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The Flow Field of the Subtropical Gyre of the South Indian Ocean

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The Flow Field of the Subtropical Gyre of the South Indian Ocean JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102,NO. C3, PAGES 5513-5530,MARCH 15, 1997 The flow field of the subtropical gyre of the South Indian Ocean L. Stramma and J. R. E. Lutjeharms1 Institut fiir Meereskunde an der Universit/it Kiel, Kiel, Germany Abstract. The mean state of the transport field of the subtropicalgyre of the South Indian Ocean has been derived for the upper 1000 m from selectedhistorical hydrographicdata. The subtropicalgyre in the southwesternIndian Oceanis stronger than the flow in the other two oceansof the southern hemisphere. Most of the water in the SouthIndian gyre recirculatesin the westernand centralparts of the basin. In the upper 1000 m the eastward transport of the South Indian Ocean Current starts with 60 Sv in the regionsoutheast of SouthAfrica. Betweenthe longitudesof 40ø and 50øE about 20 Sv of the 60 Sv recirculatesin a southwestIndian subgyre. Another major diversion northward occursbetween 60 ø and 70øE. At 90øE the remaining 20 Sv of the eastwardflow splits up, 10 Sv going north to join the westwardflow and only 10 Sv continuingin a northeastwarddirection to movenorthward near Australia. Near Australia, there is indicationof the polewardflowing Leeuwin Current with a transport of 5 Sv. In the central tropical Indian Ocean between 10øSand 20øS, about 15 $v flows to the west. The western boundary current of this subtropical gyre consistsof the AgulhasCurrent along the east coastof southernAfrica. Its mean flow is composedof 25 $v from east of Madagascarand 35 $v from recirculationin the southwestIndian subgyresouth of Madagascar,with only 5 $v being contributed from the MozambiqueChannel. A net southwardtransport of 10 $v resultsfor the upper 1000 m of the South Indian Ocean. In contrastto the triangular shapeof the subtropicalgym in the SouthAtlantic, probablycaused by the cross-equatorial flow into the North Atlantic, the area influencedby the subtropicalgym in the South Indian Ocean is more rectangular. 1. Introduction boundary regions of the South Indian that are to some extent adequately sampled. Due to a paucity of hydrographicdata the gyral circu- This is particularly detrimental to an understanding lation of the South Indian Ocean has always been con- of the dynamic processes of the region since it is be- siderably less well understood than that of comparable coming increasingly apparent that the wind-driven, an- basins such as the South Atlantic. The International In- ticyclonic circulation of the South Indian Ocean is con- dian Ocean Expedition of the early 1960sto some extent siderably different than, for instance, that of the South alleviated the contemporary undersampling,but its ob- Atlantic [Peterson and •tramma, 1991; Reid, 1994]. servational program was, notwithstanding pleas to the Reasonet al. [1996] have, for example, found evid- contrary [W•'st, 1960],concentrated in the north. In- ence in model results that modulations of the Indonesian creasedresearch interest in the Somali Current [e.g., throughflow can impact significantly on interdecadal $chottand Fiend, 1985],the AgulhasCurrent [e.g., Gor- variability in the South Indian Ocean. don et al., 1987; Lutjeharms, 1987], and the Leeuwin The platesin the atlas by Wyrtki [1971]that portray Current [e.g., Smith et al., 1991]over the past decade the dynamic topography of the Indian Ocean show two has further contributed to a geographicskewing of the persistent features for the South Indian: a large, basin- hydrographic data base. Consequently,it is only the wide circulation and a well-developed subgyre west of the MadagascarRidge (for bottomtopography, see Fig- ure l a). He has constructedmaps for 2-calendar-month 1Now at Department of Oceanography,University of Cape periods, in this way reducing the number of data avail- Town, Rondebosch, South Africa. able for each portrayal. A number of other circulation cells, of small dimensions, are also evident in Wyrtki's Copyright 1997 by the American GeophysicalUnion. [1971] maps, but most are seen in only one 2-month Paper number 96JC03455. period and are therefore probably artifacts of the data 0148-0227/97/ 96JC-03455 $ 09.00 distribution. It is perhaps of passing interest to note 5513 5514 STRAMMA AND LUTJEHARMS' SUBTROPICAL GYRE OF SOUTH INDIAN OCEAN :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ..•. S:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ';.•.:::":: Basin. k; • .'• , ::::::::::::::::::::::::::::::::::::i:i::: ::i:i:•.