GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 22, PAGES 2773-2776, NOVEMBER 15, 1997 Seasonal circulation in the South Indian Ocean Amy Ffield Lamont-DohertyEarthObservatory,Palisades, New York John Toole WoodsHole Oceanographic Institution,WoodsHole, Massachusetts Doug Wilson NOAA AtlanticOceanographic andMeteorolog.ical Laboratory, Miami, Florida Abstract. Two World Ocean Circulation Experiment hydrographic cruisesin MarchandJune1995,alongwith TopexPoseidonaltimeterdataandNationalMeteorologicalCenterwind data, estimatedan averageAgulhasCurrent volume transportin the upper 1000 m of 65 Sv. Effortsto estimatethe seasonalityof the AgulhasCurrenthaveproducedconflictingresults[Pearce dataareusedto estimate seasonal changes in theSouthIndian and Grfindlingh, 1982] in part due to different criteria and Ocean subtropicalgyre. Mean annual curvesderived from the altimeterand wind data reveal strengthening of the anticyclonic gyre in March and September, and weakening in June and December. The seasonalchangescorrespond to variationsin the wind field south of 30 øS at the equinoxesand solstices. In addition,the wind-drivengyre is furthernorthin July, andfurther southin March. These variationsin strengthand locationof the SouthIndian Ocean gyre may influenceinter-oceantransports south of Africa. Despite the inferred mean annual seasonal variationsin the South Indian Ocean gyre, volume transports methods. As part of the World OceanCirculationExperiment,high quality,highresolutionhydrographic surveysof theIndianOcean wereobtained in 1995with repetition of certainsections to capture the variability. In March 1995 the NOAA Sh!p MALCOLM BALDRIGEobtained"repeat" hyd,rographic data acrossthezonalI5W section,nominallyalong33 øS,crossing the AgulhasCurrentsouthof Durbanandcontinuingeastwardto 55 øE (Figure1). In June1995,theRV Knorroccupied the same sectionobtaining"one-time"hydrographic coverageof the I5W estimatedin 1995 from the hydrographic dataare closeto mean line. In this'letter,the two I5W cruises alongwith Topexvalues. Apparently, a mesoscaleeddy in March disruptsthe PoseidonaltimeterdataandNationalMeteorological Centerwind strongerfall gyre, whereasin June the weaker winter gyre is dataareusedto estimateseasonal changes in thesubtropic•al gyre delayedby 1 month. of the South Indian Ocean. Circulation Inferred From Hydrographic Data Introduction Wind analysesfrom the EuropeanCentre for Medium Range WeatherForecasts(ECMWF) reveal the anticyclonicforcing in the South Indian Ocean. The annual mean wind stress is westward between 10 and 35 øS, and eastward between 35 and 60 Two potential temperature sectionsacrossthe southwestern Indian Oceanreveal downwardslopingisothermsfrom the coast of SouthAfrica to 32 øE associatedwith the strongsouthward flowing AgulhasCurrent(Figure2). Eastof 32 øE the isotherms generallyslopeupwardsin June indicatingpossiblenorthward øS [Trenberthet al., 1990]. The anticyclonicforcingdrivesthe interior flow into the Indian Ocean. However in March the subtropicalgyre of the SouthIndian Oceandefinedby the strong Agulhas westernboundarycurrentflowing southnear the coast 25' 30' 35' 40' 45' 50' 55' 60' of Africa, and relatively weak northward interior flow east to I I I I I I I I ! I I I I I I I I I 1 .... ii:•.-'..:5!•:.-2-. !•i•: ß .:i•:.--"• Australia[Toole and Warren, 1993]. Away from the equatorthe ß..:.... -.:•:.--.:: -.:::s•:: ß ====================== I RMS :.::...::,:,•.... ß ............ . ......... ,, .............. .1 -25' cud of wind stressis usedin the Sverdruprelationto describethe -.>-•:" • .-.-:.:-'..:•:•:-'.:•5:•5•:•:ß:•:•B•:.-':.•:::•5".-:•:•-':'-•:•:B•:• I cm ................................................................. 1 ( ) :-":" • ................................... depth-averaged,wind-driven, ocean circulation. The 1980 to ! 