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Remote Sensing of Environm
Seasonal and interannual variability of surface circulation in the Cape
Verde region from 8 years of merged T/P and ERS-2 altimeter data
Clara Lazaro a,*, M. Joana Fernandes a, A. Miguel P. Santos b, Paulo Oliveira b
aFaculdade de Ciencias, Universidade do Porto, Porto, PortugalbInstituto Nacional de Investigacao Agraria e das Pescas (INIAP) IPIMAR, Lisboa, Portugal
Received 15 April 2005; received in revised form 27 May 2005; accepted 11 June 2005
Abstract
The characterisation of the geostrophic surface flow field around the Cape Verde Archipelago in the northeast Atlantic Ocean with satellite
altimeter data is presented. The aim is to analyse the main current systems present in the region 3-–30-N, 40-–10-Wand their seasonal and
interannual variability. A merged data set of Topex/Poseidon (T/P) and ERS-2 altimeter data for an 8-year period, beginning in June 1995,
has been used and corrected sea surface heights were computed by applying a homogeneous set of relevant geophysical corrections. ERS-2
data were crossover adjusted to T/P. Monthly maps of sea level anomalies were created for the whole period and were used in the
computation of monthly maps of absolute dynamic topography, geostrophic currents and eddy kinetic energy (EKE). The seasonal signal of
the northeast Tropical Atlantic large-scale surface circulation appears as the prevailing cause of the variability in the region, particularly in the
southernmost portion of the region being studied. This signal is also present in the flow field along the African coast and in the Guinea Dome.
Regions of highest EKE values are clearly associated with the North Equatorial Counter-Current and with the currents along the African
coast. The significant interannual variability found for 1998 seems to be associated with the 1997–1998 ENSO Pacific event, but other
anomalous periods (1996–1997 and 2001–2002) uncorrelated with ENSO are also evident.
D 2005 Elsevier Inc. All rights reserved.
Keywords: Satellite altimetry; Sea level anomaly; Absolute dynamic topography; Geostrophic surface currents; Eddy kinetic energy; Seasonal and interannual
variations; Northeast Atlantic Ocean; Cape Verde Archipelago
1. Introduction
Satellite altimetry, providing a globally and quasi-
synoptically description of the geostrophic sea surface
currents, can be used to analyse the ocean circulation and
its variability. Since 1995, with the simultaneous operation
of Topex/Poseidon (T/P) and ERS-2 missions, the estima-
tion of the scales of ocean surface circulation processes, in
particular the mesoscale, has been made possible with an
unprecedented resolution. The advantages of merging data
from different altimetry missions, which have complemen-
tary temporal and spatial resolutions, have already been
0034-4257/$ - see front matter D 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.rse.2005.06.005
* Corresponding author. Faculdade de Ciencias, Universidade do Porto,
Departamento de Matematica Aplicada, Rua do Campo Alegre, 687, Porto,
4169-007, Portugal. Tel.: +351 220100879; fax: +351 220100809.
E-mail address: clazaro@fc.up.pt (C. Lazaro).
discussed by several authors and are described, for instance,
in Ducet et al. (2000) and Le Traon and Dibarboure (1999).
Several studies have been carried out in the northeast
Atlantic Ocean in recent years, allowing the characterisation
of the large-scale ocean circulation and its seasonal
variability. These studies have been mainly supported by
numerical models, historical hydrographic data, and, more
recently, in situ observations (e.g., Fratantoni, 2001;
Mittelstaedt, 1991; Siedler et al., 1992; Stramma & Isemer,
1988; Stramma & Schott, 1999). Due to the nature of the
hydrographic and in situ data used, most of these analyses
are restricted both in space and in time. In studies of the
north Atlantic surface circulation and eddy kinetic energy
with data from satellite-tracked drifters, some authors (e.g.,
Fratantoni, 2001; Zhurbas & Oh, 2004) have pointed out
that, in contrast with other areas such as the western Atlantic
and in particular the Gulf Stream region, data are lacking for
ent 98 (2005) 45 – 62
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6246
a large area in the eastern tropical Atlantic off the western
African coast between 5- and 25-N. Therefore, the knowl-
edge of the ocean dynamics at the sea surface is still far
from being complete in this region.
Due to its global coverage, satellite altimetry is consid-
ered to contribute significantly to improve this knowledge.
Moreover, the length of presently available altimetry data
sets starts to be long enough to allow the retrieval of the
interannual signal with a unique capability. Regional studies
based on altimeter data, either from independent solutions,
data assimilation into models, or data comparison with in
situ observations, are still scarce for the study area.
The ocean variability of this region has been mainly
studied in the scope of global investigations of altimeter
data (Ducet et al., 2000). On the other hand, the northeast
subtropical Atlantic Ocean area, north of 20-N, has been the
focus of several studies using satellite altimetry, but these
are concentrated mainly on the Canary Islands and the
Azores Current regions (e.g., Efthymiadis et al., 2002;
Tejera et al., 2002; Tokmakian & Challenor, 1993; White &
Heywood, 1995).
This study focuses on the description of the seasonal and
interannual variability of the ocean circulation in the
southern region of the northeast Atlantic Ocean surrounding
the Cape Verde Archipelago, from 3- to 30-N in latitude and
from 40- to 10-W in longitude (Fig. 1), using satellite
Fig. 1. Schematic map representing the POCM_4B mean dynamic topography (con
studies, present in the region surrounding the Cape Verde Archipelago: NECC — N
Canary Current; MC — Mauritania Current; GD — Guinea Dome. Locations of th
and sections A and B selected for transport computations are also shown.
altimetry. For this purpose an 8-year time-series of merged
ERS-2 and T/P satellite altimeter data, beginning in June
1995, has been analysed.
The paper is structured as follows. In the next section the
current systems and their variability known from previous
studies are briefly described. In Section 3 the altimeter data
and the processing methodology adopted in the derivation
of the analysed oceanographic variables are explained. The
seasonal patterns of the main current systems and the most
interesting features revealed by the results, as well as the
potential of this time series in the identification of
interannual variability and wave propagation, are high-
lighted in Section 4. Finally, Section 5 presents the
discussion of the results.
2. Study area and background
The Cape Verde Archipelago is located in the northeast
Atlantic Ocean at the eastern boundary of the subtropical
gyre, at about 500 km west of Cape Verde sited in Senegal
at the NW African coast (Fig. 1). A seasonal signal of the
large-scale surface circulation has been documented as the
response to the seasonal variability of the trade winds and
the meridional displacement of the Intertropical Conver-
gence Zone (ITCZ), being also present in the flow field
tours shown every cm) and the main surface current systems, from previous
orth Equatorial Counter-Current; NEC — North Equatorial Current; CC —
e Cape Verde Archipelago, the several capes mentioned throughout the text
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 47
along the northwest African coast and in the associated
upwelling phenomenon (e.g., Stramma & Schott, 1999;
Stramma & Siedler, 1988).