•i:i:: • • rolist•Plateau 40• •Plate•u /• • I • 20 • 40 • 60 ø 80 • 100 • E 120 • 10 ø s 20 ø 30 ø 40 ø 20 ø 40 ø 60 ø 80 ø 1000 E 120 ø Figure 1. The SouthIndian Ocean with (a) the mainfeatures of the bottomtopography repres- entedby the smoothed4000-m isobath (AI indicatesthe AmsterdamIsland) and (b) the sections (lines)as listedin Table 1 and positions(dots) of hydrographicdata usedfor this investigation. that marinebiologists [Heydom et al., 1978]have hypo- The data used in the abovestudies of the large-scale thesized three main circulations in the South Indian in gyral circulation [Wyrtki, 1971; Harris, 1972; Lutje- order to accomodatethe movementof e.g., turtle species harms, 1976] have considerableoverlap so that these in this ocean. One is the full width of the South Indian results cannot be consideredto be entirely independ- with a circulation rate of years, one is a southwestIn- ent. Furthermore, the existenceof intense anticyclonic dian Ocean subgyreand the last is an intermediateone eddies in the center of the southwestIndian subgyre somewhere in between. [Gr•'ndlingh,1989; Gr•'ndlingh et al., 1991]creates a bi- From the atlasplates by Wyrtki [1971]it is clearthat asingproblem in the data. If suchan eddy were sampled the circulation of the basin has a distinct pivot in the oncein the widely spaceddata of the historicaldatabase, southwesterncorner, i.e., in the Agulhas Basin. This but not spatiallyresolved, it might havelead to the por- has subsequentlybeen confirmed by the work of Harris trayal of a falsely intensifiedsubgyre recirculation. [1972]and Lutjeharms[1976] (Figure 2a). The depth The existenceof well-developedsubgyres within the of the 10øC isotherm in this southwest Indian Ocean generalbasin-wide gyre of the South Indian circulation subgyreis about 850 m, comparedto 600 m for the ana- has a number of wider implications. One of these con- logouslocation in the SouthAtlantic gyre [Wiist, 1978], cerns the sourcesof the Agulhas Current. Early por- suggestingthe comparatively increasedintensity of the trayals of the currents that feed the Agulhas Current circulation about this point in the South Indian Ocean. are basedfor the greaterpart on measurementsof ships' STRAMMA AND LUTJEHARMS- SUBTROPICAL GYRE OF SOUTH INDIAN OCEAN 5515 •0 ø E 40 ø E 50 ø E 60' E 70 ø E o , , •./, ":i:.,a)'"',/:%/•'.. .....• ......,... ................... ::*,':. I0 ø $ ':' : •0' $ '..:??:' '!..! .• i. • . •0 ø $ ß .' ';. •// % x\ ' . l .,o-•.-x.•--: ,,/,- I • I•' x \ I ' 40' $ 10 o 20 ø 30 ø (• 1;•øø 40 ø 20 ø E 40 ø 60 ø 80 ø 100 ø Figure 2. (a) The anticyclonicsubgyre of the SouthIndian circulationin the AgulhasBasin relativeto the depth of the rre : 26.8 surfaceduring the northeastmonsoon season [after Lutjcharms,1976]. (b) The circulationof the SouthIndian subtropicalgyre extractedfrom a map of global ocean dynamictopography based on sea surfaceheight measurementsfrom the TOPEX/POSEIDON for October1992 3 to 12, [afterFu aad Christsasea,1993]. drift and in somecases sea surfacetemperatures [e.g., from a large measureof recirculation,thus allowing the MSller, 1929;Paech, 1926; Michaelis, 1923]. They show concept of a strongly developedrecirculation cell in the a simple and direct inflow into the northern Agulhas gyral circulationof the SouthIndian Oceanto gain addi- Current from the South Equatorial Current, one via tional importance. A rough estimateof the volumecon- the MozambiqueChannel, called the MozambiqueCur- tributionsto the AgulhasCurrent [Harris, 1972]from rent, the other aroundthe southerntip of Madagascar, hydrographicdata has suggestedthat about 35 Sv (1 calledthe East MadagascarCurrent. The conceptof the Sverdrup= 106m a s-•) comesfrom east of Madagascar Mozambique Current as an upstream extensionof the (not necessarilyvia the EastMadagascar Current), 10 AgulhasCurrent seemsto be wrong [Saetreaad Jorge Sv only comesthrough the MozambiqueChannel, and da Silva, 1984], the circulationin the channelin fact 27 Sv is recycledin a southwestIndian Oceansubgyre. consistingof a series of recirculation cells instead. Fur- Fu [1986],using an inversemethod solution for six hy- thermore, the surfaceconnection between the East Mad- drographicsections in the Indian Ocean, has indicated a agascarCurrent and the Agulhas Current doesnot exist southwardtransport of only 6 Sv throughthe Mozambi- either, except perhaps in a sporadic way by filaments que Channel. Gordon[1986] has suggested that the wa- andeddies [Lutjeharms et al., 1981;Lutjeharms, 1988]. ter mass compositionof the Agulhas Current indicates If thesetwo sourcesare not asimportant aspreviously that the water on the inshoreedge of the current does thought, the water of the Agulhas Current must come originatein the MozambiqueChannel. The largedegree 5516 STRAMMA AND LUTJEHARMS: SUBTROPICAL GYRE OF SOUTH INDIAN OCEAN of variabilityin the throughflow[Lutjeharms, 1972] sug- 0.4 to 0.6 mL/L. We correctedthe zonalRRS Discovery gestswhy these estimatesmay vary considerably. oxygensection at 32øSby an offsetof 0.5 mL/L. This The recirculation of the subtropicalgyre
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