551.465.11 : 551.513 (267)

A numerical modelling study of the interannual l'ariabilityin the Indian Ocean

S. K. BEHERA and P. S. SALVEKARIndian Institute of Tropical Meteorology, Pune

(Received 9 Ma rch 1994, Modified 15 November 1994)

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ABSTRACT. A simp le reduced .ravity wind-driven ocean circulation model i. used to stud y theIntennnual variability in the upper layer of the Indian Ocean (24-5-2rN and JS-&Il's -E). The mon thlymean wind stress for me period Im·l986 are used as. forcin& in the model. The model reproduces most ofthe observed CeatureJ of the ann ual qcle of the upper layer cimd.t~n in the Indi an Oceea when wu for­ced with me ten-year aweraae monthly mean wind. The cirwlation feature••nd the model upp er layer th ick­ness Ibow considerable interannual variabil ity in most part of the buin; in particular, the Somali Curre nt,the bu in wide so uthern hem isphere I)ft. th e Equatorial Currenu and the I)T'eS in the &y of Denial . Sixconllocuth'e )'I:an atartina from 1978 to 1983 which include two bad mOn300n yr:an of 1979 and 1982 arec:hoaen to stud y the in teran nual variability. February cirwlalion field .hoWi stronger Equatori al CounterCUrrenu in bad mcnscon yean., Mlereu. the currents north of Madapscar nowina up to th e African coaltare found to be . ronae!' in aood montoon yean. The IOUthward return now from the Southern Gyre inAullJa it drone an d more to IlOUth em 1.tituda hi the bad monsoon yean. The flow circulated eas tward toform another eddy euI of Sou them G)1"e. The buin ";de on: of the southern hem isphere (SH) shows lessvariability in two CODJl:eutiw normal yean than in c:onlrUl ina yean.

K.r, woNi - Reduced I n vity, Tnnsport, Indian Ocean. CiraJ)ation. Intera nnual variability.

t, Introduction

Following the work of Cox (1970), in the last twodecade. there have been cons iderable mathemati­cal modelling work Cor the Indian Ocean. Q .

Luther et aL (1985), Luth er lit. 0' Brien (1989),McCn:ary lit. Kundu (1988), Das et 01. (1987) andJensen (1990). COI (1970) used a thn:e dimeD5ionaloceanic general circulation model with se""n le""lsin ""mcal din:ction, when:as, most oC the othe r"Orken used layer models with assumption oC oneor mon: active layen over an infinitely deep motion­leu layer. These layer models with leu vemcaln:solution and les" prognostic equations n:quiren:latively small computationa l n:sources compan:dto oceanic ll"neral circulation models . Thesemod'e" do not include prognostic thermohaline

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equatio ns, bu t due to their computational efficiencythey can be used Cor several numerical experimentsto study their sensitivity to various forcings, boun­dary co:tditions and geometries. These models canalso be modified to include additional physics, e.g.•Zebiac and Cane (1987) added a mixed layeroC con­stan t depth with thermodynamics on the top oC theupper layer and coupled the ocean model 10 a sim­ple almosphen: mode l (Gill 1980) to study IheENSO phenomenon.

Luther et 01. (1985) using a one and half layermode l with two difTen:nl wind Corcings simulatedn:alistically the Somali Current and the associa tedgyn: system, Luther lit. O· Brien (1989) using 23-yearmean monthly wind da ta '(Cadet and Diehl 1984)showed the interannual variability in Somali