSO and other interannual variabilityindicators are rather weak (see table 1) The highest coeYcient of correlation is withNINO34 at 050 The temporal evolution pattern with NINO34 SST for south-eastern South Africa is shown in gure 4( f ) As in the previous case the minimumin NDVI is from December to March corresponding to the period of peak warmingin the eastern Paci c The highest correlations are with SOI (043) and WIO-SST(043) At the two-month lag the coeYcient of correlation with NINO34 increasessigni cantly to shy 075 (plt0005) Although correlations with the QBO and AtlanticSSTs were not signi cant for the two Southern Africa sites previous work by Masonand Tyson (1992) and Jury and Pathack (1993) show that the phase of QBO andAtlantic Ocean SSTs can modulate the eVects of ENSO over the Southern Africaregion When the QBO is in an anti-SOI phase as was the case during this periodit has a tendency to dampen the normal ENSO drought impacts over Southern

A Anyamba et al1856

(a) (d )

(b) (e)

(c) ( f )

Figure 4 Comparison and correlations between NDVI anomalies and ENSO indices EastAfrica (andashd) Southern Africa (endashf) NDVI anomalies over East Africa closely followthe pattern of SST departure patterns in the WIO region (a) with a signi cantcoeYcient of correlation at lag 0 (r=083) (b) There is an apparent lag of about threemonths between East Africa NDVI and NINO34 SST with a peak in NINO34 SSTin December 1997 followed by a peak in East Africa NDVI in February 1998 (c) Thelag correlation plot is shown in (d) Shown in (e) and ( f) are NDVI anomalies forNamibia and south-eastern South Africa respectively plotted against NINO34 SSTanomalies In both cases in the early phase of the event there are slightly positiveNDVI anomalies followed by near normal to negative NDVI anomalies at themaximum of Paci c warming in DecemberndashMarch This is a recognized feature ofrainfall patterns in this region during ENSO warm events with tendency for precipita-tion to be above normal during the early phase and below normal during the maturephase (Nicholson and Entekhabi 1986)

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

A Anyamba et al1854

result of the record breaking above normal rainfall received during this period someareas in coastal central and eastern Kenya and in Somalia averaged 5ndash10 times thenormal rainfall (Bell and Halpert 1998)

The temporal pro le drawn from Namibia shows a slight decrease in NDVI fromJuly to September then an increase to values above the long-term mean in November1997 during the Southern hemisphere spring season ( gure 3(b)) This is followed bya dramatic decrease in December 1997 below the long-term mean then an increaseto just above the long-term mean values in January 1998 The later part of thegrowing season (February to May) shows slightly below normal NDVI with themaximum negative anomaly in March 1998 ( gure 3(e)) One noticeable feature ofthe anomalies is the increase in NDVI prior to the beginning of the growing season(SeptemberndashNovember) and a decrease in December This character has been notedas a precursor during warm ENSO years (Eastman and Anyamba 1996) This patternis consistent with the tendency for above normal precipitation over the region duringthe early phase of a developing warm event and lower than normal precipitationduring the mature phase of a warm event (Diaz and Kiladis 1993 Nicholson andEntekhabi 1986) Long-term means and monthly NDVI values from the second sitein south-eastern South Africa are shown gure 3(c) The annual cycle de ned by thelong-term average shows a minimum in NDVI during September and a maximumin February The corresponding monthly NDVI values during the period of analysisfollow a similar pattern The earlier part of the growing season (SeptemberndashNovember) shows signi cantly above normal NDVI decreasing to slightly belownormal conditions in February 1998 and a recovery to slightly above normal con-ditions from March to May ( gure 3( f )) Overall the drought conditions were notas severe over the Southern Africa region as in previous ENSO warm events Thewestern portions of the region were slightly drier than normal especially in Namibiaand northern Botswana in December FebruaryndashApril ( gure 2) as compared withthe south-eastern region covering the Drakensburg Mountains and parts of Transvaalwhich were greener than normal The dominant regional pattern over SouthernAfrica shows a north-west (negative)ndashsouth-east (positive) dipole pattern in NDVIanomalies This pattern supports ndings by Eastman and Anyamba (1996) of thediVerent manifestations of ENSO warm event teleconnection patterns over SouthernAfrica This north-westndashsouth-east spatial pattern is also well marked in the OLRanomalies ( gure 1(b))