The islands are located at the southern limit of the Canary
Current (CC), responsible for the transport of cool water
southwards off the African coast. This current detaches from
the coast between latitudes 25- and 20-N, near Cape Blanc.At Cape Verde latitude, the CC flows southwestwards and
joins the North Equatorial Current (NEC) (Mittelstaedt,
1991). Stramma and Siedler (1988) found that the CC is
stronger in summer near the African coast while in winter it
is stronger west of the Canary Islands.
Between latitudes 5- and 10-N, the dominant feature is
the eastward flow known as the North Equatorial Counter-
Current (NECC). The NECC shows a pronounced seasonal
variation, being stronger in northern summer and autumn
(from July to December) when the ITCZ reaches its
northernmost position. During this period the NECC is a
continuous zonal flow extending over approximately the
entire Tropical Atlantic (Mittelstaedt, 1991; Stramma &
Siedler, 1988). Some studies also indicate a correlated
northward shift of the NEC during this period due to the
northward shift of the ITCZ (Stramma & Siedler, 1988).
During northern winter and spring the strong trade winds are
responsible for the weakening of the NECC, which at the
same time is pushed back to the Equator and also becomes
irregular (Mittelstaedt, 1991).
As the NECC approaches the African coast, some of its
flow derives towards north resulting in a northward flow
referred by some authors as the Mauritania Current (MC),
responsible for the carriage of warm oligotrophic equatorial
water to the tropical eastern Atlantic. The MC also shows a
seasonal behaviour associated with the NECC. In winter and
early spring the MC only reaches latitudes of about 14-N(Cape Verde, Fig. 1). At this period of the year the wind
field off the African coast between 14- and 20-N is
favourable to coastal upwelling. Cold and nutrient-rich
upwelled water is transported southwards and is responsible
for the high productivity of the region (Mittelstaedt, 1991).
In summer and early autumn, due to the strengthening of the
NECC and the relaxation of northeast trade winds, the MC
reaches latitudes of about 20-N, just south of Cape Blanc
(Fig. 1), at the same time of the occurrence of the non-
upwelling season south of this latitude (Mittelstaedt, 1991).
Siedler et al. (1992) also observed a strong increase in the
northward flow along the African coast during summer.
Another prominent feature in the study region is the
Guinea Dome (GD) a constituent of the large-scale near-
surface flow fields associated with the NEC, the NECC and
the North Equatorial Undercurrent (NEU) (Siedler et al.,
1992). The GD is located southwest of Cape Verde
Archipelago and has an associated cyclonic geostrophic
flow, confirmed by observational data (Mittelstaedt, 1991).
According to Siedler et al. (1992), it exists permanently
throughout the year at NEU levels while seasonal variations
are found above. They also suggested that the associated
flow weakens in winter, probably caused by the similar
seasonal change in the NECC. From their results the upper
thermocline centre of the structure is thought to be found at
about 9-N, 25-W in the summer months, and at 10.5-N,22-W in the winter months. Still these positions were not
accurately determined since an apparent double-cell struc-
ture was noticeable during the summer. The shape of the
dome varies according to season, an elongation of the dome
with a NE–SW direction being seen in summer, while an
approximately circular form is observed in winter.
Fratantoni (2001) found fast drifter motions (larger than
0.4 m s�1) in a relatively continuous range including the
equatorial region. Trajectories in the low latitudes suggested
rotary motions with meridional scales of several hundred
kilometres probably related with meanders of the NECC.
Slow trajectories (<0.1 m s�1) were found in the eastern
subtropical latitudes. Unfortunately, the lack of data in the
eastern tropical Atlantic surrounding the Cape Verde
Archipelago, precluded a detailed discussion of this region.
The major current systems are an important source of
eddy kinetic energy through their instabilities and it is
generally accepted that, in areas away from the influence of
topography, high EKE regions correspond to regions of
energetic currents (Reverdin et al., 2003). A conclusion is
usually drawn that EKE may be considered as a proxy for
the paths of the major currents (e.g., Heywood et al., 1994).
The highest velocity values are found for the area where the
NECC is present and consequently higher values of eddy
kinetic energy occur in the same area. At lower latitudes of
the North Atlantic Ocean, south of 5-N, values of EKE as
high as 0.15 m2 s�2 are found west of 35-W and between
longitudes 30-–25-W (Fratantoni, 2001). From the study of
Ducet et al. (2000), values of EKE up to 0.02 m2 s�2 are
found around Cape Verde Islands, the smallest values
(<0.01 m2 s�2) being obtained north of 15-N.Consequently, the area between 10- and 15-N, where the
Cape Verde Archipelago lies, can be viewed as a region of
large-scale interactions between the CC, the NEC and the
NECC. Different water masses meet near the archipelago
forming a large-scale frontal system, a potentially produc-
tive zone favourable for the aggregation of large pelagic fish
species (e.g., tunas).
Studies based on altimeter data series, mainly focusing
on the investigation of interannual variability and long wave
propagation, are hardly found for the study region.
Arnault and Cheney (1994) examined the region
variability based on 4 years of Geosat data (April 1985 to
September of 1989) in spite of the difficulty in obtaining
relevant oceanographic information from this data set. Apart
from the expected strong seasonal signal, the authors found
significant interannual variations occurring 1 year after the
Pacific El Nino event of 1986–1987. Arnault and Kestenare
(2004) analysed the variability of the surface currents in the
tropical Atlantic from 10 years (November 1992 to August
of 2002) of T/P data. From their results, anomalous
interannual events were found for 1996–1997 and 2001–
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6248
2002 and a simple relation between these events and the
Pacific El Nino events was difficult to establish.
The propagation of Rossby waves in the ocean plays a
significant role in its adjustment on annual to decadal time
scales (Killworth et al., 1997). On the other hand, these long
waves are fast enough in the tropics to allow the adjustment
of the ocean on seasonal time scales and are important in
regions where the seasonal variability is strongest (Doos,
1999; Jochum et al., 2004). Evidence of their westward
motion has been found in satellite sea surface temperature
data and, more recently, in sea surface height anomalies as
the time series of satellite altimetry became long enough to
allow these studies (Cipollini et al., 2000; Doos, 1999;
Jochum et al., 2004; Katz, 1997; Killworth et al., 1997).
An analysis concerning the wave propagation along the
equatorial Atlantic was performed by Katz (1997) using 2
years (1993–1995) of T/P and inverted echo sounder (IES)
data. The author found a westward propagating wave with
phase speeds, inferred from T/P and IES data respectively,
of �0.23 m s�1 and �0.30 m s�1, the differences being
explained to some extent by natural variations between the
measurements.
