Interannual Variation of East AsianWinter Monsoon and ENSO

Yi ZhangKennethR. Sperber

James S. Boyle

This paperwaspreparedfor submittalto theThird Conference on Esat Asia and Western Paci@ Meteorology and Climate

Chungl~ TaiwanMay 16-18,1996

December 1996

DISCLUMSR

Thisdocument was prepred as an account of work sponsored by an agency of the United States Government.Neither the United States Governrnent nor the University of California nor any of their employees, makes anywarranty, express or implied, or assumes any legid liability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or process d~closed, or representa that its use would notinfringe privately owned rights. Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States Government or the University of California. The views andopinions of authors expressed herein do not necessarily state or reflect those of the UNted States Governrnent orthe University of California, and shidl not be used for advertising or product endorsement purposes.

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Interannual Variation of East Asian Winter Monsoon and ENSO

Yi Zhan~ KennethR. SperberandJames S. BoyleProgramjbr ClimateModelDiagnosisand Intercompison,

LawrenceLivermoreNationalLaboratory,Livermore,CA 945s0

ABSTRACT

..

This paper examines the interannualvariationof the East Asian wintermonsoonand itsrelationship with ENSO based on the 1979-1995 NCEP/NCARreanalysis.Twostratificationsofcold surgesare used.Thefirstone,describedas theconventionalcoldsurges,indicatesthatthesurge frequency reaches a minimum one year after El Nino events. The second one, originatedfrom the same region as the first, is defined as the maximum wind events near the South China ,.Sea. The variation of this stratification of surges is found to be in good agreement with the SouthernOscillation Jndex (SOI). Low SOI (l@ SOI) events coincide with years of low (high) surgefrequency.

The interannual variation ofaveraged meridional windneartheSouthChinaSeaandwesternPacific is dominated by the South China Sea cold surges, and is also well correlated (R=O.82)withthe SOL Strong wind seasons are associated with La Nina and high SOI events; likewise, weakwind years are linked with EINino and low SOI cases. This pattern is restricted north of the e@tor

within the region of (oON-200N, 11O%13OOE), and is confined to the near surface layer. The surfaceSiberian high, 500 hPa trough and 200 hPa jetstream, all representing the large-scale monsoonflow, are found to be weaker than normal during El Nino years. In particular, the interannualvariation of the Siberian high is in general agreement with the SOL

1. Introduction

The East Asian winter monsoon, which isassociated with a strong Siberian high andactive cold surges, is one of the most energeticmonsoon circulation systems. The dramaticshiftof northeasterlies and the outbreak of coldsurges dominate the winter weather and localclimate in the East Asian region. The wintermonsoon and cold surges exert a strong impacton the extratro~ical and tropical planetary-

scale circdaticms (chang and Lau, 1982), andinfluence the SSTS in the tropical westernPaafic (chang et al., 1979). Their generalcharacteristics and the possible linkagebetween cold surges and tropical atmosphericand oceanic plwmoxnena have been examinedin many studies. For example, Chang and Lau(1982) showed that the outbreak of wintermonsoon surges forced short term changes in

the Hadley and Walker Circulation, the EastAsian jetstrearn and large-scale deepconvection over the equatorial western Pacific.Boyle (1986a, 1986b) found the iiequency andintensity of the monsoon surges for a givenmonth were related to the intensity of the EastAsian jetstream and the extratropical largescale circulation patterns. Lau and Chang(1987) suggestedthat theinterannualvariationof thewintermonsoonandcoldsurgesmayberelated to ENSO and tropical intraseasonaloscillations. Ding and Krishnamurti (1987)noted an eastward shift of the tropicalplanetary scale divergent circulation inassociation with the cold surges which wasvery similar to the shift of the divergentcirculationbetweenElNinoandLaNinayears.

Althoughthe wintermonsoonis knowntobe much weaker during the 1982-83 El Ninoevent (Lau and Chang, 1987), there has never

.,, . . . .