33 Correlation patterns between NDVI anomalies and ENSO indicesIn the context of ENSO teleconnections in global weather patterns are based

on the notion that there is a tendency for anomalous warming in SSTs in the easternPaci c Ocean ( gure 1(a)) to have widespread impacts over distant or remote loca-tions Speci cally this is manifested in the ampli cationdampening of the precipita-tion signal (and hence vegetation greenness patterns in our case) over widespreadlocations over the global tropics The Paci c signal is transmitted via the AtmosphericWalker Circulation System (Southern Oscillation) causing dramatic changes in thenormal patterns of precipitation in the major centres of tropical convection includingthe Amazon Basin Indonesia and Congo Basin and the surrounding areas( gure 1(b)) These changes result in above normal precipitation in some regions andbelow normal precipitation in others Typically anomalous conditions associatedwith ENSO are observed during the rainfallgrowing season for speci c tropicallocations To test whether there is any correlation between NDVI departure patterns

NDVI anomalies during the ENSO event 1855

over Africa and ENSO during the 199798 period we computed correlations betweenENSO indices and NDVI anomalies for selected sites (in sect31 above) in East andSouthern Africa

The evolution of NDVI anomalies over eastern Kenya compared with the SSTanomalies in the adjacent region in the WIO is shown in gure 4(a) for the periodJuly 1997 to May 1998 As can be seen the patterns of evolution of the two variablesco-evolve well with a peak in January (WIO-SST ) and February (NDVI) and a dropin the anomalies towards the end of the long rains growing season in May For theentire time period both the NDVI and SST show positive anomalies There is noapparent lag relationship except for the peak period in the series where the SSTreaches a maximum in January and the NDVI reaches a maximum in FebruarySimultaneous variations between NDVI and SST are observed nevertheless NDVIattests to a time response of 1ndash2 months in the semi-arid lands of Africa (Nicholsonet al 1990 Richard and Poccard 1998) The coeYcient of correlation between thetwo time series is 083 (plt0005) This shows the close coupling between WIOwarming precipitation and vegetation greenness patterns over large areas of EastAfrica during this period Signi cant correlations are also found with the globaltropical SSTs (087) and the Southern Atlantic SSTs (083) (table 1) This in partshows the in uence of an enhanced ITCZ due to above normal SST across theglobal tropics and its eVects on precipitation ( gure 1(a)) and thus vegetation overEast Africa ( gure 2)

Lag correlation with the WIO leads to a signi cant drop in correlation coeYcientsfrom 080 (lag 0) to 058 at lag 3 (see gure 4(b)) showing that at the monthly timescale the NDVI response to WIO SST is near simultaneous Correlations with otherENSO indicators at zero-lag are rather low (see table 1) However at the 1ndash4 monthlag there is a signi cant increase in correlations for example to shy 080 with NINO34with NDVI lagging NINO34 SST by about three months ( gure 4(c d)) Althoughconvective activity was already enhanced in the eastern Paci c by August 1997 thecorresponding pattern does not show up in the WIO until OctoberndashDecember duringthe short rains growing season in East Africa (Bell and Halpert 1998) This is notunexpected as the Indian Ocean response to El Nino involves basin scale warmingwith a lag of about one season (Cadet and Diehl 1984) This in part accounts forthe lagged response between NDVI over East Africa and NINO34 SST anomalies

The Namibia sample site shows a positive relationship with NINO34 SST fromSeptember to November ( gure 4(e)) but this gives way to an inverse relationshipin December and February to May during the late phase of eastern Paci c warming( gure 4(e)) Correlations with most of the ENSO and other interannual variabilityindicators are rather weak (see table 1) The highest coeYcient of correlation is withNINO34 at 050 The temporal evolution pattern with NINO34 SST for south-eastern South Africa is shown in gure 4( f ) As in the previous case the minimumin NDVI is from December to March corresponding to the period of peak warmingin the eastern Paci c The highest correlations are with SOI (043) and WIO-SST(043) At the two-month lag the coeYcient of correlation with NINO34 increasessigni cantly to shy 075 (plt0005) Although correlations with the QBO and AtlanticSSTs were not signi cant for the two Southern Africa sites previous work by Masonand Tyson (1992) and Jury and Pathack (1993) show that the phase of QBO andAtlantic Ocean SSTs can modulate the eVects of ENSO over the Southern Africaregion When the QBO is in an anti-SOI phase as was the case during this periodit has a tendency to dampen the normal ENSO drought impacts over Southern

A Anyamba et al1856

(a) (d )