Jochum et al. (2004) and Doos (1999) compared their
results, obtained from general circulation model simula-
tions, to altimeter and in situ data. The former authors found
that there is evidence of westward propagating tropical
Scheme 1. Summary of the main steps used in the altimeter data processing. Boxe
text represent mean monthly products (computed by averaging, for the whole 8-y
instability waves on both sides of the Equator, generated
from May to January. Changes in the phase speed, as the
waves propagate from east to west, are pointed out as one of
the causes for the discrepancies in the range of the phase
speed values found by various authors. Apart from direct
meteorological forcing, unstable flow fields can also be
responsible for wave generation. Therefore, a range of
possible phase speed values can be expected caused by the
variability of current properties.
Doos (1999) found that the latitudinal change in the
westward phase velocity of the waves is confirmed by
observational and model data, being however smaller than
predicted by theory. The phase speeds computed were lower
than predicted for latitudes south of 10-N and the opposite
was verified for latitudes north of 10-N.
3. Data and methodology
The altimeter data used in this study include ERS-2 and
T/P for the 8-year period from June 1995 to May 2003. The
ERS-2 data are the OPR02 Version 6 precise Ocean
Products (OPR) provided by the European Space Agency
(ESA, 1996), from cycles 2 to 85. The spatial resolution of
these data in the study region varies from 80 km at latitude
3-N to about 69 km at latitude 30-N. The temporal
s with text in italic represent final monthly products, while boxes with bold
ear period, all the grids corresponding to the same month of the year).
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 49
resolution is 35 days. The T/P data are the Merged
Geophysical Data Records (MGDR) provided by the
Archivage, Validation et Interpretation des donnees des
Satellites Oceanographiques (AVISO, 1996), from cycles
103 to 394, with a spatial resolution of 315 km at latitude
3-N and approximately 273 km at latitude 30-N and a
temporal resolution of 10 days.
These data have been used to derive a set of oceano-
graphic products, adopting a methodology described in the
following two subsections and summarised in Scheme 1.
3.1. Altimeter data processing description
All geophysical corrections have been applied to data to
compute the corrected sea surface height: ionospheric
correction, dry and wet tropospheric correction, sea state
bias (SSB), earth and ocean tides, tidal loading, pole tide
and the inverse barometric effect. When necessary, correc-
tions have been updated to introduce state-of-the-art models.
In order to obtain homogenous data sets the same models
were used for the following corrections: dry troposphere,
earth tide, ocean tide, pole tide and inverse barometer. The
criteria used in the choice of the remaining geophysical
corrections and orbit solutions were twofold: the best
models available for each mission should be used and the
T/P data, used as reference, should be kept at its highest
level of accuracy. Table 1 summarises the corrections
applied to each data set.
The ERS-2 satellite ephemeris used were the precise
DGM-E04 orbits available from Delft University of
Table 1
Geophysical corrections and models used in the processing of ERS-2 and T/
P altimeter data
Corrections/models ERS-2 T/P
Mean Sea
Surface
GSFC00.1 (Wang,
2001)
GSFC00.1 (Wang, 2001)
Orbit TuDelft DGM-E04
(Scharroo & Visser,
1998)
NASA JGM3 (AVISO,
1996)
Dry troposphere ECMWF (ESA, 1996) ECMWF (AVISO, 1996)
Wet troposphere ATSR-M model,
improved in coastal
regions (Fernandes
et al., 2002)
TMR model (yaw state
and drift effect on TMR
TBs applied, (Scharroo
et al., 2004))
Ionosphere Bent’s model (ESA,
1996)
Topex: double frequency
(filtered, (Fernandes
et al., 2002))
Poseidon: DORIS model
Sea state bias BM3 formulae (Gaspar
& Ogor, 1996)
(Chambers et al., 2003)
Earth tide Applied (ESA, 1996) Applied (AVISO, 1996)
Ocean tide NAO99 (Matsumoto
et al., 2000)
NAO99 (Matsumoto
et al., 2000)
Pole tide Applied (AVISO, 1996) Applied (AVISO, 1996)
Inverse
barometer
(Dorandeu & Le Traon,
1999)
(Dorandeu & Le Traon,
1999)
SPRT and
USO drift
Applied –
Technology (Scharroo & Visser, 1998). For ERS-2 all
corrections recommended in the CLS ERS-2 cycle valida-
tion reports have been introduced. These include the effect
on the wet tropospheric correction due to the gain fall and
drift occurred on the 23.8 GHz channel of the microwave
radiometer and the BM3 formulae for the sea state bias
correction (Gaspar & Ogor, 1996). With the aim of reducing
land effects on data near coastal zones the ERS-2 radiometer
wet tropospheric correction has been improved using
appropriate methodologies (Fernandes et al., 2002). ERS-2
data have also been corrected for the SPTR (Scanning Point
Target Response) and USO (Ultra Stable Oscillator) drift
effects and for the pole tide.
For T/P the cycle dependent drift effect in the TMR wet
tropospheric correction, including the yaw state correction
to the TMR measured brightness temperatures, has been
modelled according to Scharroo et al. (2004). The SSB
correction was derived from the Chambers et al. (2003)
model; a residual SSB correction of �3 mm was applied to
cycles 236 and greater (Topex B) (Berwin, 2003).
The Topex dual frequency ionospheric correction has
been smoothed using a second order Butterworth filter
(Kulhanek, 1976), with a cut-off period of 20 seconds
(k�120 km). The result of this filtering is similar to the
procedure referred by Imel (1994) who used a 20-s
averaging window, with the advantage that the effects of
discontinuities introduced by data gaps or land regions are
minimized.
For both T/P and ERS-2 the same ocean tide and inverse
barometer models have been used. The ocean tide model
used was the NAO99b model (Matsumoto et al., 2000),
which has been found to give smaller residuals in coastal
regions (Fernandes & Antunes, 2003). The adopted inverse
barometer model makes use of a non-constant surface
atmospheric pressure interpolated from global average
values obtained from CNES/CLS (Dorandeu & Le Traon,
1999).
Sea level anomalies (SLA) have been derived from the
corrected sea surface height values by removing the mean
sea surface from the GSFC00.1 model (Wang, 2001).
3.2. Computation and mapping of geostrophic currents
For the analysed period of 8 years (June 1995 to May
2003) monthly maps of absolute dynamic topography
(ADT) and geostrophic currents were derived using the
procedure described as follows.
ERS-2 35-day and T/P 10-day along track data (SLA
values) were split into monthly files, containing data centred
on each month of the year, including 35 days of ERS-2 and
365.25/12 days of T/P data. This procedure was adopted to
get full ERS-2 coverage and consequently to improve the
spatial resolution.
