Pmg. Oce4nol'. VoL 14, pp. 353-385, 1985. 0079-66il/8S $0.00 + .sO Printed in Great Britain. AU ~ts reerved. Copyright e 1985 hzalmon-Preu Ltd. A Model of the SeasonalCirculation in the Arabian Sea Forced by Observed Winds MARK E. LUTHER and JAMES J. O'BRIEN Me.,~ale Air-Sea Interaction Group, The Florida State Univemry, TalJahal.fee,Florida 32306, au. Abstract - Re.hs of a numericalmodel of the wind driven seasonalcirculation in the Arabian Seaare presented,with partjcular emphasison the ocean'sresponse to the mon~on winds. The model equationsare the fully nonlinearreduced gravity transportequations in spherical coordinates.The model re~lution is 1/8° in the east-westdirection and 1/4° in the north- south direction. The model basin geometrycorresponds as clo~ly as possibleto that of the north-west Indian Oceanfrom 400E to 7JoE and from 100Sto 25°N, and includesthe gulfs of Aden and Oman,and the islandsof Socotraand the Seychelles.The southernboundary and a portion of the easternboundary, from the equatorto 60S,representing the openingbetween the Maldivesand the ChagosArchipelago, are open boundaries.At other boundaries,the no- slip condition is applied.The wind stressdata used to force the modelcomes from the NOAA NationalClimate Center'sTD-9757 Global Marine Sumsdata, which consistsof monthly mean winds compiled on 1° squaresfrom over 60 yearsof ship ob~rvations. Thesedata are inter- polatedin time using the meanand Iust lIVe Fourier harmonicsat eachpoint, and then inter- polatedlinearly to the model grid. The model equationsare integratedin time using centered finite differencesin time and space(a leap-frogscheme), with lateral friction treated by a Dufort-Frankel scheme. After a one year spin up, the model settlesinto a regular periodic seasonalcycle, even though the ~lution to the model equationsis locally highly nonlinear,with largenonlinear eddiesdeveklping in the samelocation at the sametime of year from one year to the next. The developmentof the model SomaliCurrent systemwith the onset of the (northern hemi- sphere)summer mon~on is consistentwith the avajIableob~rvations in the region.The model reproducesmany of the observedfeatures in this region, such as the two-gyre circulation pattern, and the timing and movementof thesefeatures corresponds well with their real world counterparts.The model also showsan eastwardjet forming in late Juneto early July at 1JoN, just to the east of Socotra. This jet is fed by flow comingout of the great whirl. The break down of the two..gyrepattern occursin mid to late August.when the southerngyre breaksup into severalsmaner eddies, the northem-mostof which coalesceswith the greatwhirl. Numer- ous small cycklnic eddiesdevelop aklng the Arabian coast, from the Gulf of Omaninto the Gulf of Aden, in early to mid August,and persistwell into the winter monsoon.The model shows that it is possibleto simulatevery complicatedflows, if one has sufficient wind data, usingfairly simplemodels with a realisticbasin geometry. 1. INTRODUCTION THE NORTHWESTIndian Ocean,the Arabian Sea,is unique amongthe world's oceansin that the winds over the ocean basin reversesemiannually, blowing from the southwest during the northern SUJJUDer,and from the northeast during the northern winter (the southwest and northeast monsoons).The periodic reversalsin the winds drive correspondingreversals in the currents of the upper ocean.These seasonalreversals of the-wmd and surfacecirCulation over such a vast area are remarkablewhen comparedto the seasonalsignals in the Atlantic or Pacific Oceans.The Somali Current has surfacevelocities and masstransports that exceedthose of the Gulf Stream but that change direction every six months. There are numerous eddies in the Arabian and Somali basins that recur on a seasonalbasis. The Indian Oceanis thus an ideal place in which to study the time-dependentnature of the responseof the ocean to changing winds. 353 354 M. E. LUTHER and,], J. O'BRIEN The currents in the Arabian Seabasins evolve rapidly into a very complexpattern of eddies with the onset of the southwestmQnsoon. Associated with theseeddies are very strong hori- zontal temperaturegradients and current shears.BRUCE (1973, 1979, 1983) and BROWN, BRUCE and EVANS (1980) have descn'bed.theeddies associatedwith the Somali Current systemin some detail. Recent attention has focused on the two gyre nature of the Somali Current onset. SWALLOWand FIEUX (1982) have reviewedthe availablehistorical daU and found that the two gyre circulation pattern, with one clockwisegyre situatedsouth of SoNand another between SoN and lOoN, is usually present in June. It is the northern gyre that was dubbed the 'great whirl' by FINDLAY (1866). More recent observations,especially those during the 1979 Indian OceanExperiment, have