1

been a systematic investigation on therelationship between ENSO and theinterannual variation of the winter monsoonand cold surges. The purpose of this study is toexamine the interannual variation of the EastAsian winter monsoon and cold surges and todetermine whether there exists a consistentrelationship between the winter monsoon andENso.

2. Data

We use the twice-daily surface and upper-air fields from the NCEP/NCAR reanalysis(Wayet al., 1996) for the period of 1979-1995.The surface quantities include temperature,wind, and sea-level-pressme (SLP); upper airfields are winds at 925 hpa, 850hPa, 700 hpa,and 200 hpa and geopotentialheight at 500hPa.The advantage of this data set is that thereanalysis used a “fro~~” state-of-the-art

analysis/forecast system to perform the dataassimilation throughout the whole period,thus circumventing problems with previousNWP analyses due to changes in techniques,models and data assimilation. Also, thisdynamically consistent reanalysis offers good

horizontal (2.5°x2.50) and vertical resolution(17 levels).

3. Winter mean circulation and cola surges

The mean winter monsoon circulation hasbeen documented by many authors (e.g., Boyleand Chang 1984; Lal.1 and Li 1984; bU2 andChang 1987; Boyle and Chen 1987) and’ isbriefly discussed here to serve as a backgroundfor the rest of the paper.

The dominant surface feature of the wintermonsoon is the Siberianhigh.The winter meanSLP averaged for 1979/80 - 1994/95 is shownin Fig. 1. This surface pressure system with

..-

Flg.1. l’he averaged East Asian mean sea-levelpresmresupefipmed onthesurface(10meter)windforwinter(DJF)of 1979/30-1994/95.UnitihPaandrns-l.

2

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central pressure in excess of 1036 hpa, coversthe entire East Asian region and yieldsnortheasterly flow over a large part of EastAsia. At 500 hPa, the flow pattern is dominatedby the coastal East Asian trough. The intensityof this trough is quasi-geostrophicallylinkedtothe surface Siberianhigh. The most prominentfeature at 200 hPa is the East Asian jet locatednear southern Japan. This jet, the strongest onthe globe, is associated with intensebarocliniaty, large vertical wind shear andstrong cold advection. It is maintained by theCoriolis torque acceleration exerted on theageostrophic wind near thejet entrance region.Its strengthis inherently related to the intensityof surface Siberian high and the 500 hPa

trough. These features characterize the large-scale three-dimensional winter monsooncirculation.

Fig. 1 also presented the winter mean (DJF)surface wind averaged for 1979/80 - 1994/95.This Cli.rnatological surface wind representscontribution from both the stationary wintermonsoon circulation and the cold surges.

North of 300N, the surface wind blowsanticyclonically along the north and east

periphery of the Siberian high where thestationarypressuresystemcontributesmoretothe wind field. south of 300’N where theSiberianhighpressureexertslessinfluence,thewind maximumnear the South ChinaSea canbe largely attributedto the cumulativeeffectsof the cold surges.one of the mainfeaturesofthe surface wind is that the westerlyflownearJapan bifurcates.The eastwardbranchmergeswith the Aleutian 10W,while the southernbranch joins the tradewind belt. Cold airintensity and thickness USUally reduce as theairmass moves southward. The monsoon coldair is thus a relatively shallow phenomenon inlow latitudes.

The cold surge is the most importanttransient disturbance embedded within themean winter monsoon circulation. Theoccurrence of a cold surge is characterized bya southward movement of a surfaceanticyclone and an associated abrupt 24-48hours surface air temperature drop in theaffected regions. The typical scenario of a cold

air outbreak is the Siberianhigh and the majortrough near the east coastal region reach acertain intensity. Further west over thecontinent, an upper level short waveundergoes very strong development as itmoves eastward. Eventually the short wavedevelops into a major trough and replaces theold quasi-stationary trough near the coastalregion. During this process, the surfaceanticyclone moves southeastward and a coldsurge occurs (Staff members of AcademiaSinica, 1957). It should be pointed out that thebuild-up and maintenance of the Siberianhighpressure is a necessary condition for theoccurrence of cold surges. Radiative cooling,persistent cold air advection (usually blockingover Urals) throughout the troposphere andlarge-scale descending motion all contribute tothe maintenance of the cold Siberian high(Ding and Krishnamurti,1987).