Each ERS-2 file was crossover adjusted to the corre-
sponding T/P data file, considered as the reference data set,
using tilt and bias parameters. This methodology allows the
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6250
reduction of ERS-2 orbital error (Rummel, 1993) and more
generally any inconsistencies between the two missions (Le
Traon et al., 2003). Before adjustment the mean and the root
mean square (rms) error of the differences at the crossover
points are of the order of 4–10 cm and 11–15 cm,
respectively. After adjustment the mean is zero and the
rms is reduced to 7–8 cm. These results are in agreement
with those obtained by Le Traon et al. (2003).
The ERS-2 adjusted and T/P data were joined together,
creating monthly files of merged along-track data.
Monthly grids of sea level anomalies were computed
using kriging. A data spacing of three arc minutes has been
used (0.05-).To reduce measurement noise, the SLA grids were
filtered using a low-pass Butterworth filter of second order
with a cut-off half-wavelength of 100 km. The adopted
small data spacing was selected to minimize the sampling
effects of the frequency domain filter.
ADT grids were generated by adding to each SLA grid a
20-year mean dynamic topography modelled by the global
ocean circulation model Parallel Ocean Climate Model 4B
(POCM_4B) (Tokmakian, personal communication) (Fig.
1). The model is a version of the Semtner and Chervin
(1992) global ocean model derived from Bryan’s multilevel,
primitive equation formulation and with an average reso-
lution of one-quarter degree (Stammer et al., 1996). The
methodology adopted to derive the ADT maps is one of the
possible methods that can be used while a sufficiently
accurate global geoid model is not available (Le Traon &
Morrow, 2001).
From the ADT grids, sea surface currents along the zonal
(u) and meridional (v) directions were computed, using the
known geostrophic equations given by the following
formulae (Knudsen, 1993):
u ¼ � cf R
flfflu
ð1Þ
v ¼ cf Rcosu
flfflk
ð2Þ
where f is the absolute dynamic topography, u and k are the
latitude and longitude respectively, A =2xesinu is the
Coriolis parameter, c is the normal gravity, R is the Earth
mean radius and xe is the Earth’s angular velocity.
Based on u and v grids, monthly maps representing the
magnitude of the geostrophic currents were also computed.
These will be referred as magnitude maps. Both geostrophic
surface current components were computed for each ADT
grid and the resultant geostrophic current was represented as
a vector overlaid on the ADT and magnitude maps. Due to
the different magnitudes of the geostrophic velocities in the
northernmost and southernmost regions of the study area,
the geostrophic velocities were scaled by a linear function of
latitude. In this way, the velocities for latitude 30-N are
represented by arrows four times greater than they would be
if the velocity scale was maintained in the meridional
direction. This procedure allows a better analysis of the
ocean circulation patterns in the northernmost region of the
study area.
3.3. Computation of transport and eddy kinetic energy
Transport computations for the upper 100 m layer were
carried out along specific sections, perpendicular to the
mean current direction. A meridional section of 500 km
length at 25-W and a zonal section of 150 km length at
16-N were selected as representative of the NECC and MC
flows respectively, represented by symbols A and B in Fig.
1. Transport computations were performed by integrating
the current velocity over the width and depth of each section
(Tomczak & Godfrey, 2003), assuming no velocity change
with depth. In the computations along the meridional
section A only the zonal (u) velocity grids were used;
along the zonal section B only the meridional (v) velocity
grids were used. Positive values represent eastward and
northward transport, respectively.
Assuming geostrophy only, monthly maps describing
EKE per unit mass were computed from surface time-
varying geostrophic velocity components (Ducet et al.,
2000):
EKE ¼ 1
2uV2 þ vV2
��ð3Þ
where uV and vV are the geostrophic velocity components,
given by formulae (1) and (2) replacing f by fV, the latter
representing the sea level anomaly variable.
3.4. Computation of mean monthly maps and Hovmoller
diagrams
For the purpose of studying the seasonal signal in the
region, mean monthly maps were computed by averaging,
for the whole 8-year period, all the grids corresponding to
the same month of the year. In this way, twelve maps
were generated for each analysed oceanographic variable
(SLA, ADT, EKE, geostrophic velocity components (u,
v), and magnitude of geostrophic velocity), representing
the mean conditions for each month, from January to
December.
Longitude-time plots (Hovmoller diagrams) were com-
puted along selected latitude profiles and for the entire study
area width. In spite of the irrelevance of the propagation
angle for linear planetary waves, data were analysed just for
zonal profiles, as evidence of different orientations were
found only rarely (Killworth et al., 1997). For each variable
three types of Hovmoller diagrams were generated, repre-
senting the total, the seasonal and the deseasonalized signals
respectively. The seasonal signal was computed considering
the 8-year average of each month, as referred above; the
difference between the total and the seasonal signal is
considered the deseasonalized signal.
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 51
4. Results
4.1. Seasonal variability
This subsection describes the mean and seasonal
characteristics of the surface circulation in the study region.
The interannual variability will be discussed in the next
subsection. In order to simplify the analysis, the year is
divided in four boreal seasons: January to March (winter),
April to June (spring), July to September (summer) and
October to December (autumn).
In the analysis below various levels of information have
been used. First, the whole sequence of monthly maps of
SLA, ADT, EKE and magnitude of the currents were used in
animated sequences in order to observe the features
described as a function of time. Second, the mean maps
computed for each month of the year, representing the
seasonal signal, were analysed.
From these, maps representative of each season were
selected. Mean ADT maps for March, June, August, and
December are presented in Fig. 2; the corresponding maps
of mean SLA are shown in Fig. 4. Mean geostrophic surface
velocities overlaid in the corresponding mean magnitude
Fig. 2. Mean monthly ADT maps for the following months: (a) March (winter); (b
shown every 1 cm (solid lines represent ADT�0 m; dashed lines represent ADT
maps are shown in Fig. 3 for some representative months
when the most interesting features occur (February, June,
October and December).
4.1.1. The North Equatorial Counter-Current and Maur-
itania Current
The results show that the surface circulation has a strong
seasonal pattern particularly evident in the southernmost
region of the study area.
In early spring, starting in April, the North Equatorial
Counter-Current (NECC) appears as a zonal flow in the
southeastern part of the study region (east of 25-W and
south of 5-N). During this season, the area occupied by
this zonal flow gradually expands westwards and north-
wards, reaching 35-W in June, being generally centred on
latitudes 5- to 6-N (Figs. 2b and 3b). In June, it is a
narrow flow, 300–400 km wide, with an almost zonal
direction covering the whole range of longitudes from 35-to 15-W, with mean velocity of 0.30 m s�1, although its
core velocity can be as high as 0.60 m s�1, particularly
between longitudes 32-–22-W (Fig. 3b). East of 20-Wtwo main branches of the NECC can be observed (Fig.