revealedmore details of the development of the two gyre system. SWALLOW, MOLINARI, BRUCE, BROWN and EVANS (1983) havedescribed the reversalof' the Somali Current with onsetof the southwestmonsoon during 1979 uSingtemperature, salinity and current measurements.These observations are ~arized inSCHOTT(1983). , Very little attention has been paid to the northeastmonSOon, from Decemberto January, primarily becauseit is so much weakerthan the southwestmonsoon and thereforeless dramatic.. BRUCE (1983) describesthe long term monthly meanwind stresspatterns obtained from ship observations,for both monsoons,and relatesthese to the observedocean circulation patterns. Thesesame wind data for the Indian Oceanare describedin an atlasby HASTENRATH and LAMB (1979). The numerous eddies in the northern Arabian Sea have been observedby CAGLE and WHRITNER (1981) using satellite infrared imagery(Fig. 1). They find that the eddiesappear to be controlled both by the wind forcing and by topographic features associatedwith the coastlinegeometry. The eddies persistfor long periods of time.and appearto strengthenand weakenwithout significant advection. We have developed a wind-driven numerical model to investjgate the seasonalcycle of currents in the Arabian Sea. The nonlinear model includes a realistic baSingeometry and is forced by observedmean monthly winds on a high resolution mesh in sphericalcoordinates. There are several earlier models of the Indian Ocean.COX (1970) developeda multi-layer model that was driven by idealizedseasonal winds and temperature.The model was integrated for two centuriesto arrive at a steadyannual signal. The slow responsetime of the very deepest layers in the model necessitatedthe very long integration. HURLBURT and THOMPSON (1976) were the first to demonstratethe ~y nonlinearnature of the eddiesthat form in the Somali Current during the onset of the southwestmonsoon. COX (1976) found that local forcing by the wind dominatesduring the onset of the monsoon,but that remote forcing by equatorially trapped wavesmay alter the flow at later times. COX (1979) found that the move- ment of the eddies was strongly influenced by the basin geometry. UN and HURLBURT (1981) showed that the spin-up of the Somali Current system could be effectively modeled using only the fIrst baroclinic mode. They also demonstratedthat the temporal and spatial distribution of the wind stressis crucial in determining the location and movement of the eddiesin the Somali Current. ANDERSONand MOORE(1979) consideredthe Somali Current as a free inertial cross-equatorialjet, forced by the southernhemisphere trades and fed by the South Equatorial Current (SEC).and the EastAfrican CoastalCurrent (EACC). All previousmodels have consideredonly the responseof the Indian Oceanto very idealized wind forcing, consideringprimarily the responseof the westernboundary region to the sudden onset of the southwest monsoon. No models to date have looked at the eddiesfound in the northern Arabian Sea.The model we presenthere examinesthe responseof the Arabian Seato A modelof' the ~naI ckcuJationin the ArabianSea 155 FIG. 1. Interpretation of infrared Imageof the coastal resion south of the Gulf of Oman. Strong fronts are indicated by ~Iid lines, weak fronts by dashedlines. A strong&w is indi- cated, ~clated with a major feature which propagatesfrom ~e upwellingoff the south tip of the Island of Madrah. A cokt cycbnk: eddy is iodated to the north of this tounge-Uke feature.A sequenceof warm and cokl eddiesIa indicatedin progrellbn down the coastof the Arabian Peninsula,and major features~clated with points of land are repeatedalong the coast. Image was taken on 26 October 1980 by the NOAA-6 satellite. From CAGLE and WHRITNER (1981). act.ualobserved winds over the entire seasonalcycle. Other models of the onset of the Somali Current have consideredonly the spin-up of an oceanwhich is initially at rest in responseto an mpulsively applied, constant wind stress.As we will show, the put ~agJn's circulation influences die onset of both the (northern hemisphere)RImmer and winter Somali Current, so that to accuratelymodel either, we must model the entire seasonalcycle. We pre~nt here the results of one particular model simulation. and compare theseremIts with recent ob~rvations. We focus our attention on the westernboundary region,particularly the Somali Current regime, becausethat is where the most prominent variations occur and where the great majority of ob~rvations are made. This is not to say that currents in other areasof the Arabian Sea are not important, but the ob~rvations there are very lhnited. We prefer to concentrate here on a de-=ription of the model and to emphasizethe agreement between the model reRllts and observations.We will leave a detailed analy.sisof the