4. Interannual variation

In order to investigate the possible linkagebetween the interannual variation of the EastAsian winter monsoon and ENSO, the 1979-1995 winter average southern OscillationIndex (SOI) is plotted in Fig. 2.

20

1.0 [

a“ -

g .1.0 -

!! ‘.20

-20 -\ ,Wf

.40 -

-s.0 -

-6.079m81s283e.48swwm Sssoslwwslsm

Yo4n

Fig. 2. Southern oscillation index. Anomaly ofpressure difference between Tahiti and Darwin.

Although the Southern Oscillation is not astanding phenomenon, the evolution ofmonsoon circulation during ENSO phases isignored given that the winter monsoon is notexamined on a monthly basis. The two major

3

El Nino events of 1982/83 and 1986/87 exhibitlow SOI. Negative departuresof the SOI alsooccurduringthe1989/90and1991/92seasons.The 1988/89LaNina event has SOIin excess of2 hpa.

4.1 Cold surges

19-w -

--- -Iwomb

17 -— su41@E. mb

161514

s13

u 12 .,B 11 \

i :

10a \87 - v

6 -

s

43 -2-

79aoal S2s2041J!3we7ea Mlmvlwmmsm

Fig.3. Coldsurgefrequen~-from 1979/80-1994/95.Unit number of events per season.

The variation of cold surge frequency isfirst examined. h interesting feature in Fig. 3is that minimum frequency events occur oneyear after the El Nino years. To test therobustness of this restdt, we investigated thesensitivity of the cold surge frequency to theSLP threshold in the cold surge criteria (Zhanget aL, 1996). Aside from an expected declineincold surge frequency with a stricter SLPcriterion, the interannual variations werefound to be insensitive.Further, the cold surgefrequency from BeijingMeteorological Center(BMC) during 1980-1984 shows the samevariation pattern as in Fig. 3. Data from BMCalso indicates that cold surge frequencyreaches a minimum one year after the 1957/58and 1965/66 El Nino events. Thisphenomenonagrees with the 1-1.5 year lag response of thenortheast China surface air temperature to theSST near the eastern equatorial Paaficsuggested by Bao et al. (1989).

As mentioned earlier, the definition of acold surge is regionally dependent. The coldsurges identified above, which are in goodagreement with the observations from BMC,typically influencethebulkofEastAsiaand thecoastal regions. However, a subset of these cold

surges can reach to the South China Sea. Thisis illustratedin Figs. 4(a) and(b), time series ofmeridional wind in three regions for the 1988/89 season, along with the SLP near the Siberianarea. The phases of SLP variation are exactlyreversed to those of meridional wind nearsouthern China, which are followedchronologically by the similar variationpatternsnear Taiwanandin thevicinityof theHainan Island. The lags between the variationpatterns in each region, which represent thesoutheast propagation of surface anticyclones,are around 1-3 days.

The typical synoptic scenario of thepropagation is as follows, before the outbreakof a cold surge, the intensification of theSiberian high simultaneously strengthens thenortheasterly flow near southern China. Theoutbreak of the cold surge pushes theanticyclonicflow southeastward and increasesthe northerlies near Taiwan region. The windthen subsequently penetrated further into thenorthern part of the South China Sea and thecold surge ended.

According to the weather forecasters (H,personal communication, 1996) of Hainan

Island (180N, 11OOE),whether a cold surge willaffect the Island and the surrounding oceansdepends on its path and intensity. When thisregion is under the influence of a cold surge,the most conspicuous weather phenomenon isthe strong northeasterly wind. The sevenstrong wind events (northerly >7 m/see) nearthe South China Sea in the 1988/89 season, asindicated in Fig. 4 (a), is a subset of the elevencold surges identified during the season(Zhang et al., 1996). Strong wind events forother years are also examined (not shown). Wefound that, with no exception, every event isassociated with a cold surge outbreak from theextratropics.