3b). One branch turns in the northeast direction and the
) June (spring); (c) August (summer); and (d) December (fall). Contours are
<0 m).
Fig. 3. Vectors representing surface geostrophic currents derived from mean monthly ADT grids, superimposed on the corresponding magnitude maps for (a)
February, (b) June, (c) October, and (d) December. Note that the vectors were scaled by a linear function of latitude accordingly to the scale on the right.
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6252
other extends eastwards until 12-–13-W where part of the
flow turns northwards, along the African coast and joins
the previous branch. The result is a poleward current,
commonly known as the Mauritania Current. This current
reaches velocities of about 0.25 m s�1, in particular
between latitudes 14-–17-N, north of Cape Verde. In June
this flow can reach the latitude of Cape Timiris (20-N), butwith smaller velocities, of about 0.10 m s�1.
In July, the NECC occupies the whole study area, south
of 8-N, and is wider. It is during the period June–July when
the NECC is fastest. The Mauritania Current is found along
the African coast, up to Cape Blanc, with mean velocity
values similar to those observed in June. It is during July
and August that the Mauritania Current reaches its north-
ernmost position.
During August and September the NECC reaches its
northernmost position, approximately 11-N (Fig. 2c). Its core
velocity is then smaller, with values lesser than 0.45 m s�1.
In October, the NECC clearly becomes irregular and
typically forms two narrow branches west of 30-W, a
stronger flow centred on approximately 4-N, with
velocities reaching 0.80 m s�1 in some years and a
weaker flow further north, at about 8-N (Fig. 3c). A
mean zonal flow with velocities of 0.25 m s�1 is still
visible at longitudes between 30- and 20-W. In November
the NECC becomes narrower and velocities reaching 0.80
m s�1 are common west of 30-W. These high velocity
values are associated with large meanders that start to
develop between latitudes 5- and 10-N in late autumn.
There is still evidence of an eastward transport between
30-–20-W (Fig. 3d). In early winter, the meanders can
have wavelengths of about 700 to 800 km and amplitudes
of 300 to 400 km. Due to their shape a southward
meridional transport can be found along some profiles
west of 30-W and the highest velocities are found in
these meanders (Fig. 3a). During this season the
continuous eastward flow between longitudes 30-–20-Wis less pronounced but the NECC is already visible as a
zonal flow east of 20-W. The intensity of the Mauritania
Current decreases during autumn, reaching lowest lati-
tudes, in general south of Cape Verde, by late winter
(Figs. 2a and 3a).
The transport values found for sections A and B are in
agreement with the described seasonal behaviour of the
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 53
NECC and the MC. The mean transport along section A is
maximum in June, reaching 12.9T2.4 Sv (1 Sv=106 m3
s�1). From this point onwards it decreases until November
(5.4T2.2 Sv), raising again and reaching a relative
maximum in January (8.9T1.9 Sv). This transport in early
winter is related to the large meanders associated with the
instability of the NECC at this time of the year. From now
onwards the current transport decreases being minimum in
March (2.3T2.2 Sv). The transport along section B reveals a
similar seasonal behaviour with maximums in June
(1.9T1.1 Sv) and early autumn (1.3T0.6 Sv). The minimum
occurs in December (�0.1T0.8 Sv) associated with the
weakening of the Mauritania Current.
4.1.2. The Canary Current and the North Equatorial
Current
In what concerns the Canary Current (CC), results show
that during the period from December to June (winter and
spring), it appears as a strengthened southward flow and is
wider, presenting flows along and off the African coast,
west of Canary Islands. It is located, in general, north of
20-N, with mean velocities less than 0.10 m s�1. In winter it
reaches the lowest latitudes, as a southward flow along the
Fig. 4. Mean monthly SLA maps for the following months: (a) March (winter); (b
shown every 1 cm (solid lines represent SLA�0 m; dashed lines represent SLA
African coast between capes Blanc and Verde, with
velocities up to 0.20 m s�1 (Fig. 3a). The weakening of
the CC is verified after July, when the NECC and its related
northward flow are seasonally intensified and confined to
the African coast, being weakest in autumn.
Several cyclonic and anticyclonic eddies are shown in
the southern vicinity of the Canary Islands. Both kinds of
features are likely to occur independently of the season, but
an anticyclonic eddy is commonly seen southeast of the
archipelago from January to May. Cyclonic eddies are more
common in summer and autumn.
In the winter the NEC is less well defined in ADT maps.
Although it appears in the POCM4B mean dynamic
topography (Fig. 1), the positive anomaly present in the
SLA maps at the latitude range 5-–20-N west of the Cape
Verde Archipelago almost masks the mean signal of the
current (Figs. 2a and 4a). In spring and summer it is well
defined as a southwest flow east of the Archipelago (Figs.
2b, c, 4b and c). The magnitude of the current does not
suffer significant variations throughout the year, with mean
values lower than 0.1 m s�1. Associated with the NEC,
there is evidence of a southwestward transport between
latitudes 10- and 20-N during the whole year, although
) June (spring); (c) August (summer); and (d) December (fall). Contours are
<0 m).
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6254
more difficult to infer during winter. In spring the NEC
reaches its southernmost position. During these months a
southwest transport is also visible south of 10-N, whichmeans that in summer and autumn the NEC migrates
northwards following the similar displacement of the
NECC.
At the Cape Blanc latitude, 250 to 300 km off the African
coast, the NEC seems to split in two branches, both flowing
southwest, one north of Cape Verde islands and another
flowing south just east of the islands. This is more evident
during the first half of the year. South of the archipelago,
part of the latter flow recirculates northwards strengthening
the Mauritania Current, particularly during the spring and
summer months (e.g., Fig. 3b). A nearly permanent
anticyclonic eddy, with diameter as large as 300–400 km
is found south or southwest of the Cape Verde islands, with
Fig. 5. Hovmoller diagrams representing the seasonal EKE signal for latitudes: (a) 4
the African coast. Please note the change in the range of the colour scale for lati
mean velocity of 0.10–0.15 m s�1 reaching maximum
values in spring (Fig. 3b).
There is a large-scale cyclonic circulation in the area
between 10- and 15-N that in the most intense months
(summer until late autumn) can reach longitudes up to
35-W. The results show that this circulation seems to be the
interaction of the NEC, mainly the branch that flows south
of Cape Verde Archipelago, the NECC and the northward
flow along the African coast, the Mauritania Current.
4.1.3. The Guinea Dome
Seasonal variations of the Guinea Dome (GD) are also
found. The cyclonic circulation associated with the GD is
clearly seen in ADT maps from May onwards until
December. In late spring the GD centre is located at
approximately 9.5-N and its centre moves westwards
-N, (b) 8-N and (c) 27-N. Blank values, represented in white, correspond totude 27-N.
