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 studythe 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]

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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.

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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 altimeter data [Go•i et al., 1997]. The altimetry RAFOS floats are measuringocean trajectoriesfor the first measured the anomalousheight field of the surveyedregion time in the Agulhas Current, in Agulhas rings that enter the and indicatedthe approximateposition of the three rings.The SouthAtlantic, and in the BenguelaCurrent and its extension. ringswere identified and carefullymapped by shipboardmea- In addition, conductivity-temperature-depth(CTD), expend- surements.The availabilityof altimetry enabledthe ring survey able bathythermograph(XBT), and velocityprofiles are being componentof the cruiseto be accomplishedvery efficiently. collectedto documentthe water massand velocitystructure of the BenguelaCurrent, its extension,and severalAgulhas rings. In this paper, resultsobtained during Agulhas ring surveys 3. Rings performed on the R/V SewardJohnson in September 1997 Each ring was initially surveyedwith XBTs and the hull- [Roubiceket al., 1998] are presentedand discussed.The data mounted ADCP to provide a measure of size and shape. A consist of XBT surveys, conductivity-temperature-depth- radial section of five CTDO-LADCP stations was then made in oxygen(CTDO) stations,direct currentmeasurements from a a northwesterlydirection from the center to the outer edge of lowered acousticDoppler current profiler (LADCP) and a each ring to study the water masscharacteristics of the core hull-mountedADCP, satellite-trackedsurface drifter trajecto- and the structureof the velocityfield. The main thermoclineof ries, and TOPEX/POSEIDON satellite altimetry measure- each ring showed a 200- to 300-m bowl-shapeddepression. ments. In addition, a total of seven RAFOS floats and six Plate 1 showsthe positionof the three ringsas depictedby the 20,974 GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT

. Ring ø

30øS

32øS

e o

ADCPo25-200 rn Velocity ø ø XBT Depth of 10 ø Isotherm TOPEXbpper Layer Thickness (Sep 07)

o 34øS 8OE 10øE 12øE Plate 2. Ring l. Vectors are acousticDoppler current profiler (ADCP) upper ocean velocity averaged between25 and 200 m alongthe cruisetrack. The upper layer thicknessfrom TOPEX/POSEIDON altimetry is depicted in the colored backgroundimages. Red shadescorrespond to greater thickness,green shadesto lesserthickness. The locationsof the TOPEX/POSEIDON data pointsare indicatedby smallcircles. The 10øC isothermdepth determinedfrom the expendablebathythermograph (XBT) surveysis contouredin white.

altimeter-derivedupper layer thicknessmap correspondingto valuesare 9, 8, and8 cms -• (or 8, 7, and7 km d-I) for rings September16, 1997 (midcruise),and the hydrography-derived 1, 2, and 3, respectively.These translationspeeds fall between depth of the 10øCisotherm. The agreementbetween altimetry previouslyobserved extreme values of 5 and 15km d-• [Gogi and hydrographyin the locationof the ringsis remarkableand et al., 1997]. Figure 2 also showsthat the ringsappear to move is a validation of the method used to derive the upper layer faster when they are over deeper regionsand slowerover the thicknessof the ocean from the sea surfaceheight observed shallowerareas, suchas the Walvis Ridge. with the altimeter [Go•i et al., 1997]. The horizontal structureof the rings, as well as the surface Similar upper layer thicknessmaps were constructedafter velocities,can be seenin Plates2, 3, and 4. Ring 1 (Plate 2) was the cruise,one per day,and usedto track thesethree ringsback significantlyless circular than the other two. We believe this is in time to their origin (see Figure 2). Our analysisreveals that due to its closeproximity to Vema Seamount,which risesup to rings 1, 2, and 3 were shed at the retroflectionregion in Feb- a depth of 12 m near 31ø41'S, 8ø20'E. Rings 2 and 3 were ruary 1997, July 1996, and April 1996, respectively.This sug- relativelyclose to eachother (400 km, two ring diameters)and geststhat their agesat the time of the cruisewere 7, 14, and 17 may have been interactingwith each other at the time they months, respectively.The computed mean translation speed were observed. GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT 20,975

*-1 30øS

32øS

ADCP 2•5-200rn Velocity XBT Depth of 10 ø Isotherm o TOPEXUl•per Layer Thickeness (Sep 14)

o 34øS , , , I i , 4øW 2 øW Plate 3. Same as Plate 2, but for ring 2.

The trajectoriesof satellite-trackedsurface drifters launched corridor" [Garzoli and Gordon, 1996; Gohi et al., 1997], al- near the center of each of the three observedrings are shown though ring 1 was locatedon the northern edge of the corridor. in Figure 3. The ring trajectoriesderived from altimetry are alsoshown for comparison.Three drifterswere launchednear 3.1. Ring 1 the center of ring 1. Two of them were immediately ejected The altimeter data availablejust prior to the cruiseshowed from the ring core, possiblyowing to deformationexperienced that ring 1 was centered near 32øS, 10øE on August 12-19, by ring 1 during its closeencounter with the Vema Seamount. 1997. An XBT survey was conducted to determine the size, The remaining drifter in ring 1 looped for 1 year. This is the shape,and preciselocation of the ring. The surveylocated ring longesttime an Agulhas ring has been tracked with drifters. 1 in the Cape Basin, near Vema Seamountat 31.0øS,9.4øE on The trajectoryin ring 1 showedevidence of a changein trans- September 9, 1997 or about 1000 km from the Agulhas Ret- lation directionand ring shapeas ring 1 approachedthe Walvis roflectionnear 38øS,17øE. A CTDO/LADCP sectionwas per- Ridge. Two drifters launchedin ring 2 looped simultaneously formed along a radial line outwardfrom the ring center. Plate within the ring for about 120 daysbefore they failed. A single 5 showsa crosssection of temperature, salinity, oxygen,and drifter launched in ring 3 was expelled after three or four velocityperpendicular to the sectionalong a radial extending rotationsabout the ring center and subsequentlydrifted west- northwestwardfrom the center of the ring. The depth of the ward in the Benguela Current extension.The trajectoriesfol- 10øCisotherm was 630 m at the ring center,compared to 400 m lowedby the three ringswere in the previouslyidentified "ring outsidethe ring (Figure 4). The diameter of the 10øCisotherm 20,976 GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT

