JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. C5, PAGES 10,391-10,422, MAY 15, 1997 Seasonal variations of thermocline circulation and ventilation in the Indian Ocean Yuzhu You Center for Climate SystemResearch, University of Tokyo, Tokyo Laboratoired'Oc•anographie Physique, Museum National d'Histoire Naturelie, Paris Abstract. Two seasonalhydrographic data sets, including temperature, salinity, dissolved oxygen,and nutrients, are used in a mixingmodel which combines cluster analysis with optimum multiparameteranalysis to determinethe spreadingand mixing of the thermoclinewaters in the Indian Ocean.The mixingmodel comprises a systemof four majorsource water masses, which were identifiedin the thermoclinethrough cluster analysis. They are IndianCentral Water (ICW), North IndianCentral Water (NICW) interpretedas agedICW, AustralasianMediterranean Water (AAMW), andRed SeaWater (RSW)/PersianGulf Water (PGW). The mixingratios of these watermasses are quantifiedand mapped on four isopycnalsurfaces which spanthe thermocline from 150 to 600 m in the northernIndian Ocean, on two meridionalsections along 60øE and 90øE, andon two zonalsections along 10øS and 6øN. The mixingratios and pathways of the thermocline watermasses show large seasonal variations, particularly in the upper400-500 m of the thermocline.The mostprominent signal of seasonalvariation occurs in the Somali Current,the westernboundary current, which appears only duringthe SW (summer)monsoon. The northward spreadingof ICW intothe equatorialand northern Indian Ocean is by way of the SomaliCurrent centeredat 300-400 m on the c•0=26.7isopycnal surface during the summermonsoon and of the EquatorialCountercurrent during the NE (winter) monsoon.More ICW carriedinto the northern IndianOcean during the summermonsoon is seenclearly in the zonalsection along 6øN. NICW spreadssouthward through the westernIndian Ocean and is strongerduring the wintermonsoon. AAMW appearsin bothseasons but is slightlystronger during the summerin the upper thermocline.The westward flow of AAMW is by wayof theSouth Equatorial Current and slightly bendsto the northon the c•0=26.7isopycnal surface during the summermonsoon, indicative of its contributionto thewestern boundary current. Outflow of RSW/PGWseems effectively blocked by the continuationof strongnorthward jet of the SomaliCurrent along the westernArabian Sea duringthe summer,giving a rathersmall contribution of only up to 20% in the ArabianSea. A schematicsummer and winter thermocline circulation emerges from this study. Both hydrography andwater- massmixing ratios suggest that the contributionof the waterfrom the SouthIndian Oceanand from the Indo-Pacificthroughflow controls the circulationand ventilationin the westernboundary region during the summer.However, during the winterthe wateris carriedinto theeastern boundary by theEquatorial Countercurrent and leaks into the eastern Bay of Bengal, fromwhere the water is advectedinto the northwestern Indian Ocean by theNorth Equatorial Current.The so-calledEast Madagascar Current as a southwardflow occursonly during the summer,as is suggestedby bothhydrography and water-mass mixing patterns from this paper. Duringthe winter (austral summer) the current seems reversal to a northwardflow alongeast of Madagascar,somewhat symmetrical to the SomaliCurrent in the north. 1. Introduction Indian Ocean can reach as deep as 500 m, while Wyrtki [1973] suggestedthat impact of the southwestmonsoon is noticeableas The main thermoclinelayer of the oceanis in generaltreated as deep as 1000 m at the Somali Current region. Tchernia [1980] a layer that is not subjectto seasonalvariability, becauseit is proposedan even deeper effect of seasonalvariation on water situatedwell below the seasonalsurface layer. Betweenthese two massproperties down to 2000 m. Probablythe most profound layersthere is a densitytransition which Defant [ 1961] calledthe effect of seasonal monsoons is on the Somali Current, which "barrierlayer." However, since climatology of the IndianOcean is appearsonly duringthe summer.The greatdifferences among the dominatedby two distinct reversal monsoons,the southwest(or estimatedtransports from the Pacific to the Indian Ocean, ranging summer) monsoon and the northeast (or winter) monsoon, from 1.7 Sv [Wyrtki, 1961] to 18 Sv [Cox, 1975, after Gordon, seasonalvariations throughout the main thermoclineof the Indian 1986]. The latter also suggeststhat strongseasonal variation must Ocean are significantand cannotbe ignored. Colborn [1975] play a role in thermoclinestructures. found that monsoonal effect on thermal structure of the northern Several factors, including distinct seasonalmonsoons and the effects of a complicated geography, the northern Indian Ocean Copyright1997 by the AmericanGeophysical