GEOPHYSICALRESEARCHLETTERS,VOL. 26, NO. 11,PAGES1613-1616,JUNE 1, 1999
Responseof the Equatorial Indian Ocean to an unusual wind event during 1994 P.N. Vinayachandran •, N.H. Saji• andToshio Yamagata •'2 Abstract.
Unusual
westward
wind anomalies
were observed
equatorialIndianOceanwouldalsobe differentfrom a nor-
in the Indian Oceanduring1994. The responseof the oceanto mal year and, possibly,strengthen the anomalouscondition theseanomaliesis studiedusingan oceangeneralcirculation throughair-seacoupledprocesses. An event in 1994 that model forced with interannual winds. Model results show that
the eastwardequatorialjets in the Indian Ocean were weak during 1994. Consequently,the sealevel in the easternIndian Ocean was unusually low, the thermoclineand the mixed layer were shallow, and the subsurfaceflow remained eastward throughoutthe year. Anomalies of sea surface temperature(SST) and sea level in the easternIndian Ocean were enhancedby upwellingfavorablewinds along the coast of Indonesia.The model resultsagreewith observationsand TOPEX/POSEIDON data. We suggestthat the dynamical responseof the ocean to wind anomaliesplayed a role in generatingthe SST anomaliesduring 1994 and enhancedthe large-scaleair-sea interaction,independentof the E1 Nifio Southern Oscillation.
developed alongsucha scenario is thesubject of thisarticle. 2. The unusual wind event in 1994
In a normal year two surgesof westefiiesoccuralong the equatorialIndian Ocean; first during April-May and second during October-November. An unusually strong and persistentwestwardanomalyof the winds occurredduring 1994 in the equatorialIndian Oceaneastof 60øE (Figure 1). Strongsoutheasterlies occurredalong the coastof Indonesia, which extendednorth of the equator.Thesewindspushedthe line of zero zonalwind stressnorthof the equatorresultingin an easterly component along the equator, which is quite unusualduringspringandfall. The wind anomaly during 1994 has severalfeaturesthat make
it
distinct
from
interannual
anomalies
over
the
equatorialIndian Ocean during other years. Besidesbeing strong,the 1994 anomalyis remarkablefor its unprecedented During the transition period between the summer and persistence. The westwardanomaliesstartfrom April and last winter monsoons,i.e. during spring (April-May) and fall for 8 monthsaffectingbothspringandfall equatorialjets. The (October-November),strongeastwardjets (calledWyrtki jets) next strongest event occurred in 1982 where westward are excited in the equatorial Indian Ocean [Wyrtki, 1973]. anomaliespersistedfor as many as 5 monthsstartingfrom Thesejets are bestdevelopedin the upperoceanbetween60- July, but anomalieswhich exceededone standarddeviation 90øE within 2.5ø on either side of the equatorwith speeds existed only during the 3 months of July, October and oftenexceeding120cm/s.The jets accumulatewarm water in Novemberwhereasthe 1994 anomaliespersistedfrom April the easternequatorialIndian Oceancausinghigher sea level, throughOctober. Yet anotherstrong(but eastward)anomaly warmerseasurfacetemperature(SST) and deepermixed layer developedduring 1988 April, but it did not persistbeyonda and thermoclinein contrastto its westerncounterpart.The jets month. E1 Nifio during 1994 was moderatein contrastto a also affect the oceancirculationin the Bay of Bengal [Mc strongE1 Nifio during 1982-83, suggestinga unique air-sea Creary et al., 1996 and referencestherein] and around Sri coupledphenomenonover the Indian Ocean during 1994 as Lanka [Vinayachandranand Yamagata,1998], the pathof the notedby Nicholls [ 1989]. 1. Introduction
Southwest
Monsoon
[Vinayachandran et
Current
al.,
in the eastern
1999],
Indian
and the
Ocean
Indonesian
throughflow[Yamagataet al., 1996] by meansof Kelvin and Rossbywave propagation. The Wyrtki jets are driven by the westerly winds in the equatorialIndian Ocean[O'Brien and Hurlburt, 1974] which peak during the transitionperiod between the monsoons. Therefore,anomaliesin thesewesterliesare expectedto affect the onset time, strengthand duration of the jets. Consequently, the sea level and thermal structurein the eastern
3. Ocean response The responseof the oceanis investigatedusingan Indian Ocean (30øS-30øN; 30øE- 115øE) model basedon the Modular
