MauMlnl. (1996), 47. I. :n ·3U

$51.461.2 : 551.553.21 (267)

Upper ocean responses in the cent ral western equatorial Indian Oce anduring southwest monsoon season

M. G . JOSEP H. P. V. I1AR E ESII KUMAR a nd P. MAO HUSOODANANNaval Physical and Oceonogl'Gl'h ir Laboratory Thrikkak ara. Cocliin

(Received /9 October /99• . Modified 2/ April /995)

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ABSTRACT. L"PJ'(T OCt-Oll1 ell.' tn) respo nse u nde r the pre-un set. onset and active rcghucs o f south west(SW) monsoona l forcing at n°N. ()(Jo E in IhI' Ind ian Oce an W3'i ana lysed. uliHsilli' tim e series d ata (01l('i.1('.1duri ng Indo-Sovi et Monsoon E.'(perill1~nt.1973 (ISMEX-73). Oceanic response under th e pn>.otuhl'l domina­lio n o f the wind stress mome ntum a nti onset domin urion ofbuoynucy flux ([1,,1 was apparent in shoa liua:1warming and deepenin g/cooling (12 m/U.!i8C in 4 Jap,1of'M ixed Laycr Depth (MlD). The pl"'-un~1 increasewas follo wed b)' au onset decrease in belo..... layer th enuohufine/d enshy l:rnJil'l1l ani' di sappcaruucc of Suh ­lJUl"flice Salin ity Maximum (SSMI, COfresponJin~l)', MlD and in h ral con le n t U ICJ,ll O) Wl'le mon: co r­relnh~'ll 10 l\ a nd Os ' U'r rwr OCI":ln rt"SPOI1~ Jurin¥ nClh-e 1"C8ime I11nnifl"slt'l1 in .Jl"l' pening!cuoli ng elf)mIl 8Cin 6 J11)"S ) of MlD u nd('r do milHtOl Pflli..tu,; lilln o f turb ulel11kin l,tk ent> rJ'y by ~inJ slress e-XCl"pl (or Ihr con­\'l""'1 in'ly d ominanl mixinwat the l:lI..'lIinniui a nd end. With redud ion in bl"lo.....-la)"Cf thennoh aline/Jensit),, nhlie nt a nd absence lI( SSM. Ihr ",'olTelmi lllU bl'lwt"l'n MLO. Ilg, ~in&1 SI I~Ss. Qs a nJ HCUl D be CHIUt'in~illn iJic n.nt d ue to incl\'ased aJ\'e"'1h>tflux during a ct in." regime . One Jimen~ion(llllil11ulalion o( mi.'t'J 10)'l' rpnra mclel'S shO\wd L"'s son ah le 0eret"lUrnl.

Key word, - InJi fll1 Oce an. $tnl1hWC5t lUunliOOll, Wind Sll\"S5, Il eal 1111~ , Mi., t"d la)'er, fl ral nllltl' nt.Com'latiun s, Simulat ion .

,

I. Introduction

In the north Indian Oc ea n. monsoona l infl uen­ces are do mina nt in imparti ng significan t changesin the thermo haline/density a nd current fields ofIhe upper oce an. Consequently. the uppe r ocea nvariability differs with the intens ity a nd extent ofthe forcings by the southwest a nd nort heast mon­soons in different areas . Howeve r. in the westernnortb Indian Oeoan and the Arab ian Sea.sou thwest (SW) mo nsoon plays th e promin en t rolein the transfer of mass. momentum a nd hea tthrough the aiNea interface. This. in tu rn effectsthe drastic cha nges in the upper ocea n ther­mohaline a nd advective fields during M ay-Sep­tember. as has been observed earlie r. Studies o nclim a tological scales (Me Phaden 1982. Molinari

4 -618 IMD/ 95(m

'" al. 1986. She lye 1986. Rao .'1 al. 1(89) suggested 501075% correlatio ns helween lhe surfac e mixcd layerchiuacterislk s a nd the verlical fluxes of energy andmass throug h Ihe naviface in western In di an Ocean,nonh of equator. Some of these a lso indicated thatthe correla tions increased by including la teralfluxes d ue 10 advec tio n and effects of entrainmentand wind stress convergence/dive rgence. Synopticsca le verification of these a rguments for IheArilbh,n Sea and weste rn eq ualoria l InJian Oceanduring soulhwes t monsoon have bee n made bymeans of one dimensiona l simula tions. usingtime series data sets from MONSOON-77 andMONEX-79. ac hieving various degrees of agree­me n ls ancl disagreeme nts (Rao 1986. Rao et al.1990. Joseph I?I al. 1990. Sa nil Kumar et al. 1991 ).However. presence ofsignilicantzonal variability in the