Union. being blockedby the Eurasianlandmass, the lack of a Subtropical ConvergenceZone, and an interoceanicthroughflow including its Papernumber 96JC03600. distinct hydrological front, lead to the formation of a unique 0148-0227/97/96JC-03600509.00 thermoclinewater- massstructure in the Indian Ocean.Sverdrup 10,391 10,392 YOU: INDIAN OCEAN SEASONAL THERMOCLINE CIRCULATION et al. [1942] identified three water massesand discussedtheir basiclarge-scale structure of the thermoclinein the Indian Ocean formation mechanisms in the main thermocline of the Indian north of 40øS are listed in Table 1, which also includes a Ocean: (1) Red Sea Water (RSW), formed in the Red Sea; (2) compendium of the same water masseswith different names Indian Central Water (ICW, also called SubtropicalSubsurface accordingto variousauthors. A relatively recentgeneralized Water by Warren et al. [1966] and SubantarcticMode Water by IndianOcean temperature - salinity(T-S) diagramof Emeryand McCartney [1977]) formed in latitudes 40ø-45øSduring late Meincke [1986] is shownin Figure 1. There is only one water winter by convectiveoverturning [Wyrtki, 1973; Colborn, 1975]; massin Table 1 thatwe havenot mentionedyet, the NorthIndian and (3) Indian Equatorial Water (IEW), formed in the western Central Water (NICW), which will be discussedlater. equatorial region through some unspecified mechanism. A closer examination of these water masses, based on their Marhayer [1975] added two more water massesto the Indian regionsof formation,suggests that the main thermoclineof the Ocean north of 40øS:Bengal Bay Water (BBW), which is only IndianOcean north of 40øSis actuallydominated by only three found in the surface layer, and Timor Sea Water (called source water masses, ICW, AAMW, and RSW/PGW. $harma Australasian Mediterranean Water (AAMW) by You and [1976], Quadfasel and Schott [1982], and You and Tomczak Tornczak[1993]), which originates in the deep basins of the [1993] have arguedthat the IEW of Sverdrupet al. [1942] is Indonesianarchipelago. The latter was missedby Sverdrupet al. actuallya mixtureof water massesfrom the northernand southern [1942] becausethey lacked data for the easternIndian Ocean Indian Ocean and from the Pacific. The BBW definedby [Colborn, 1975]. In a recentreview, Emery and Meincke [1986] Marhayer [1975] is the resultof excessprecipitation and river included anotherwater mass,Arabian Sea Water (ASW), which is runoff over evaporationresulting in a low salinity(between 32 influenced by outflow of Persian Gulf Water (PGW) and is and 33 psu). This low salinity causesit to be isolatedfrom the distinguishedfrom RSW. These water masseswhich form the water in the main thermoclineby a sharphalocline located Table 1. An Inventory of Water Mass Definitionsin the Thermoclineof the Indian Ocean Water Mass Study T-S Characteristics Notes T, øC S, psu Indian Central Water (ICW) a Sverdrupet aI. [1942] 7.00- 16.00 34.70- 35.70 Mamayev [ 1975] 16.00 35.60 Emery and Meincke [1986] 8.00- 25.00 34.60- 35.80 Warren et aI. [ 1966] 7.00- 15.00 34.70- 35.50 McCartney [ 1977] Formedby late winter convectiveoverturning at 40ø-45øS Australasian Mediterranean You and Tomczak[1993] 5.55- 14.89 34.52- 34.77 Water (AAMW) b Mamayev [1975] 25.00 34.50 Emery and Meincke [1986] 8.00- 23.00 34.40- 35.00 Wyrtki [1961] After Warren [ 1981] Rochford[ 1966] North Indian Central Water Gordon [1986] Defined as thermocline (NICW) water below the surface water at the Bay of Bengal. Youand Tomczak[1993] 7.80- 15.72 34.84- 35.10 The same as Gordon but defined as aged ICW Red Sea Water (RSW) Sverdrupet aI. [1942] >7.00 >35.00 Definedas oxygenminimum core. Mamayev [ 1975] 23.00 40.00 Defined as intermediate water. Bengal Bay Water (BBW) Mamayev [1975] 25.00 33.80 Defined as surface low- salinity water. Emery and Meincke [ 1986] 25.00- 29.00 28.00- 35.00 Defined as surface low- salinity water. Arabian Sea Water (ASW) Emery and Meincke [1986] 24.00- 30.00 35.50- 36.80 Indian EquatorialWater Sverdrupet aI. [1942] 5.00- 16.00 34.90- 35.20 (IEW) Mamayev [1975] 25.00 35.30 Emery and Meincke [1986] 8.00- 23.00 34.60- 35.00 a. CalledSouth Indian Subtropical Water by Mamayev[ 1975],South Indian Central Water by Emeryand Meincke[ 1986], SubantarcticMode Waterby McCartney[1977], and SubtropicalSubsurface Water by Warrenet aI. [1966]. b. CalledTimor Sea Water by Mamayev[1975], Banda Intermediate Water by Rochford[1966], Banda Sea Water by Wyrtki [1961], and IndonesianUpper Water by Emery and Meincke [1986]. YOU: INDIAN OCEAN SEASONAL THERMOCLINE CIRCULATION 10,393 BBW defined as an independentsource water massbecause it lacks a ' ' /• ' ! ' / commonformation process. They found that when they did not ..,./ include NICW in their mixing model they derived a totally /I _-?'- different water