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Three Agulhas Rings Observed During the Benguela Current Experiment Silvia L

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Three Agulhas Rings Observed During the Benguela Current Experiment Silvia L JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. C9, PAGES 20,971-20,985, SEPTEMBER 15, 1999 Three Agulhas rings observed during the Benguela Current Experiment Silvia L. Garzoli,• Philip L. Richardson,2 Christopher M. Duncombe Rae,3 David M. Fratantoni, 2 Gustavo J. Gofii, • and Andreas J. Roubicek • Abstract. A field program to study the circulation of the Benguela Current and its extensioninto the southeasternAtlantic Ocean has completedthe surveyand instrument deploymentphase. We report here new observationsof three Agulhasrings north and west of Cape Town, South Africa. Three mesoscaleanticyclonic rings initially identified by meansof TOPEX/POSEIDON altimetry were surveyedwith expendable bathythermographs,conductivity-temperature-depth-oxygen profiles, direct current measurementsfrom a lowered acousticDoppler current profiler, a hull-mountedacoustic Doppler current profiler, and satellite-trackedsurface drifters. Characteristicsof the rings are presented,and their originsare discussed.Two are typical Agulhasrings surveyedat different times after their generation;the third Agulhas ring has an anomalouswater mass structurewhose most likely origin is the SubtropicalFront. 1. Introduction from the main current. The fate of Agulhas rings and their importanceto northward transport of intermediatewater are The South Atlantic Ocean is a major conduit for warm important issues. upper layer water that flows northward acrossthe equator in Several studiesprovide estimatesof the volume and heat compensationfor the coldersouthward flowing North Atlantic transportcarried by these rings from the Indian Ocean to the Deep Water. Gordon et al. [1992] concludedthat the major Atlantic Ocean. On the basis of hydrographicdata, Gordon proportionof deepwater exportedfrom the North Atlantic is [1985] and Gordon et al. [1987] suggesteda net transfer be- balancedby northward flow of lower thermocline and inter- tween5 to 15Sv (1 Sv= 106 m 3 s-•). Gordonand Haxby [1990] mediate water through the South Atlantic Ocean. This large- used hydrographicdata to estimate the transport of Indian scale thermohaline circulation, the so-called conveyor belt Ocean Water by Agulhas rings to be between 10 and 15 Sv. [Broecker,1991], is responsiblefor the northward heat flux Byrne et al. [1995] used maps of surface height anomalies throughthe SouthAtlantic. Within the subtropicalregion the derived from Geosat data to track six Agulhas rings per year northwardheat flux estimatesrange from 0.5-1.3 PW [Peterson and estimated a total value of at least 5 Sv of Indian Ocean and Stramma, 1991; Saundersand King, 1995]. Water entering the South Atlantic via those rings. van Balle- The Benguela Current is a broad, northward flow adjacent gooyenet al. [1994] observeda total of 14 anticyclonicrings to southwestern Africa that forms the eastern limb of the generatedby the Agulhas Current over the 2-year period of SouthAtlantic SubtropicalGyre. At 30øSthe entire current is their study. They estimated a volume flux of Indian Ocean confinedbetween the African coastand the Walvis Ridge near Water warmer than 10øCinto the SouthAtlantic via the Agul- the Greenwich meridian. The Benguela Current has two pri- has eddy field of 6.3 Sv and a correspondingheat flux of 0.045 mary sourcesof water, the South Atlantic Current and Indian PW. Ocean Water that rounds the southern tip of Africa via the From 1992 to 1994 the Benguela Sourcesand Transports AgulhasCurrent [Gordonet al., 1992]. Experiment (BEST) [Garzoli and Gordon, 1996] studiedthe Indian Ocean Water enters the South Atlantic by both a flow of the easternboundary of the SouthAtlantic Subtropical ring-sheddingprocess at the western end of the Agulhas Ret- Gyre. One of the main resultsof that programwas the confir- roflection and by intermittent streams or plumes of Indian mation that transients,mostly in the form of Agulhasrings, are Ocean Water. Ship and satellite data have revealed the pres- a major componentof the Benguela Current. Observations ence of numerouslarge (300- to 400-km diameter) rings of revealedthat at 30øSthe BenguelaCurrent transportconsisted AgulhasWater embeddedin the BenguelaCurrent [i.e., Olson of a nearly steadyflow confinedbetween the South African and Evans, 1986; Lutjeharmsand van Ballegooyen,1988; Gor- coastand approximately4øE (amounting to 10 Svin the mean) don and Haxby, 1990;Duncombe Rae, 1991]. These ringsform and a more transientflow (3 Sv in the mean) between4øE and as the western part of the Agulhas Retroflection pinches off the Walvis Ridge [Garzoliand Gordon,1996]. Duncombe Rae et al. [1996] concludedthat a minimumof four to six rings of •AtlanticOceanographic and