(b) (e)

(c) ( f )

Figure 4 Comparison and correlations between NDVI anomalies and ENSO indices EastAfrica (andashd) Southern Africa (endashf) NDVI anomalies over East Africa closely followthe pattern of SST departure patterns in the WIO region (a) with a signi cantcoeYcient of correlation at lag 0 (r=083) (b) There is an apparent lag of about threemonths between East Africa NDVI and NINO34 SST with a peak in NINO34 SSTin December 1997 followed by a peak in East Africa NDVI in February 1998 (c) Thelag correlation plot is shown in (d) Shown in (e) and ( f) are NDVI anomalies forNamibia and south-eastern South Africa respectively plotted against NINO34 SSTanomalies In both cases in the early phase of the event there are slightly positiveNDVI anomalies followed by near normal to negative NDVI anomalies at themaximum of Paci c warming in DecemberndashMarch This is a recognized feature ofrainfall patterns in this region during ENSO warm events with tendency for precipita-tion to be above normal during the early phase and below normal during the maturephase (Nicholson and Entekhabi 1986)

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

NDVI anomalies during the ENSO event 1855

over Africa and ENSO during the 199798 period we computed correlations betweenENSO indices and NDVI anomalies for selected sites (in sect31 above) in East andSouthern Africa

The evolution of NDVI anomalies over eastern Kenya compared with the SSTanomalies in the adjacent region in the WIO is shown in gure 4(a) for the periodJuly 1997 to May 1998 As can be seen the patterns of evolution of the two variablesco-evolve well with a peak in January (WIO-SST ) and February (NDVI) and a dropin the anomalies towards the end of the long rains growing season in May For theentire time period both the NDVI and SST show positive anomalies There is noapparent lag relationship except for the peak period in the series where the SSTreaches a maximum in January and the NDVI reaches a maximum in FebruarySimultaneous variations between NDVI and SST are observed nevertheless NDVIattests to a time response of 1ndash2 months in the semi-arid lands of Africa (Nicholsonet al 1990 Richard and Poccard 1998) The coeYcient of correlation between thetwo time series is 083 (plt0005) This shows the close coupling between WIOwarming precipitation and vegetation greenness patterns over large areas of EastAfrica during this period Signi cant correlations are also found with the globaltropical SSTs (087) and the Southern Atlantic SSTs (083) (table 1) This in partshows the in uence of an enhanced ITCZ due to above normal SST across theglobal tropics and its eVects on precipitation ( gure 1(a)) and thus vegetation overEast Africa ( gure 2)

Lag correlation with the WIO leads to a signi cant drop in correlation coeYcientsfrom 080 (lag 0) to 058 at lag 3 (see gure 4(b)) showing that at the monthly timescale the NDVI response to WIO SST is near simultaneous Correlations with otherENSO indicators at zero-lag are rather low (see table 1) However at the 1ndash4 monthlag there is a signi cant increase in correlations for example to shy 080 with NINO34with NDVI lagging NINO34 SST by about three months ( gure 4(c d)) Althoughconvective activity was already enhanced in the eastern Paci c by August 1997 thecorresponding pattern does not show up in the WIO until OctoberndashDecember duringthe short rains growing season in East Africa (Bell and Halpert 1998) This is notunexpected as the Indian Ocean response to El Nino involves basin scale warmingwith a lag of about one season (Cadet and Diehl 1984) This in part accounts forthe lagged response between NDVI over East Africa and NINO34 SST anomalies

The Namibia sample site shows a positive relationship with NINO34 SST fromSeptember to November ( gure 4(e)) but this gives way to an inverse relationshipin December and February to May during the late phase of eastern Paci c warming( gure 4(e)) Correlations with most of the ENSO and other interannual variabilityindicators are rather weak (see table 1) The highest coeYcient of correlation is withNINO34 at 050 The temporal evolution pattern with NINO34 SST for south-eastern South Africa is shown in gure 4( f ) As in the previous case the minimumin NDVI is from December to March corresponding to the period of peak warmingin the eastern Paci c The highest correlations are with SOI (043) and WIO-SST(043) At the two-month lag the coeYcient of correlation with NINO34 increasessigni cantly to shy 075 (plt0005) Although correlations with the QBO and AtlanticSSTs were not signi cant for the two Southern Africa sites previous work by Masonand Tyson (1992) and Jury and Pathack (1993) show that the phase of QBO andAtlantic Ocean SSTs can modulate the eVects of ENSO over the Southern Africaregion When the QBO is in an anti-SOI phase as was the case during this periodit has a tendency to dampen the normal ENSO drought impacts over Southern