986 ECMWF windsforce an annualmeanSverdruptransportin theSouthIndianOceanreaching 100Sv(1 Sv= 1 x 106m3/s)at the tip of Africa [Trenberth et al., 1990]. The integrated Sverdrup transportvalue is assumedto be closed by a western boundarycurrent,in this casethe AgulhasCurrent. At the southerntip of Africa, Gordonet al. [ 1987] estimated the baroclinicAgulhasCurrent transportrelative to the seafloor 25' 30' 35' 40' 45' 50' 55' 60' to be 95 Sv, closeto the amountneededto balancethe Sverdrup circulation. Much of this flow is locatedin the upper ocean. Figure 1. Root-mean-squared seasurfaceheightanomalies in Strammaand Lutjeharms[1997], usinghistoricalhydrographic the southwest IndianOceansmoothed by a 5ø by 5ø gaussian filter. The contourinterval is 6 cm. CTD (circles)and XBT (crosses)stationlocationsfor the March 1995 cruiseare shown, Copyright 1997bytheAmerican Geophysical Union. alongwitha subset of thecriss-crossing tracksOfthealtimeter data. (CTD station locations for the June 1995 cruise are at both circle and crosslocations.) Papernumber97GL01253. 0094-8534/97/97GL-01253505.00 2773 2774 FFIELD ET AL.: SEASONAL CIRCULATION IN THE SOUTH INDIAN OCEAN b) June 1995 a) March 1995 .0 • ' .... ' ' • ............ .' • .... ! .... I .... transportsare integratedfor the upper 1000 m (only near the Madagasca.r Ridgeat 44 øE doesthe bottomtopography rise I slightlyabove 1000 m). ?•::•i[ '"'"'""•'••••:..... .•••• ,, 18 The cumulativegeostropic transports (Figure3) quantifythe 100 :• !••Jl•l'• -•i.---:.- 2OO 200 300 300 400 4OO 500 500 6OO - 600 700 700 ' ........ I .... 35 I .... 40 45 I .... I .... 35 50 I .... 40 I .... 45 I 50 .flow patternsimplied by the potentialtemperaturesections (Figure2). In JunetheAgulhasCurrentflowsswiftlysouthward with61 Sv in theupper1000m, similarto thehistorical average valueof 64 Sv reportedby StrammaandLutjeharms[1997]. In March the eddy obscuresthe offshoreedge of the Agulhas Current.Integrated to 32 øE - thewesternedgeof thecyclonic eddy- the cumulative.transport is 7 Sv lessin March(54 Sv) than in June (61 Sv). However, when integratedto 36.7 øE - the easternedgeof thecycloniceddy- thecumulative transport is 3 Sv morein March (60 Sv) thanin June(57 Sv). In the interior, between 32 and50 øE,theflowis weakwith0 Svnettransport in March,and4. Sv net northwardtransport in June. Thesenet interiorflows are not significantlydifferentfromeachother. Longitude(o East) Figure 2. Potentialtemperaturesectionsnominallyalong 33 øS for the a) March 1995 and b) June 1995 cruises. The contour interval is 1 øC. CTD and XBT data are usedin a), and CTD data are'used in b). a) MeanAnnualAverage 0.00 15 situationis more complicatedwith the isothermsbelow 200 db revealing a cyclonic mesoscaleeddy centeredat 34.5 øE, and possiblenorthwardthen southwardflows centeredat 42 øE. Both thesefeaturesare centeredover shallow 1000 m topography:the MozambiqueRidge at 35 øE and the MadagascarRidge at 44 øE. Cyclonic eddies have been previously observedin this region [Griindlinghet al., 1991; Griindlingh, 1988]. To compare volume transports across the zonal sections, geostrophic velocities are estimated. Rather than attempt to determine the absolute transport across the sections, the calculationsare performedin a way to insuregoodcomparisons between the different occupations. In this way baroclinic transportscan be directly comparedto eachother,and errorsdue to bottom topography,stationspacing,and small velocitiesare reduced: 1) the integrationstartsslightly offshoreAfrica where the bottom depth is greater than 1000 m, 2) only stations occupiedduringbothcruisesare included,3) a level of no motion at 2000 dbarsor the deepestcommondepthis selected,and 4) the • 0.25 0.50 0.75 1.00 10 E v • o I <t: -5 U• -10 -15 95.00 30 95.25 b) 1995 95.50 95.75 96.00 20 0 o -20 -30 -40 -60 e---•---30 35 '-' Figure 4. Sea surfaceheightanomaly(SSHA) differenceacross the "Agulhas"(31.1 øS, 30.4 øE - 32.2 øS, 32.5 øE, .solidline), "• +'"-June1995 40 45 50 LongitudeEast (o) Figure3. Cumulative geostrophic transport in theupper1000m