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 55
between longitudes 22- and 23.5-W. During this season, the
GD exhibits a more circular shape (Fig. 2b). A significant
westward displacement of the GD centre is clear during
summer and autumn, being accompanied by a slight
northward shift during this period. In August, the develop-
ment of a double-cell structure is already evident (Fig. 2c).
During this period it is centred on approximately 10.5-N,27-W and it reaches the northwesternmost position 11-N,29-W in October, probably due to the similar northward
migration of the NECC. The GD clearly weakens from
November onwards, being inconspicuous during winter.
4.1.4. Eddy kinetic energy
Also inspected was the seasonal variation of eddy kinetic
energy (EKE) for each season. Geostrophic time-varying
vectors were overlaid in EKE grids to distinguish the sense
of movement, cyclonic or anticyclonic, and not only the
maxima or minima of energy.
Higher energies are found south of 10-N corresponding
to the area where the mesoscale variability is highest, where
values up to 0.30 m2 s�2 can be found mostly in autumn,
while the rest of the area does not show EKE values larger
than 0.010 m2 s�2. During the winter, EKE values are
smaller than 0.020 m2 s�2 in the region south of 10-N and
east of 30-W. Values up to 0.050 m2 s�2 can be found west
of 30-W, associated with the previously mentioned eddies
and meanders which reach their maxima in late autumn, but
are still present although weaker at this time of the year
(Figs. 2a and 5a). During spring, EKE values in the range
0.20–0.35 m2 s�2 can be found east of 24-W at latitude
4-N. These values increase westwards as the season
progresses, in a similar fashion with the behaviour of the
NECC (Fig. 5a). It is interesting to note that at latitude 8-Nthe corresponding EKE values for this season are consid-
erably lower, below 0.012 m2 s�2. In summer, EKE values
present an almost zonal pattern at 4-N. The highest EKE
values found at latitude 8-N in late summer and early
autumn are representative of the northward displacement of
Fig. 6. Hovmoller diagrams representing the seasonal SLA signal for latitudes: (a
latitude and every 1 cm for the last two (solid lines represent SLA�0 m; dashed
the NECC during this period. This is well depicted in Fig.
5b. The most energetic features in the study region are found
west of 30-W in autumn. These are clearly associated with
the large meanders and eddies which are associated with the
NECC instability.
The region north of 10-N is generally low energetic (Fig.
5c), apart from the region close to the African coast, where
EKE values in the range of 0.010–0.020 m2 s�2 are
observed in some winter months, associated with the
strengthening of the Canary Current.
In spring the Mauritania Current has generally EKE
values of 0.010 m2 s�2, which can be higher (0.025 m2 s�2)
west of Cape Verde and in the southwest African coast. In
summer it is clear that EKE values similar to those found for
the Mauritania Current in spring are now seen at higher
latitudes up to Cape Blanc.
As referred above, eddies south of Cape Verde Archipe-
lago are strongest in spring when they can reach EKE values
up to 0.020 m2 s�2, whether they are cyclonic or anti-
cyclonic.
4.2. Interannual variability
In the analysis of the interannual variability various
levels of information have been used. Apart from the whole
series of monthly maps of SLA, ADT, EKE and magnitude
of the currents, used in animated sequences, transport
anomalies along sections A and B and Hovmoller (time–
longitude) diagrams were analysed.
The inspection of the referred animated sequences
revealed anomalous events mainly in the years 1996–
1998 and 2001–2002.
Hovmoller diagrams representing the variation of the
described oceanographic variables for the 8-year period
were created for various latitudes. Here only some examples
of SLA diagrams are shown for representative latitudes.
Hovmoller diagrams were created for the full and
residual deseasonalized signal. As mentioned before, it
) 4-N, (b) 14-N and (c) 27-N. Contours are shown every 2 cm for the first
lines represent SLA<0 m).
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6256
was found that the seasonal signal is dominant in the study
region. This is evident by comparing the Hovmoller
diagrams for example, for latitude 4-N (Figs. 6a and 7).
For the analysis of these diagrams, an extended region from
40-W towards 7-E is considered for the latitude 4-N and
from 40-W towards east, until the African coast is found, for
the remaining latitudes.
At 4-N two seasonal positive anomalies can be seen,
moving westwards (Fig. 6a). The first one crosses the study
area from February until September and seems to be
associated with a planetary wave with a phase speed of
approximately �0.3 m s�1. In this wave a change in the
phase velocity can be observed around longitude 30-W. The
Fig. 7. Hovmoller diagrams representing (a) the full SLA signal and (b) the deseas
lines represent SLA�0 m; dashed lines represent SLA<0 m).
second one crosses the same region from late autumn to
early winter, in approximately two and a half months, with a
mean speed of �0.8 m s�1 (Figs. 6a and 7a). In the residual
anomaly map (Fig. 7b) it can be seen that the first Rossby
wave of the year (February–September) is weaker in the
years 1997 and 1998. The behaviour of the 1998 wave is
associated with 1997–1998 El Nino Southern Oscillation
(ENSO) condition, while the 1997 one is related to an
anomalous condition not associated with ENSO, as it will be
further discussed in Section 5.
During 1998 another positive anomaly propagates west-
wards with similar phase speed but at a different time of the
year (from December 1997 to June 1998). During these 2
onalized SLA signal for latitude 4-N. Contours are shown every 4 cm (solid
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 57
years (1997 and 1998) the positive anomaly in the
westernmost side of the study region is stronger in late
spring and early summer.
In Fig. 8 the full and the deseasonalized signal
Hovmoller diagrams for latitude 14-N are shown. The
seasonal signal present at this latitude is depicted in Fig. 6b,
where an annual Rossby wave crossing the study area from
November until May, with a change in the phase velocity
around longitude 25-W, is well represented. Another
interesting feature which can be seen in Fig. 6b is the
variation of the SLA near the African coast, with minimum
values in February–March and maximum values in
November, associated with the seasonal behaviour of the
Mauritania Current. The most relevant features shown in the
deseasonalized SLA diagram (Fig. 8b) are the anomalous
positive and negative anomalies occurring respectively from
Fig. 8. Hovmoller diagrams representing (a) the full SLA signal and (b) the deseaso
lines represent SLA�0 m; dashed lines represent SLA<0 m).
late 2001 to the middle of 2002 mainly east of 35-W and
during autumn 1999 and winter 2000 in the half eastern side
of the region.
The seasonal SLA signal for latitude 27-N is represented
in Fig. 6c. The full and deseasonalized SLA diagrams are
shown in Fig. 9a and b, respectively. At this latitude there is
no evidence of seasonal propagation of planetary waves.