Because the surface meridional wind is themost important indicator of the cold surgeactivity in Hainan area, we define the SouthChinaSeacold surge as thenumber of days thatmaximum northerly wind >7 m/s in the region

of (1OON-2OON,11O%12OOE).Similardefinitionhas been used by Chang and Chen (1992).Based on this definition, the South China Sea

4

II

5

3

1

-1

-3

-5

-7

-9

-11

, , I ,

.,3 LNov

ill --- Tdwanmeridlondwind

!i’I — HainarImeridionalwind

I 1 ,

?c Jan Feb Mar Apr

Iml. .,>. l”””’”””””’’”””’ “’”’’’”’’’”””’”’’’’’’”ITime (month)

1055

1050

1045

1020

1015

1010

. .

!!

14

lm51 ”’’’’’””’’”’’”’”’”’” “’’’’’”f I

Nov Dec Jan Feb Mar AprTime(month)

Fig.4. (a)Twice dailytimeeeriesof area averagedmeridiona windfromregion nearsouthemCtia

(102.5”E-107.5”E,45”N-5O”N), nearTaiwan(120”E-125%22.5”N-27.5”N),andin the vicinityof theHainmIeland (11O”E-115”E; 1o”N-2O”N). Unikm/see. (b) Twicedailytimeserieeof SLP near Siberia(l12.5”E-117.5”E,250N-3CPN)from Novemberof 1987toMarchof1988.UNRhPa.A five-pointrunningaveragehas

beenappliedto allthefields.

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cold surge frequency is shown in Fig. 5. Theinterannual variation of cold surge frequencyis in good agreement with the pattern of SOI(correlation coeffiaent = 0.$5). Low cold surge

100

26 -

L :a

I :so

7s

m -

66 -

2079 W818ZES248S 202702 Wmmwfuwsw

Fig. 5. Cold surge frequen&-k the South China Sea,numberof daysthemaximumnortherlywind near

the South China Sea (110%-120%, 1O?N-2LY’N)exceed7 m/s. Unk numberofdaysperseason.

frequenaes coinadewith El Nino and low SOIevents; high frequenaes accompany with LaNii and high SOI years. This agrees withobserved strong wind record of Hainan Island(H, personal communication, 1996) for wintersof 1950-1980, which indicates that cold airactivity is much reduced during the years of1957/58,1965/66,1968/69,are El Nino events

4.2 Mean mm”dional m“nd

and 1972/73, all

K

Frequency of the South China Sea coldsurges not only bears good relationship withthe SOI, but also largely controls themeridional wind variation over the SouthChina Sea and over even larger areas. Fig. 6shows time series of mean meridionalwind forincreasingly larger regions. The patterns in allthe three regions are similar to that of SOL Inagreement with the cold surges, strongernortherlies occur during La Nina and high SOIseasons, and weaker northerlies occur duringEl Nino and low SOI years. The correlationcoeffiaent between the SOIand the meridional

wind in the South China Sea (10~-

2OON,11O%-12O%)is 0.82. By examining thesame time series as in Fig. 6 over many nine-grid-point rectangular boxes near the westernPacific, we found that this SOI-likepattern ofthe wind does not exist in any of the areas

outside the region of (@N-200N,100%-130’%).-5r

.— (1ON.2CN,11OE-12OE)

.1.s--- (5N-mN.lo7.sE-122.5El— (WON,1OOE-1*

.1 1’792081 =03246s8s 87298Sm91a22294952e

Yun

Fig. 6. Winter (NDJFM)area averaged northerlywindover threeregionsnear the SouthChinaSeaandwesternPacific.Unk m s“*.

-5.5

-0.5 ~79m8162212M25m 2722 299091 s293242s

Yaws

Fig. 7. Winter (NDJFM)area averaged northerlywindovertheSouthChinaSea(110%-115%, 10’%1-2@N) at 925hpaand850 hpa.Unikm S-*.