usinga subsetof the March 1995 (opencircles)and June1995 (soliddiamonds) stationdatanominallyalong33 øS. "Indian" (32.2 øS, 32.5 øE - 33.2 øS, 112.4 øE, thick dashedline), and "SouthwestIndian" (32.2 øS; 32.5 øE - 33.2 øS, 50.0 øE, thin dashedline). In a) the mean annual average SSHA differences are smoothedby a 6 monthgaussianfilter, with starsmarkingthe solsticesand equinoxes,and in b) the 1995 SSHA differencesare smoothedby a 2 monthgaussianfilter with the occurrences of the March(opencircle)andJune(soliddiamond) cruises lndicated. FFIELD ET AL.' SEASONAL CIRCULATION IN THE SOUTH INDIAN OCEAN However, the energetic +/- 20 Sv flows centered over the MozambiqueRidge at 35 øE andthe MadagascarRidge at 44 øE in March contrastwith the +/- 5 Sv amplitudeflows observedin June. Circulation Inferred From Altimeter Data Fully processedTopex/Poseidonsea level heightdata were obtained from the NASA Ocean Altimeter Pathfinder Data Projectfor the period,October1992 throughApril 1996. The data are averagedto 0.2 degreesspatialresolutionalong each 2775 clearly reveal a seasonalcirculation in the South Indian Ocean (Figure zt). On Figure 4 negative values imply anomalous southward flow (or reduced northward flow if the mean flow is larger and northward) and positive values imply anomalous northward flow (or reduced southward flow if the mean flow is larger and southward). The seasonalcirculation is maximum with stronger southward "Agulhas" and stronger northward "Indian"flows impliedduringMarch and September(starson Figure 4a). In other words a strongergyre. In contrastduring June and December, the oppositesituationis observed: weaker southward "Agulhas" andweaker northward "Indian" flows.The satellitetrack and the threeyear, 1993 to 1996, meansubtracted. same geasonalityis found in the mean-annualcurve constructed Consequently, the contributionof the geoidandthe meanocean for the "Southwest Indian". This strengtheningand weakeningof the SouthIndian Ocean circulationto the sealevel heightvaluesis effectivelyremoved. The remaining data are time series of sea surface height gyre may be driven by seasonalvariationsin wind, becausethe correspondto equinoxes(March and September), anomalies(SSHA) at approximately10 day intervals. The root- strengthenings mean-square(RMS) of the SSHA data illustratesthe high and weakeningscorrespondto solstices(June and December). variability(> 12 cm)of thesouthwestern IndianOceanassociated The monthlymeanpositionof the centerof the subtropicalhigh withtheAgulhasReturnCurrent,theeastern edgeof theAgulhas reaches maximum southerly positions in March (35 øS) and September (32 øS),andmaximumnortherlypositions in June(30 Current,andnumerousmesoscaleeddies(Figure 1). (30 øS)[VanLoon,1971,Fig.7]. At 90 øE, The slopeof the seasurface- whichcanbe calculatedusing Os)andNovember SSHA data - is proportionalto geostrophicflow at the sea 50 øS, the annualcycle of mean zonal wind stressalso reveals in March(3.5dyncm-2)andSeptember (3.7dyncmsurface.Gcopotential topography at theseasurface,500 m, and maximums in May (2.6dyncm-2)andDecember (1.9dyn 1000m all revealan anticyclonicgyrein the SouthIndianOcean 2), andminimums etal., 1989,Fig.23]. Onereport of geostrophic westof 80 øE [Wyrtki,1971,plates388, 389, 390]. Thissuggests cm-2)[Trenberth thatcirculationrevealedby SSHA dataat the seasurfacemirrors volume transport of the Agulhas Current relative to 1000 m deepercirculationpatterns. In orderto directlycomparethe suggeststhis seasonality:23 Sv in January-February,38 Sv in SSHA datato theresultsof thetwo hydrographic sections,SSHA April, 32 Sv in July-August-September, 27 Sv in Octoberdataintersecting the sectionsare examined.The largestRMS November. However, when the geostrophic velocities are variability along the gectionsis at 32.5 øE, which is also referenced to observed surface currents the seasonality [Griindlingh, 1980,averages compiled fromhisFig. approximatelythe transitionpoint betweenthe southward disappeared flowing AgulhasCurrentand the eventualnorthwardinterior 6]. Despite the mean annual seasonality, at the times of the flow to the east. To investigatethe variability of the current flows, the differencesin SSHA acrossthreelongitudeintervals hydrographic cruises in March and June, 1995, the SSHA areestimated:the "Agulhas" (31.1øS,30.