A Anyamba et al1856

(a) (d )

(b) (e)

(c) ( f )

Figure 4 Comparison and correlations between NDVI anomalies and ENSO indices EastAfrica (andashd) Southern Africa (endashf) NDVI anomalies over East Africa closely followthe pattern of SST departure patterns in the WIO region (a) with a signi cantcoeYcient of correlation at lag 0 (r=083) (b) There is an apparent lag of about threemonths between East Africa NDVI and NINO34 SST with a peak in NINO34 SSTin December 1997 followed by a peak in East Africa NDVI in February 1998 (c) Thelag correlation plot is shown in (d) Shown in (e) and ( f) are NDVI anomalies forNamibia and south-eastern South Africa respectively plotted against NINO34 SSTanomalies In both cases in the early phase of the event there are slightly positiveNDVI anomalies followed by near normal to negative NDVI anomalies at themaximum of Paci c warming in DecemberndashMarch This is a recognized feature ofrainfall patterns in this region during ENSO warm events with tendency for precipita-tion to be above normal during the early phase and below normal during the maturephase (Nicholson and Entekhabi 1986)

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

A Anyamba et al1856

(a) (d )

(b) (e)

(c) ( f )

Figure 4 Comparison and correlations between NDVI anomalies and ENSO indices EastAfrica (andashd) Southern Africa (endashf) NDVI anomalies over East Africa closely followthe pattern of SST departure patterns in the WIO region (a) with a signi cantcoeYcient of correlation at lag 0 (r=083) (b) There is an apparent lag of about threemonths between East Africa NDVI and NINO34 SST with a peak in NINO34 SSTin December 1997 followed by a peak in East Africa NDVI in February 1998 (c) Thelag correlation plot is shown in (d) Shown in (e) and ( f) are NDVI anomalies forNamibia and south-eastern South Africa respectively plotted against NINO34 SSTanomalies In both cases in the early phase of the event there are slightly positiveNDVI anomalies followed by near normal to negative NDVI anomalies at themaximum of Paci c warming in DecemberndashMarch This is a recognized feature ofrainfall patterns in this region during ENSO warm events with tendency for precipita-tion to be above normal during the early phase and below normal during the maturephase (Nicholson and Entekhabi 1986)

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

NDVI anomalies during the ENSO event 1857

Table 1 Zero lag correlations between NDVI anomalies for East Africa Namibia and south-eastern South Africa with various ENSO indices Correlations marked in bold aresigni cant at plt0005

VariableENSO indices

NINO34 SOI NAT SAT TROP QBO30 EPWI WIO

East Africa NDVI 035 shy 043 030 083 087 shy 040 shy 068 083Namibia NDVI 050 shy 005 shy 017 006 009 035 shy 006 017SE South Africa NDVI 0176 043 025 shy 014 shy 035 035 shy 002 shy 047

Africa In general although the drought during this event was not as severe as duringprevious events low NDVI values were observed over Southern Africa during theDecember 1997ndashMarch 1998 period which is supposed to be the period of maximumprecipitation in the region (Ropelewski and Halpert 1987 1989) The spatial patternsand the evolution of NDVI anomalies during this period correspond well withobserved rainfall anomalies and with the general relationship to the ENSO cycle forthis region (Bell and Halpert 1998)

4 Summary and conclusionsThe 199798 ENSO warm event was the strongest event recorded in the 20th

century both in terms of the SST departure patterns and the associated magnitudeof the climatic anomalies worldwide In terms of the land biosphere response overAfrica the most anomalous conditions occurred over equatorial Eastern Africa ina band bounded by 10deg Nndash10deg S Positive NDVI anomalies covered the region fromOctober 1997 through May 1998 and thus by inference a persistence of abovenormal rainfall over the entire region The normal dry season (DecemberndashFebruary)recorded high rainfall totals Correlation results indicate that the above normalNDVI appears to be a direct result of the anomalous positive SSTs in the WIOregion (~+30degC above normal) and a lagged response to eastern Paci c warmingThe positive rainfall response over Eastern Africa resulted in the blooming of vegeta-tion in semi and semi-arid areas of the region and improvement in pasture conditionsIn some areas the above normal rainfall resulted in widespread ooding especiallyalong coastal eastern and north-eastern Kenya and in Somalia causing extensivedamage to property claiming hundreds of lives and leaving thousands homeless