OceanModel version2.211[Pacanowski,1996]. The model
has realisticcoastlineand bottomtopography.All boundaries are closedand a spongelayer is appliednear the southernwall of the model ocean.Horizontal grid spacingis 0.33ø in both latitude and longitudeand there are 25 levels in the vertical. The model is forced by Hellerman and Rosenstein[1983] winds for 5 years and then by the Florida State University IInstitute forGlobalChange Research, Frontier Research System (FSU) monthlymeanwindsfor the period1990-1996.Surface salinity in the model is relaxed to monthly climatological for Global Change,Tokyo,Japan. 2Department of EarthandPlanetary Physics, Graduate School of value with a time scaleof 30 daysand surfaceheat flux is Science,The Universityof Tokyo,Tokyo,Japan. parameterized by a schemethat combinesclimatologicalheat flux with sea surfacetemperature.The performanceof the Copyright1999by theAmericanGeophysical Union. model (at a lower horizontal resolution) has been evaluated beforeand found to reproducethe circulationin the Bay of Papernumber1999GL900179. 0094-8276/99/1999GL900179505.00 Bengalreasonablywell [Vinayachandranet al., 1999]. 1613
0.5
-0.5
Figure 1. Wind stress (dynes cm '2)anomalies (vectors) andSST[Reynolds andSmith, 1994] anomalies (color filled contours at 0.5øC interval) during May1994andOctober 1994 overtheIndian Ocean. Thewindstress iscalculated fromLegler etal. [1989] using aconstant drag coefficient of0.0013. Theanomalies arefromthemean during theperiod 1970-96. -
Theequatorial jet simulated bythemodel forthe1990-96 periodis shown in Figure2. Normally thespring jet lies
-
between60-90øEwith its speedin the rangeof 100-125
-
cm/sec. It develops during April,reaches itspeakintensity in
-
Mayanddecays byJune. In 1994, thespeed ofthespring jet
-
extentwas only up to 80øE.An eastward flow which developed eastof80øEduring March1994wasquitedifferent fromotheryears andisnotcomparable tothejet.Thefalljet wasabsent duringOctober1994andtheflow eastof about
-
70øEremained westward. Thougha weakjet witha maximum
-
waslessthanhalf itsnormalvalue(Figure2) anditseastward
" speed of about50 cm/sec developed duringNovember, it is -_._ apparent fromFigure2 thatits spatialas well as temporal '
A conventional eastwardEquatorialUndercurrent (EUC),
-
drivenby an east-west pressure gradient, is observed in the
-
IndianOceanduringFebruary-April [Knox,1976].A reverse
-
undercurrentwith eastwardflow at the surfaceand subsurface
" -
westward flow is seenduringAugust-September [Taft and Knauss,1967]. The model resultsshow similar features,
650E
75"E
85øE
LONGITUDE
950E
exceptduring1994-95(Figure3). The speedof themodel
-
EUC is about60 cm/sduring1993and 1996whereasduring 1994and 1995theEUC wasweakwith speedsup to 20 cm/s. The reverse(i.e. westward)undercurrent is alsoreproduced
--
bythemodel during thesummer (e.g.1993,1995and1996in Figure3). During1994,however, the subsurface flow was
-
eastward fromJanuary1994to February1995(Figure3); the
-
reverseundercurrentdid not occurduring 1994.
.•
Reppin et al. [1999] noted from direct current measurements duringJuly1993- September 1994thatthefall
--
jet during1993wasverystrong withaneastward transport of
-
35 Sv whereasthe springjet of 1994wasa very weakone witha transport of only5Sv.Themodelresults areconsistent
-
with theseobservations. SimulatedEUC is alsocomparableto the observations but weaker;the observedEUC during April
--' --
1994attains a speed of about40 cm/sec whereas in themodel it is only20cm/sec (Figure3). Theobservations alsoshowed thattheEUCweakened duringMay-July butrecovered during
2 -
August. A similar recovery of eastward subsurface flowis simulated bythemodel during August-September (Figure 3).
-
55øE
extent was much shorterthan other years.
-
Figure2. Interannual variation of the simulated equatorial jetsduring 1990-1996. Zonalcomponent ofvelocity averaged from 2øS to 2øN is contoured at an interval of 25 cm/s. Eastwardflow is shownin red and westwardflow in blue.
VINAYACHANDRAN ET AL.: INDIAN OCEAN RESPONSE TO THE 1994 WIND EVENT
1615 120 100
20---
80
40
60
60
'
40 20
100
0
x
-20
120
-,.
-40 -.