This can be explained by the relatively weak and low
energetic flows present in this region of small variability in
the wind field. Instead there is a seasonal pattern of higher
and lower SLA during the periods September to January and
February to August, respectively. These are related to
anticyclonic and cyclonic eddies present at this latitude.
Fig. 9b shows that there is clear evidence in some years of a
westward movement of both kinds of eddies. For example in
the years 2000 and 2002 there is a cyclonic eddy in the
nalized SLA signal for latitude 14-N. Contours are shown every 4 cm (solid
Fig. 9. Hovmoller diagrams representing (a) the full SLA signal and (b) the deseasonalized SLA signal for latitude 27-N. Contours are shown every 4 cm (solid
lines represent SLA�0 m; dashed lines represent SLA<0 m).
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6258
eastern part of the region, propagating westwards from the
beginning of the year until September. Roughly in this
period of the year, in the same region, there is an
anticyclonic eddy with the same phase propagation. The
westward movement of these eddies is clearly seen in the
animated sequence of ADT maps.
5. Discussion
Eight years of merged T/P and ERS-2 data have been
used to characterise the seasonal and interannual variability
of the northeast Tropical Atlantic. The period of analysis is
believed to be long enough to contain the most important
ocean–atmosphere coupled events inducing anomalous
oceanographic variations, such as the strong El Nino, which
occurred in 1997–1998, and La Nina in 1998–1999.
Therefore the analysed seasonal signal is influenced by
both normal and extreme conditions. The use of merged
data from T/P and ERS-2 significantly increases the
capability of mapping mesoscale features compared to the
use of T/P only. In this study, monthly maps were created in
a compromise between keeping the full spatial sampling
capability provided by ERS-2 rather than the higher
temporal sampling of T/P.
The results show that the dominant signal in the study
area has a seasonal pattern, accompanying the seasonal wind
field variability known for the region. The interannual
signal, although typically weaker than the seasonal one, is
not negligible in some years, being strongest at lower
latitudes, where maximum values of the seasonal cycle are
also found.
In what concerns the geostrophic circulation, pronounced
seasonal variations are found for the North Equatorial
Counter-Current and the Mauritania Current as a result of
the north–south migration of the Intertropical Convergence
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 59
Zone (ITCZ) and corresponding changes in wind-stress curl.
The NECC is stronger in summer and autumn (from July to
December) and its zonal pattern is most evident in late
spring and summer. From this study, the NECC is fastest
during June and July. In late summer the continuous zonal
flow reaches its northernmost position. In October large
meanders start to develop west of 30-W and the establish-
ment of the mean flow position is difficult, a result also
reported by Verdy and Jochum (2005). From the analysis of
the results of an ocean general circulation model of the
Tropical Atlantic Ocean, these authors found that west of
32-W the advection of relative vorticity by the mean flow
and by eddies are not negligible in the vorticity budget,
pointing it as the main difficulty in the development of a
theoretical framework for the NECC and a challenge to
observational studies. These energetic meanders can be
responsible for a southward heat transport near the equator
reported by, e.g., Lamb (1981), Carissimo et al. (1985), and
Oort and Vonder Harr (1976). According to some authors
(Stramma & Isemer, 1988) this transport is still controver-
sial since is not present in their results. These large
anticyclonic meanders are still common in the westernmost
part of the study region during January and February. In late
winter and early spring, the NECC is either not present or
weakened west of 20-W. This latter condition is reached in
some years, when an eastward flow, although weaker during
this period, is present at these longitudes throughout the
year. These results confirm those obtained by studies based
on historical hydrographic data (e.g., Mittelstaedt, 1991;
Stramma & Siedler, 1988), and altimetry data (Arnault &
Kestenare, 2004).
The seasonal behaviour of the NECC can be interpreted
by the analysis of the mean SLA maps shown in Fig. 4. In
March and April a clear zonal negative anomaly is present
in the NECC region, responsible for a time-variable
westward flow, thus reducing the eastward NECC pre-
dominant flow (Fig. 4a). During these two months the
NECC is either weak or not present west of 20-W (Figs.
2a and 4a). On the other hand the positive zonal anomaly
found in the southeastern part of the region during late
spring contributes to the strengthening of the NECC during
this period (Fig. 4b). These positive anomalies intensify
and progress westwards and northwards during summer, in
a similar fashion as the described NECC behaviour during
this period (Fig. 4c). During late summer and autumn the
positive anomalies lose their zonal characteristic and so
does the NECC (Fig. 4d).
The strengthening of the Mauritania Current, which is
evident in summer, follows the behaviour of the NECC.
Cross-correlation between monthly transport values for
sections A and B were computed. A maximum value of
0.72 was found for lag of 1 month. During this period the
MC, which is originated from NECC branches, reaches the
northernmost latitude up to Cape Blanc. It is responsible for
the transport of warm oligotrophic equatorial waters to the
tropical Atlantic and occurs simultaneously to the suppres-
sion of coastal upwelling south of the mentioned cape. The
intensity of the MC decreases during autumn reaching the
lowest latitudes, south of Cape Verde, by late winter.
Also directly related with the seasonal variability of the
NECC is the variability of the position, shape, and intensity
of the Guinea Dome. The GD and its associated cyclonic
circulation are clearly depicted in ADT maps from May to
December. During this period, the GD shape varies from a
nearly circular to a double-cell structure with its centre
showing a northwestward migration. These results differ
from those obtained by Siedler et al. (1992). According to
these authors the circular-shaped GD centre in winter was
located north of its corresponding position in summer. From
our results, the GD is inconspicuous during winter and
migrates northwards during summer, following the analo-
gous migration of the NECC. Also, our results indicate that
the GD can be found as west as 29-W, in summer,
considerably west of the position given by the mentioned
authors (25-W).
The Canary Current does not show a strong seasonal
variability, although it is more intense and wider during
winter and spring and weaker and closer to the African coast
from July onwards. The southwestward transport related to
the North Equatorial Current is visible throughout the year
between latitudes 10- and 20-N, while less defined in ADT
winter maps. At Cape Verde latitude results indicate the
existence of two branches of the NEC during the first half of
the year, one turning southwestwards north of Cape Verde
Archipelago and the other becoming a southwest flow south
of the archipelago, both appearing to join together again at
lower latitudes.
Consequently, the area between 10- and 20-N, where theCape Verde Archipelago is located, can be regarded as a
region of interactions between the main current systems
described. Since these transport different water masses and
meet in the surroundings of the archipelago, a large-scale
frontal system is formed, and a potentially productive zone
can be expected near the islands. Favourable conditions for
the aggregation of large pelagic migratory fish species can
arise, sustaining the fisheries activity of the country.