The extensive ooding also created the conditions for the outbreak of variousdiseases including malaria and Rift Valley Fever (Linthicum et al 1999) whichaVected human and livestock populations over large areas of East Africa Slightlygreener than normal conditions over southern Africa occurred from October toNovember 1997 but drier than normal conditions prevailed later in the season fromJanuary through May 1998 predominantly over the western part of the region Theeastern portion of the region was slightly greener than normal The magnitudes ofthe negative NDVI departures over Southern Africa were not as pronounced asduring similar ENSO events in the past including 198283 198687 199293 and1995 At the moment we hypothesize that this could have been in part a directresult of strong equatorial easterly winds dumping excess moisture from a warmerthan normal equatorial WIO thus dampening the expected drought response Inaddition the counteracting in uence of the QBO cannot be ruled out as shown byprevious research The magnitude of the warming in the WIO during this period

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

A Anyamba et al1858

was unprecedented in recent history and necessitates serious attention Since thevariability in SSTs in both the Indian and Atlantic Oceans in uences rainfall overAfrica and may exacerbate or dampen the likely teleconnection eVects associatedwith global ENSO manifestation incorporation of these SSTs in regional ENSOpredictions would be a useful early warning tool for Eastern and Southern Africa

AcknowledgmentsThis research was funded under EduTech contract NAS5662 and USRA

NAS5-98181 and is a contribution to an interagency programme in support ofUSAIDFEWS

ReferencesAndres L Salas W A and Skole D 1994 Fourier analysis of multi-temporal AVHRR

data applied to a land cover classi cation International Journal of Remote Sensing15 1115ndash1221

Anyamba A and Eastman J R 1996 Interannual variability of NDVI over Africa and itsrelationship to El NinoSouthern Oscillation International Journal of Remote Sensing17 2533ndash2548

Asrar G 1989 Theory and Applications of Optical Remote Sensing Wiley Series in RemoteSensing (New York John Wiley amp Sons)

Bell G D and Halpert M H 1998 Climate Assessment for 1997 Bulletin of AmericanMeteorological Society 79(5) 1ndash50

Cadet D L and Diehl B C 1984 Interannual variability of surface elds over the IndianOcean during recent decades Monthly Weather Review 112 1921ndash1935

Cihlar J St Laurent L and Dyer J A 1991 The relation between normalized diVerencevegetation index and ecological variables Remote Sensing of Environment 35 279ndash298

Diaz H F and Kiladis G N 1993 Atmospheric teleconnections associated with theextreme phases of the Southern Oscillation In El Nino Historical and PaleoclimaticAspects of the Southern Oscillation edited by H F Diaz and V Markgraf (New YorkCambridge University Press) pp 7ndash28

Eastman J R and Anyamba A 1996 Prototypical patterns of ENSO-related drought anddrought precursors in Southern Africa The Thirteenth Pecora Symposium Proceedingsheld at Sioux Falls South Dakota USA 20ndash22 August 1996 (Sioux Falls SouthDakota USGS)

Eastman J R and Fulk M A 1993 Long sequence time series evaluation using standard-ized principal components analysis Photogrammetric Engineering and Remote Sensing53 1649ndash1658

Farrar T J Nicholson S E and Lare A R 1994 The in uence of soil type on therelationships between NDVI rainfall and soil moisture in semiarid Botswana IINDVI response to soil moisture Remote Sensing of Environment 50 121ndash133

Gray T I and Tapley D B 1985 Vegetation health natures climate monitor Advances inSpace Research 5 371ndash377

Hastenrath S 1990 Decadal-scale changes of the circulation in the tropical Atlantic sectorassociated with the Sahel drought International Journal of Climatology 10 459ndash472

Holben B 1986 Characteristics of maximum-value composite images for temporal AVHRRdata International Journal of Remote Sensing 7 1417ndash1434

Huete A R and Tucker C J 1991 Investigation of soil in uences on AVHRR red andnear-infrared vegetation index imagery International Journal of Remote Sensing 121223ndash1242

Jury M R and Pathack B M R 1993Composite climate patterns associated with extrememodes of summer rainfall over Southern Africa 1975ndash1984 Theoretical and AppliedClimatology 47 137ndash147

Justice C O Holben B N and Gwynne M D 1986Monitoring East African vegetationusing AVHRR data International Journal of Remote Sensing 7 1453ndash1474