-60
140
-80
160
-100
' M
' J
-120
I S
D
M
1993
J
S
D
1994
M
J
S
D
M
1995
J
S
1996
Figure 3. Interannualvariationof the equatorialundercurrent in the model.Zonal velocityat the 0ø,80øEis shownwith color shadingat an intervalof 20 cm/s. Eastwardand westwardflows are shownin gradesof red and blue respectively.Interannual variationof the depthof the warmpool (definedby 28øCisotherm)anddepthof the thermocline(definedby 20øCisotherm)at 0ø,95øE are also shown.
5.0øN
0.0 ø
.tO C• C•
m •
)October, 1994
--
5.0os
.
,, 5.0øN
5.0øS
60øE
80øE
100øE
60øE
80øE
100øE
Figure 4. a). Time-longitudesectionof seasurfaceheightanomalyfrom TOPEX/POSEIDONaveragedfrom 2øS-2øN.A 5pointHanningsmoothingin time is appliedto the 10-daydatabeforeplotting.b) Horizontaldistributionof the SSH during October,1993 in the equatorialIndianOcean.c). Sameas in b) but for October,1994. In all panelscontourintervalis 5cm and positiveanomaliesare shownin red andnegativeanomaliesare shownin blue.Landareasare shownin green.
1616
VINAYACHANDRAN
ET AL.: INDIAN OCEAN RESPONSE TO THE 1994 WIND EVENT
Normally the warm pool in the eastem equatorialIndian Ocean [Vinayachandranand $heyte, 1991] deepensto about 80 m during May-June and November-December,which is due to the warm water pile-up by the equatorialjets [Wyrtki, 1973]. During 1994 the isothermsstayedat a shallowerdepth than their normal position (Figure 3). The warm pool in particularwas thinnerby about 10-20m. This is clearly due to the reducedwarm water build-up consequentto the weak jets and upwelling causedby strongalongshorewinds along the coastof Indonesia.ObservedSST (Figure 1) and heat storage in the upper ocean(0-400m) show positiveanomaliesin the westem equatorialIndian Ocean and negativeanomaliesin the east during 1994 [http://jedac.ucsd.edu; Meyers 1996] which is consistent with our results.
Further
evidence
for the event
in
1994
can be seen in
suggest thattheNichollsdipole[Nicholls,1989],whichseems to be independent of the SouthernOscillationmay be related to this uniquephenomena.
Acknowledgements. Thanksto Dr. P. Y. Le Traonfor providing the altimeterdata.The altimeterproduceshavebeenproducedby the CLS SpaceOceanography Divisionasa partof theEnvironment and ClimateAGORA EC project(ENV4-CT9560113).Figures2-4 were generated usingFERRET. References Hellerman,S., and M. Rosenstein,Normal monthlywind stressover the world ocean with error estimates.J. Phys. Oceanogr., 13, 1093-1104, 1983.
Knox, R. A., On a long seriesof measurements of Indian Ocean equatorial currents nearAdduAtoll.,Deep-SeaRes.,23, 211-221,
satellite-derivedsea surfaceheight (SSH) anomaly data. We 1976. used SSH anomaliesfrom TOPEX/POSEIDON at 10-day intervals on a 0.5øX0.5ø horizontal grid [Le Traon et al., Legler,D. M., I. M. Navon,andJ. J. O'Brien,Objectiveanalysisof pseudo-stress overthe IndianOceanusinga direct-minimization 1998]. In a normal year, the SSH anomaly in the eastem approach, Mon. WeatherRev.,117,709-720, 1989. Indian Ocean is positive during spring and fall (Figure 4a) Le Traon, P. Y., and F. Ogor, ERS-1/2 orbit improvementusing with a dropin sealevel duringsummer,whereasit is negative TOPEX/POSEIDON: The 2cm challenge,J. Geophys.Res., 103, 8045-8057, 1998. duringwinter.Suchpattemswere seenduring1992-1993and McCreary, J.P., W. Han,D. Shankar,andS. R. Shetye,Dynamicsof 1995-1996. However, in 1994, a large negative anomaly the eastIndia coastalcurrent2. Numericalsolutions.J. Geophys. persistedin the eastern(eastof about85øE) equatorialIndian Res., 101, 13,993-14,010, 1996. Ocean (Figure 4b,c). As discussed above, the unusual Meyers,G, Variationof Indonesianthroughflowand the E1 Nifio Southern Oscillation, J. Geophys. Res.,101, 12,255-12,263,1996. weaknessof the Wyrtki jet led to a lack of the usualpiling up and Australianwinter rainfall, of water. The maximum south of the equatorindicatesthat Nicholls,N., Sea surfacetemperatures J. Climate, 2, 965-973, 1989. upwelling causedby the strongalongshorewinds along the O'Brien, J. J. and H. E. Hurlburt,Equatorialjet in the Indian Ocean: coastof Indonesiais alsoimportantin this process. Theory,Science,184, 1075-1077, 1974. 4. Discussion