MeteorologicalLaboratory, NOAA, Agulhasorigin entered the Cape Basinper year during BEST Miami, Florida. and were responsiblefor the transferof 0.007 PW and 2.6-3.8 2WoodsHole OceanographicInstitution, Woods Hole, Massachu- Sv betweenthe two basins.Gofii et al. [1997] usedBEST data setts. and TOPEX/POSEIDON-derived maps of upper layer thick- 3SeaFisheries Research Institute, Cape Town, South Africa. nessto estimatethat five,four, and sixrings were shedfrom the Copyright 1999 by the American GeophysicalUnion. Agulhas Current during 1993, 1994, and 1995, respectively, Paper number 1999JC900060. with an averageof 1 Sv contributedby each to the volume 0148-0227/99/1999JC900060509.00 transportof the BenguelaCurrent. Arhan et al. [this issue] 20,971 -10' -fi" 0 ø G' 1l¾ 1R' -15'" ='=" "==' --" ==" '=== =='= ===' --" '"' "==' === ........... .• ß .•, , •;..•....... ..•..,•............. ........... ........... •..• .......................... : ............,...•.... -20'I" --•.6•?,.4:•--" • DRIFTERS ' f--_'%;.;{.Y_'., "::'• t.. •,.,,. '"':'......."•;•.:.:•. J 20' ' ß,x;•:•'.......... ' J •,•,•' '>"MOORINGS ,/'"' ?.,•',"."t ' ' '5'-, \ ",N,", '"•;•?!..'%1.. .;?.. • _ ,, ,?,o ; i t, 7 .: .•.1. .'i:'"'.....•'"'- • •i' t -,.•",t . .. N,,,•'; •, •::• •.il•... I ....•':•.. ' •,.,/'•,,,• I •,•, •¾ ' .:...;..,. ,-'.,/T I \'•",. l'k I :.•.:...•" :;::::j.-- / , t :•t! • : '?•....... [ "• --• •** ..... •:.•,•'..•..:.•:.•<-'•-•-'.%,. .:t•..... I ...o •.;,..:" ..•} "• • kt•? t • •,' •"-..,':, \ ...........•..:. :..: ". - -. ' ..:,, •-'•t ,.-•,.• I ""-'k .... ß ½-, ,• '- .... •• ,, .•:.x':.'-'..]/*;';t.,,•.Z••'•' - J . i• Cape',,-Town "•..'..... -,,-, ! o _LL.•, ' • '' •• - ,:, "" .40ø•:• .......•':•-'•__i:•;.•/•: • ..,•':.":;•..::• • .: • • -• •'•: -..• .......• ........ •.;.40 ø -10' - 5....... 5 .... I I- '........ 115" ..... 20-- Figure 1. Cruise track and location of the stationsoccupied during the R/V SewardJohnson cruise, Sep- tember 4-30, 1997. The RAFOS and ALFOS floatswill drift for 2 years.Three Agulhas ringswere found and surveyednear 31.0øS,9.4øE; 31.5øS, 2.3øW; and 29.8øS,6.1øW. ....•..• .•....•-:;.'-; ' -.-.. 30oS •. ..• :.. ....•.• • • .% •.•.;;--;• "•. ..... • ..... ;'•v• .• ..... e .....ß ß • .... •.-,, •. .-.. f ..:'•, "-•?"• 'RING• .•."Ex• 3 • '•;"•,•x © . L. & '..E".:•; •-'•' • •.•:•-• ½• '• •T•'-;•% •" •a.'• ' {•' •-•". •- • • '•2 ' '• .. .:• • ...... ,•/,'- -•x..... .• x•. >-• ...• 10øW 0 ø 10øE 20øE 30øE Figure 2. Pathsof rings 1, 2, and 3 as revealedfrom the upper layer thicknessmaps derived from altimetry. Open circlesshow the mean location of rings each month. The altimetry-derivedring locationsduring the cruiseare shownwith solid circles,and thosefor November 1997 are shownwith solid squares.The spacing betweenmonthly open circlesis smallerfor rings2 and 3 near the Walvis Ridge, implyinga slowerring speed. Topographiccontours are drawn every 1000 m. GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT 20 973 (m) 720 600 480 360 240 ß 12o 10øW 5øW 0 ø 5øE 10øE 15øE 20øE Plate 1. Locationsof the three rings as depictedby the altimeter upper layer thicknessmap corresponding to September16, 1997.Also shownare the cruisetracks (blue line), hydrography-deriveddepths (meters) of the 10øCisotherm (red contours),and bathymetry(black contours). describethree Agulhasrings observed on World Ocean Circu- surfacedrifters were launchedin the ringsin order to measure lationExperiment (WOCE) hydrographicsections and tracked their movement and evolution. This data set is one of the most the rings with altimetry back to their formation. Their rings completeever obtainedof Agulhasrings. were inferred to be formed from the retroflection in March 1994 (ring 1) and May 1993 (rings 2 and 3). Estimatedages were 11 months(ring 1) and 21 months(rings 2 and 3). 2. September 1997 Cruise The presentBenguela Current Experimentaims to quantify The R/V SewardJohnson departed Cape Town, South Af- the flow of intermediate water carried into the South Atlantic rica, on September4, 1997, and arrived in Recife, Brazil, on by the Benguela-Agulhassystem and to track its extensionto September30, 1997 (Figure 1). The main fieldworkconsisted the northwestwith an array of 32 subsurfaceRAFOS floats of 44 CTDO-LADCP stationsto 2000 m depth or the bottom deployednear a depth of 700 m. This experimentis part of an alongthe nominallines 30øS (15øE-7øW) and 7øW (33øS-18øS). internationalcooperation among the United States,Germany, The locationsof the stationsare indicated in Figure 1. XBT and SouthAfrica calledKAPEX (Cape of Good Hope Exper- surveyswere carried out to identify and map three Agulhas iment) [Boebelet al., 1998],whose main focusis the exchange rings encountered.The XBT surveyswere guided by maps of and advectionof water at intermediatedepths (700-1000 m) the depth of the 10øC isotherm derived from TOPEX/ around southern Africa. During KAPEX, more than 100 POSEIDON
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