The meridional wind variation is a shallowphenomenon, occurring primarily at thesurface.This is demonstratedin Fig. 7 wherethewindvariationover the South China Sea isplotted at the 925 hPa and 850 hPa levels.Althoughthe patternsat both levels still bearcertain resemblance to the SOI pattern, thebiennial oscillation is the dominate mode. At700 hPa and above (not shown), the

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geostrophic flow is decidedly zonal over theentire East Asian continent and the wind in thenorthern part of the South China Sea is undergreat influence of the spIit jetstream south ofthe Tibetan Plateau. The signature of the SOIhasbeen totallylost. Clearly, thisSOI-likewindpattern is horizontally restricted within the

region of (OON-2O%L1OO%-13O%)andvertically confined to the near surface layer.

Based on the above discussions, it is clearthat the variation of area averaged meridionalwind near the South China Sea and westernPaafic hinges on the variation of the SouthChina Sea cold surges. The South China Seacold surges share the same origin as theconventionally defined cold surges. The areaaveraged meridional wind near the SouthChina Sea is thus greatly controlled by thenortherlies propagated from the extratropics.

4.3 Large-scale features

As mentioned earlier, the surface Siberianhigh, 500hPa trough, and the 200 hPa jetstreamcharacterize the winter monsoon large-scalecirculation and their strength are inherentlyrelated to each other.

,.~79ma18283e485ms7ea s99091s2s3e4mm

Fig. 8. Number of da~y%at the Siberianhighpressure exceeds 1050 hPa in the Siberiaregion(45%1-55~, 90%110%). Unk numberof daysperseason.

Fig. 8 presentsthe numberof days that thecentralpressureis greaterthan 1050I@a in theregion of (450N-5YN, 9OOE-11OOE).Althoughthis may not be the only measure of the

Siberianhigh intensity, thebroad aspect of theinterannual variation of this measure agreeswith the SOI. The variation patterns indicatethat, in general, the Siberian high is stronger(weaker) in La Nina (El Nine) and high (low)SOI years. The mean meridional windaveraged over the southeast periphery (5W20N, 160%180%) of the Siberian high,another way to represent the intensity of thehigh pressure, shows similarpattern as that ofFig. 8 (not shown).

The variation of the 500 hPa geopotentkd

height anomaly along 135% is calculated.Although the overall variation pattern (notshown) isnot comparable with the SOI,the’500hPa trough is weaker than normal during the ‘1982/83, 1986/87 and 1991/92 El Nino events.The 200 hPa East Aian jet is also examined.Although the East Asian jet is subject topropagation or expansion in the east-westdirection during differentphases of ENSO, thejet intensity (at 140%) is found abnormallyweak during allthree majorl?lNino events (notshown). Thisresult is consistentwith the weakSiberianhigh and 500 hpa trough.

Overall, the large-scale monsooncirculation is weaker than normal during allthe three major El Nino events. In particular,the variation pattern of the Siberian highgenerally agrees with the SOI.

5. Summary and discussion

The study of the East Asian wintermonsoon and cold surges from the 1979-1995NCEP/NCAR reanalysis has revealed severalnoteworthy features involving theirinterannual variability. The two stratificationsof cold surges are revealing when consideringthe relationship between ENSO and theinterannual variation of the cold air activity.The first stratification yields result that isconsistent with the in situ observation fromBMC. The interannual variation of cold surgefrequency shows that minimum surgefrequencyoccurs one year after El Nino events.The second stratification, based on themaximum meridional wind event near the

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South ChinaSea, indicatesthattheinterannualvariation of the South China Sea cold surge isin good agreement with the SOI. High coldsurge frequency is found during La Nina andhigh S01 events, low frequency is associatedwith El Nino and low SOI events.