Using an oceanmodel and availableobservations,we have described the dynamic responseof the equatorial Indian Oceanto the wind anomaliesduring 1994. Suchanomaliesare
Pacanowski,R., MOM2 Version 2.0(Beta): Documentation,user's guide and referencemanual,GFDL Ocean TechnicalReport 3.2, 329pp, 1996. Potemra, J. T., and R. Lukas, Seasonal to interannual modes of sea level variability in the westernPacific and easternIndian Oceans,
Geophys.Res.Lett., 26, 365-368, 1999. Reppin,J., F. A. Schott,J. Fischer,andD. Quadfasel,Equatorialcurrentsand transportsin the uppercentralIndian Ocean:Annualcyfailed to causenormalWyrtki jets and reversedthe subsurface cle andinterannualvariability,J. Geophys.Res.,(accepted),1999. flow. The thermal structurein the equatorial Indian Ocean, Reynolds, R. W., and T. M. Smith, Improved global sea surface particularlythe zonal contrastof the SST was muchaffected. temperatureanalysisusing optimuminterpolation,J. Climate, 7, not well
documented
so far.
The
unusual
wind
anomalies
The anomaliesalongthe coastof Indonesiawere enhancedby upwelling favorable alongshorewinds. Potemra and Lukas [ 1999] found thatnegativesealevel anomaliesoccurredalong the coast of Indonesiaduring 1994 and 1996 and positive anomaliesduring 1993 and 1995; which they attributedto the changes in alongshore winds. EOFs of SST also show strongestanomaliesalong the coast of Indonesia(R. Lukas, personalcommunication).Thus, the large scale winds over the Indian Ocean appearto have two anomalypattemsthat affect the dynamicsof the equatorial Indian Ocean in a significant manner: first is the easterly anomaly which is related to weakeningof the Walker circulationand secondis the strongupwellingfavorablealongshoreanomalywhich is more or less related to the strengtheningof the Hadley circulationover the eastempart of the Indian Ocean. Reppin et al. [ 1999] arguesthat the observedcurrentanomaliesin the equatorialIndian Ocean are well correlatedwith the ENSO. It is intriguingthat the easterlywind anomalies[Legler et al., 1989] were weaker and short lived during 1982 and 1997 when the E1 Nifio was stronger.The extremely anomalous atmospheric and oceanic states during 1994 despite the moderateE1 Nifio suggestthe existenceof a unique oceanatmosphere-landcoupled systemin the Indian Ocean. We
929-948, 1994.
Taft, B. A., and J. A. Knauss,The equatorialundercurrentof the Indian ocean as observedby the Lusiad expedition. Bulletin of ScrippsInstitutionof Oceanography, 9, 163pp,1967. Vinayachandran,P. N., and T. Yamagata,Monsoonresponseof the sea
around
Sri
Lanka:
Generation
of
thermal
domes
and
anticyclonicvortices,J. Phys.Oceanogr.,28, 1946-1960,1998. VinayachandranP. N., Y. Masumoto,T. Mikawa and T. Yamagata, Intrusion of the southwestmonsooncurrent into the Bay of Bengal,J. Geophys.Res.,(accepted),1999. VinayachandranP. N., andS. R. Shetye,The warm pool in the Indian Ocean, Proc. Indian Acad. Sci. (Earth & Planetary Sci.), 100, 165-175, 1991.
Wyrtki, K., An equatorialjet in the IndianOcean.Science.181,262264, 1973.
Yamagata,T., K. Mizuno, and Y. Masumoto,Seasonalvariationsin the equatorial Indian Ocean and their impact on the Lombok throughflow,J. Geophys.Res.,101, 12,465-12,473,1996. P. N. Vinayachandranand N.H. Saji, Frontier ResearchSystem for Global Change,Seavans-N,7F, 1-2-1 Shibaura,Tokyo 105-6795, Japan.(e-mail:
[email protected],
[email protected]. or.jp) T. Yamagata, Department of Earth and Planetary Physics, GraduateSchoolof Science,Universityof Tokyo, Tokyo 113-0033, Japan.(e-mail:
[email protected]. ac.jp) (ReceivedDecember14, 1998;revisedFebruary18, 1999; acceptedFebruary23, 1999)