Katz R W 1988 Use of cross correlations in the search for teleconnections Journal ofClimatology 8 241ndash253

NDVI anomalies during the ENSO event 1859

Kaufman Y J and Holben B 1993 Calibration of the AVHRR visible and near-IR bandsby atmospheric scattering ocean glint and desert re ection International Journal ofRemote Sensing 14 21ndash52

Lamb P J and Peppler R A 1991 West Africa In Teleconnections Linking WorldwideClimate Anomalies edited by M H Glantz R W Katz and N Nicholls (New YorkCambridge University Press) pp 121ndash189

Lamb P J Peppler R A and Hastenrath S 1986 Interannual variability in the tropicalAtlantic Nature 322 238ndash240

Lindsey J A 1988 Southern Africa rainfall the Southern Oscillation and a southernhemisphere semi-annual cycle Journal of Climatology 8 17ndash30

Linthicum K J Anyamba A Tucker C J Kelly P W Myers M F and PetersC J 1999 Climate and satellite indicators to forecast Rift Valley fever epidemics inKenya Science 235 1656ndash1659

Los S O 1993 Calibration adjustment of the NOAA AVHRR Normalized DiVerenceVegetation Index without recourse to Channel 1 and 2 data International Journal ofRemote Sensing 14 1907ndash1917

Los S O Justice C O and Tucker C J 1994 A global 1deg by 1deg NDVI data set forclimate studies derived from the GIMMS continentalNDVI dataInternationalJournalof Remote Sensing 15 3519ndash3546

Mason S J and Tyson P D 1992 The modulation of sea surface temperatures and rainfallassociations over Southern Africa with solar activity and the Quasi-BiennialOscillation Journal of Geophysical Research 97 5847ndash5856

McPhaden M J 1999 Genesis and evolution of the 1997ndash98 El Nino Science 283 950ndash954Myneni R B Los S O and Tucker C J 1995 Satellite-based identi cation of linked

vegetation index and sea surface temperature anomaly areas from 1982ndash1990 forAfrica Australia and South America Geophysical Research Letters 23 729ndash732

Nicholson S E Davenport M L and Malo A R 1990 A comparison of vegetationresponse to rainfall in the Sahel and East Africa using Normalized DiVerenceVegetation Index from NOAA-AVHRR Climate Change 17 209ndash241

Nicholson S E and Entekhabi D 1986 The quasi-periodic behavior of rainfall variabilityin Africa and its relationship to the Southern Oscillation Archive fur MeteorologieGeophysik und Bioklimatologie A 34 311ndash348

Nicholson S E and Kim J Y 1997The relationship between El Nino-Southern Oscillationto African rainfall International Journal of Climatology 17 117ndash135

Ogallo L J 1988 Relationships between seasonal rainfall in East Africa and the SouthernOscillation Journal of Climatology 8 31ndash43

Rasmusson E M and Carpenter T H 1982 Variations in tropical sea surface temperatureand surface wind elds associated with Southern Oscillation El Nino MonthlyWeather Review 110 354ndash384

Richard Y and Poccard I 1998 A statistical study of NDVI sensitivity to seasonal andinterannual rainfall variations in Southern Africa International Journal of RemoteSensing 19 2907ndash2920

Ropelewski C F and Halpert M S 1987 Global and regional scale precipitation patternsassociated with El NinoSouthern Oscillation Monthly Weather Review 115 1606ndash1626

Ropelewski C F and Halpert M S 1989 Precipitation patterns associated with the high-index phase of the Southern Oscillation Journal of Climate 2 268ndash284

Ropelewski C F and Halpert M S 1996 Quantifying Southern Oscillation-precipitationrelationships Journal of Climate 91043ndash1059

Saji N H Goswami B N Vinayachandran P N and Yamagata T 1999 A dipolemode in the tropical Indian Ocean Nature 401 360ndash363

Sellers P J 1985Canopy re ectancephotosynthesisand transpirationInternationalJournalof Remote Sensing 8 1335ndash1372

Tucker C J 1979 Red and photographic infrared linear combinations for monitoringvegetation Remote Sensing of Environment 8 127ndash150

Tucker C J and Newcomb W W 1994 AVHRR data sets for determination of desertspatial extent International Journal of Remote Sensing 17 3547ndash3565

Tucker C J and Sellers P J 1986 Satellite remote sensing of primary productionInternational Journal of Remote Sensing 7 1133ndash1135