• øE to 32.2 øS,32.5 differencesare all within 3 cm of the mean (Figure4b). In March of thegyreappears disrupted by øE),the "Southwest Indian"(32.2øS,32.5 øEto 33.2øS,50.0 øE; 1995theexpectedstrengthening eastern edgeof thehydrographic sections), andthe"Indian"(32.2 the mesoscaleeddy crossingthe section, whose circular SSHA øS,32.5 øE to 33.2 øS, 11•.• øE,easternedgeof the SouthIndian depressionsignaturecan be tracedfrom March 9 to April 20. In Ocean). Mean annual curves constructed for the SSHA differences a) Average Sverdrup Transport 0.00 75 '•' 70 between 0.25 34 and 45 øS 0,50 0.75 1.00 June 1995 the expected weakening of the gyre seems to be delayed 1 month. b) Latitude of Maximum Transport 0.00 36 0.25 0.50 0,75 1.00 38 o n. 65 i-.. 60 E o 55 Figure 5. Ina)themean annual curve oftheaverage Sverdrup transport between 34and45øSalong 32.5øEisdrawn fortheSouth IndianOceangyre,with starsmarkingthe solstices andequinoxes.In b) themeanannualcurveof the latitudeof maximumSverdrup transportalong 32.5 øE is drawn, along with the 1995 curve showingthe occurrences of the March (open circle) and June (solid diamond) cruises.Thedataaresmoothed bya 6 monthgaussian filter,andthemeans areindicated by dashed lines. 2776 FFIELD ET AL.: SEASONAL CIRCULATION Circulation Inferred From Wind Data The data-assimilating, operational, T-62 model wind field from the National Meteorological Center is used with a drag coefficient of 1.4, to compute the Sverdruprelation at -1.5 degree spatial resolution and weekly averaged temporal resolution. The Sverdrup circulation is estimatedevery week from 1991 through 1995, by integrating volume transports calculated from the Sverdrup relation westward from the Australian and New Zealand's coasts between 20 and 50 øS. The AgulhasReturnCurrentis locatedbetween34 and45 øS, and feeds the northward interior circulation of the South Indian gyre. The meanannualcurveof the averageSverdruptransport between 34 and 45 øS, at 32.5 øE is constructed for the South indian gyre (Figure5a). The averageAgulhasReturnCurrentis estimated to be 65 Sv. Seasonally, the wind-driven Agulhas Return Current follows the same semi-annual seasonality revealedby the altimeterdata. For example,in boththe altimeter data and the wind data, March (70 Sv) and September(70 Sv) correspondto maximumgyres,and June(55 Sv) and December (65 Sv) to minimum gyres (stars on Figures 4a and 5a). The centerof the subtropicalanticycloniccirculationalsovarieswith season(Figure5b). The latitudeof maximumSverdruptransport along32.5 øE is furthestnorthin July (38 øS) andfurthestsouth in March (43 øS). This meridional movement of the gyre is partiallyresolvedby historicalhydrographic sections crossing30 øS, 65 øE, that show a shift in the circulation southwardbetween January and March, and northward during June and July [Strammaand Lutjeharms,1997]. In March 1995 the position of the wind-driven SouthIndian Ocean gyre is anomalously north relative to the mean annual curve (Figure 5b). Whereas, in June 1995 the northward movementof the gyre appearsdelayedone month. Discussion and Conclusions Altimeter and wind data usedto estimateseasonalchangesin the anticyclonic subtropical gyre of the South Indian Ocean reveal a strongergyre in March and September,and a weaker gyre in June and December. The Sverdrup transport of the Agulhas Return Current suggeststhat seasonalvariationsrange between +5 to -10 Sv about the 65 Sv mean. In addition, the meridionalpositionof the gyre is furthestnorthin July (38 øS), and furthest south (43 øS) in March. The meridional movement