Higher EKE values are found south of 10-N, clearly
related with areas where both mesoscale variability is higher
and the strongest currents are found, namely the NECC and
the MC. The highest values are found during autumn, west
of 30-W, when the instability of the NECC is evident in the
westernmost part of the study area. The comparison between
the EKE, ADT and magnitude monthly maps allow to
conclude that EKE can be used as a useful indicator of the
paths and intensity of these currents.
The SLA deseasonalized Hovmoller diagrams created for
different latitude profiles clearly reveal the existence of
years showing anomalous conditions.
In deseasonalized SLA Hovmoller diagram for latitude
4-N a sequence of anomalous positive and negative SLA
values can be identified, from the beginning of 1996 to the
summer 1999 with apparent westward propagation.
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–6260
The sequence of positive and negative SLA in the
equatorial Atlantic, prior to the 1997–1998 Pacific ENSO
event, have been reported as the result of an anomalous
event which occurred in the tropical Atlantic in the period
1995–1997. This Equatorial Atlantic Oscillation (EAO)
was analysed by Handoh and Bigg (2000) using altimetry
amongst other data. During this event, which started in the
autumn of 1995 and continued until the autumn of 1997,
time sequences of distinct anomalies in sea surface temper-
ature (SST) and sea surface height (SSH) were observed in
the equatorial band. This event seems to be the long-lived
oceanic response to a westerly wind anomaly in the western
Atlantic during the autumn (October–December) of 1995.
Warm (early 1996 to summer of 1996) and cold (early 1997
to summer of 1997) phases were reported (Handoh & Bigg,
2000). The authors also identified a sequence of Kelvin
waves during the period of this event, followed by the
propagation of slow westward waves. Since there was also
evidence of westward propagation with similar speed in the
SST field and in atmospheric convection data, the authors
called these westwards propagating coupled-mode waves
instead of Rossby waves. Handoh and Bigg (2000) thus
found that the climate of the tropical Atlantic during 1996
and 1997 was dominated by a coupled atmosphere–ocean
mode of behaviour and suggested that this climate
oscillation in the Atlantic was independent of the ENSO.
In the deseasonalized Hovmoller diagram for the equator
(not shown here), both positive (warm) and negative (cold)
phases reported by Handoh and Bigg (2000) were identified.
In addition, as referred by the authors that it should be
expected but not observed in their results, another sequence
of positive (after summer 1997 until the end of the year) and
negative (since the start of 1998 until late spring) SLA are
identified. This could be a consequence of the higher spatial
resolution present in our merged T/P and ERS-2 data set
compared to the T/P data used by those authors.
The anomalous Atlantic event referred above explains
the described interannual variability between 1995 and mid-
1998. Assuming that the North Atlantic oceanic response to
the Pacific ENSO events has an approximate lag of 4–5
months, as reported by Enfield and Mayer (1997), the 1997
ENSO event should be the responsible for the positive SLA
observed between June and September 1998, clearly visible
at the equator and still present, although less well defined at
the latitudes of our study region. On the contrary, the
response of the tropical Atlantic to the 1998 La Nina event
is well depicted in the Hovmoller diagram of 4-N, betweenspring and early summer of 1999 (Fig. 7).
In the time-sequence of ADT maps, an intensified NECC
is seen in the spring of 1997 and 1998, resulting, in June, in
a continuous zonal flow covering the width of the study
region. In regular years this situation is only reached in July
while in June it is confined at longitudes east of 35-Wyears.
This seems to be associated with the earlier northward
displacement of the ITCZ, reported by Enfield and Mayer
(1997), causing an early spring transition.
This behaviour of the NECC is related with the
anomalous positive anomalies found at the NECC band of
latitudes during the warm phase of the EAO event and the
1997–1998 El Nino. Also intensified is the GD in the
summer of 1997 due to the strong SLA negative anomalies
seen in the GD region for the 10-N profile (not shown). The
negative anomalies found west of 30-W, during late autumn
of 1997 and early winter of 1998, are responsible for the
weakening, visible in ADT maps, of the large anticyclonic
eddies that in general are seen west of this longitude. This
seems to be related to the cold phase observed in the
summer of 1997.
The anomalous negative anomalies found west of 20-Win the spring of 1996 and 1999 can be associated with the La
Nina event. This corresponds to an eastward shift of the
NECC zonal flow at this time of the year.
During October 1997, following the cold EAO phase,
extensive phytoplankton blooms (¨2�105 km2), charac-
terised by chlorophyll a concentrations ¨40 times greater
than the adjacent waters were observed in offshore
oligotrophic waters approximately 1200 km from the coast
(Fernandes, submitted for publication).
Hovmoller diagrams of EKE for 4-N reveal an anom-
alous higher energetic period during the year 1996 (March
to October) extending over the whole range of longitude of
the study area with highest values in the western part.
Although not evident in the corresponding deseasonalized
SLA Hovmoller diagram this anomaly corresponds to the
period of occurrence of the first Rossby wave of the year.
No increase in EKE values was found in 1997.
In the period 2001–2002 another anomalous event is
depicted in the deseasonalized SLA diagrams at all latitudes.
Anomalous positive anomalies can also be seen in the
westernmost side of the study region from December 2001
to March 2002, associated with the eddy intensification. In
particular, in the region north of 10-N, this seems to be the
most remarkable anomalous period (Figs. 8 and 9).
In this study the mean dynamic topography (MDT) has
been modelled by a 20-year mean run of the POCM_4B
model. Comparing the magnitude of the model MDT
values and the altimetry-derived SLA it is clear that the
magnitude of the former is at least of the same order of
magnitude of the SLA values found for most of the
periods. In spite of being a region of weak oceanic
variability, and has not been the subject of attention such
as the western tropical Atlantic, it is a challenge for
techniques such as satellite altimetry which proved to be
effective in detecting the seasonal and interannual varia-
bility of this small magnitude. With the recent develop-
ments in geopotential modelling from the satellite gravity
missions, with particular emphasis to GRACE (and GOCE
in the future) it is expected that independent models of
MDT will be derived. In parallel with the advances in SLA
variability determined from satellite altimetry, these models
will certainly improve the knowledge of the absolute
dynamic topography.
C. Lazaro et al. / Remote Sensing of Environment 98 (2005) 45–62 61
Acknowledgments
This study has been funded by FCT project POCTI/
36095/CTA/2000. The ERS-2 altimeter data have been
provided by ESA under the scope of the project ‘‘Remote
Sensing Applied to Fisheries in the Cabo Verde Region’’,
AO3-265. The T/P data have been provided by AVISO. The
authors also want to acknowledge Robin Tokmakian for
providing the 20-year mean dynamic topography from
POCM_4B model used in the study.
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