The variation of mean meridional windnear the western Pacific hinges on the SouthChina Sea cold surges. A in the case of thesurges, the interannual variation of the wind isalso well correlated with the SOI.Strong windyears are associated with LaNina and high SOIevents; weak wind years are found during ElNino and low SOI events. This pattern ofvariation is restricted north of the equator

within (0~-2@N,100%-13@E), and COXlfinedto the near surface layer. The interannualvariation of the Siberian high, measured bymeans of both pressure and wind, is in generalagreement with the SOL Both the strength ofthe 200 hpa jetstream and the 500 hpa troughare found to be weaker than normal during ElNino events, although their interannualvariation patterns do not agree with the SOI asclosely as that of the Siberianhigh.

Given the close relationship between theSOI and the interannual variation of monsoonmeridional wind and cold surges, it is mturalto wonder why some of the monsoon relateddisturbances, either originated from or locatedin the extratropics, exhibit variation patternssimilar to the SOI, and what are the physicalmechanisms between the ENSO/wintermonsoon interaction.

During a typical El Nino year, theconvective activity in the maritime continent isusually reduced. E the cumulus convection inthis region is one of the most importantmonsoon energy sources as suggested byRamage (1971), the suppression of themaritime convection should be responsible, atleast partly, for the weaker than norrmdextratropical monsoon circulation during ElNino events. With the available observedoutgoing longwave radiation (OLR) data fromthe National Oceanic and AtmosphericAdministration, we have computed the OLR

anomaly in the maritime continent (110%-

120%, 5%J-5%) from 1979-1992. The overall

8

interannualvariationpattern (not shown here)is not as close to the SOI as that of the SouthChina Sea cold surges or the !Xherian high.However,negative anomaly is found duringthe 1988/89 La Nina season; likewise,positiveOLR anomalies are found during the 1982/83and 1986/87 El Niinoseasons. The correlationcoefficientbetween the OLR anomaly and theSOI is IR I=0.5.

On the otherhand, evidenceindicated thatthe winter monsoon and cold air activity cangreatly affect the tropical convective activityand SST. Slingo (1996, personalcommunication) found that Indonesianconvection is enhanced when cold surgespenetrated far into the tropics. Chang et aL(1979) have shownthat cold surges are capableof influencingthetropicalSST.Butwhether thecold surges can significantlyaffect the tropicalregion, as has been discussed, depends on theintensity and path of the cold surges. It mayalso relate to the background large-scalecirculation.Theoreticalstudieshave suggested(Branstator 1983, Lau and Li 1984) that theinteraction between the tropical convectionand midlatitude circulation depends criticallyon the relative position of the convection andextratropical quasi-stationarywaves.

It has also been suggested that theinteraction between cold surges andconvective activities near the maritimecontinentwill in turn modify the extratropicaland tropical synoptic and planetary scalecirculations (Chang and Lau, 1980).Apparently, further studies are much neededto delineate the mechanism of the interactionbetween the winter monsoon and convectionnear the maritime continent.

In summary, the current study hasindicated that the interannual variation of theSouth China Sea cold surges is in goodagreement with the SOI.The South China Seacold surges share the same origin with theconventionally defined cold surges. They alsodominate the variation of the area averagedmeridional wind near theSouth China Sea andthe western Pacific. The conventionallydefinedcold surges,on theother hand, reachedminimum frequencies one year after El Nmoevents. However, physicalmechanisms mainly

.

responsible for the interaction between ENSOand winter monsoon are still not wellunderstood, especiallyproblems on ifandhowthe winter monsoon cold air activity affectstheENSO through its impact on tropical SSTpattern and convective activities in themaritime continent. Simulations of coupledocean models and GCM simulations withspecii5ed observed SST, such as simulationsfrom the Atmospheric Model IntercomparisonProject (AMP), should provide a goodopportunity to tackle these problems.

Acknowledgments. we wish to thank Y.-L.Zhang and S.-J. Chen of Peking University,China for providing the observed cold surgedata. Tian-Fu Li of Hainan MeteorologicalBureau provided valuable information andoffered his forecasting experience. MichaelFiorino facilitated access to the NCEP/NCARreanalysis. Thiswork was performed undertheauspices of the U.S. Department of EnergyEnvironmental science Division at theLawrence Livermore National Laboratoryunder contract W-7405-ENG-48.

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