and strength of the wind-driven gyre may have important implications for inter-oceantransports,by possiblyinfluencing the water mass sources drawn into the South Indian Ocean subtropicalgyre and by modulating volume transportssouthof Africa. OCEAN in the southwestIndian Ocean. For example, even thoughthe integratedtransportvalues acrossthe hydrographicsectionsare similar in March and June, energetic +/- 20 Sv flows were observedin March centeredover the MozambiqueRidge at 35 øE and the Madagascar Ridge at 44 øE. No notable transport features were seen there in June. Theresponse of thesouthwestern Indian Ocean toseasonal variation in wind forci.ng is clearly complicatedby the major ridge systemsof this basinthat attaindepthsshallowerthan 1000 m. Onlybarotropic wavemotions propagate sufficientiy fastto transmitinformation about seasonallyvarying wind stressacross mid-latitude oceans. Baroclinic waves are too slow. Yet, barotropic motions can be reflected and/or scattered into baroclinic modes at tall bathymetry. Better understandingof theseprocessesmay help explain the high SSH variability of the southwesternIndian Ocean, and shed light on the seasonal transport field of the Agulhas Current and its associated interbasinexchanges. Acknowledgments. This work was supportedby the National Oceanic and Atmospheric Administration grant NA66GP0267. The Lamont-DohertyEarth Observatorycontributionnumberis 5648. References Gordon,A. L., J. R. E. Lutjeharms,and M. L. Griindlingh,Stratification and circulationat the AgulhasRetroflection,Deep-SeaRes., 34, 565599, 1987. Griindlingh, M. L., On the volume transport of the Agulhas Current, Deep-SeaRes., 27, 557-563, 1980. Griindlingh,M. L., Review of cycloniceddiesof the MozambiqueRidge Current,S. Aft'. J. Mar. Sci., 6, 193-206, 1988. Griindlingh, M. L., R. A. Carter, and R. C. Stanton,Circulation and water properties of the southwest Indian Ocean, Spring 1987 Prog. Oceanog.,28,305-342, 1991. Pearce,A. F., and M. L. Griindlingh, Is there a seasonalvariationin the AgulhasCurrent?,J. Mar. Res.,40, 177-184, 1982. Stramma,L., andJ. R. E. Lutjeharms, The flow field of the subtropical gyre of the SouthIndian Ocean,J. Geophys.Res., 102, 5513-5530, 1997. Toole, J. M., and B. A. Warren, A hydrographicsectionacrossthe subtropicalSouthIndian Ocean,Deep-SeaRes.,40, 1973-2019,1993. Trenberth,K. E., W. G. Large,andJ. G. Olson,The meanannualcyclein globaloceanwind stress,J. Phys.Oceanogr.,20, 1742-1760,1990. Trenberth,K. E., J. G. Olson,and W. G. Large, A globaloceanwind stressclimatology based on ECMWF analyses,NCAR Tech. Note, NCAR/FN-338+STR, 93 pp., Nat. Center for Atmos. Res., Boulder, CO, 1989. Van Loon, H., A half-yearly variationof the circumpolarsurfacedrift in the SouthernHemisphere,Tellus,23, 511-516, 1971. Wyrtki, K., Oceanographicatlas of the International Indian Ocean Expedition,531 pp., Nat. Sci. Found.,Washington,D.C., 1971. A. Ffield, Lamont-DohertyEarth Observatory,Palisades,NY 10964- Despite the inferred seasonal changes, baroclinic volume transportsestimatedfrom the March and June1995 hydrographic data are close to the historical IN THE SOUTH INDIAN mean. In March 1995 a mesoscale eddy disruptsthe expectedstrongerfall anticycloniccirculation and the South Indian gyre movesunusuallyfar north during the same time interval. In June 1995, the expectedweaker winter anticyclonic circulation and northwardmovementof the gyre is delayed by 1 month. These results mask the far more complicatedreality indicatedby the large sea level height RMS 8000. (email: affield@ldeo.columbia.edu) J. M. Toole, Mail Stop 21, Departmentof PhysicalOceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02543. (email:jtoole@whoi.edu) W. D. Wilson, NOAA Atlantic Oceanographicand Meteorological Laboratory, 4301 RickenbackerCauseway,Miami, FL 33149. (email: wilson@aoml .noaa.gov) (Received March13, 1997;accepted April 18, 1997.)
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