i ',ng

30os -

100 cm/s

32øS

ADCP 25-200 rn Velocity o o XBT Depth of 10o Isotherm TOPEX Upper Layer Thickness (Sep 17) o

t ...... I ...... t i ! ..... o t 8'W 60W Plate 4. Same as Plate 2, but for ring 3.

at 500 m wasapproximately 200 km; the ringwas elliptical with 4 cm s-• at 1000m andthen decreasing to the bottomof the a major axisoriented northwest-southeast. profile (2000 m). Backtrackingwith TOPEX/POSEIDON altimetry suggests In comparisonwith surroundingwater, ring 1 was slightly that ring 1 was shedduring February 1997 and took 7 months warmer and carried a small salinitysurplus (Figure 4). The to reach its surveyedposition. Ring 1 appearsto be a typical entire water column above the Antarctic Intermediate Water, Agulhasring in its first year after shedding,that is to say,with indicatedby the salinityminimum, had an oxygendeficit com- a singlethermostad near the surfaceand a simple,depressed paredto waterbeyond the ring (Plate 5). This is consistentwith centralwater thermocline[Duncombe Rae, 1991].The vertical the characteristicsof the Agulhas Current [Chapmanet al., profile (Plate 5) showedweak vertical gradientsof tempera- 19871. ture, salinity,and oxygenin a near-surfacelayer of 200 m in extentwith characteristicvalues of temperature(16.15øC), sa- 3.2. Ring 2 linity (35.59),and oxygen(5.62 mL L-•). No other major thermostadsoccurred within the water columnof this ring. The Ring 2 was found near 31.5øS,2.3øW on September14, 1997. LADCP velocity profiles (Plate 5) indicate maximum near- This ring had translated1900 km from its presumedformation surfacevelocities larger than 30 cm s-•. The maximumtan- site near 38øS,17øE and had alreadycrossed over the Walvis gentialvelocity observed was 36.7 cm s -•. Themean profile of Ridge at the time of our survey.Altimetry showedthat ring 2 all stationsinside the ring (stations7-12, not shown)shows the had left the retroflectionduring July 1996, 14 monthsprior to tangentialvelocity decreasing from 23 cms- • at thesurface to the survey,which gives a meanvelocity of 7 km d-•. The GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT 20,977

translationspeed appeared to decreaseover the WalvisRidge 25 23 19 Teml•perature½4C) 12 6 3 1 (Figure 2). ø Ring 2 was the largestof the three ringsencountered: The diameter of 10øC at 500 m was approximately350 km. The 10øCisotherm reached 732 m at the center, comparedto a ooo backgrounddepth of around450 m (Figure4 andPlate 6). The ring had two near-surfacemixed layers with similarcharacter- 1500 istics,60 and 100 m in vertical extent, with temperaturesof , , i ,, , , 2000 I I IIII I I I I I I I1•I I I I 16.85øCand 16.65øC,salinities of 35.71 and 35.70, and oxygens .... of 5.40and 5.15 mL L -z, respectively.Between 440 and 580 m I Illil I I i I I II1•1 I I I 2500 I I IIII I I I I I I In• i, I I I I depththere occurreda thermostadat 12.2øCwith salinitybe- -5 0 5 10 tween 37.07 and 35.10. This layer had a high oxygencontent Salinity(psu)

5OO ,, ,, ooo

500

2000 I II III I I I I I I IIIII I I I I I

I II III I I I I I I IIIII I I I I I

... 250O I IIIII I I I I I I •H,I, I I I I -5 0 5 10

Sigma-t

500

1000

1500

2000' '

2500

øW Longitude øE Figure 4. Vertical sectionsof temperature,salinity, and den- sityversus pressure derived from CTD castsalong 30øS (Figure 1). Rings1 and 2 are indicatedby the closelyspaced stations 32øS and the depressionof isothermsnear 9øE and 2øW.

(5.35mL L -z) approachingthat of surfacewaters and was 90% (0.9) saturated.A subsurfacevelocity maximum (station 19) was observednear 800 m, presumablyrelated dynamicallyto the depressionof the thermoclineunderneath the deeperwell- mixedlayer (Plate 6). The meanprofile (stations 20-24) hasa meantangential surface velocity of 16.3cm s-z anda second maximumof 20 cm s-z at 300 m. The velocitydecreases to a value of 4 cm s-z at 1000 m and remains constant to 1500 m similar to the mean tangentialprofile for ring 1. The hydrographicsurvey of ring2 indicatesthat thisring was different from rings 1 and 3 and unlike most previouslyob- servedAgulhas rings. A typicalAgulhas ring has a deep ther- mostadthat is more saline,warmer, and lower in oxygenthan the surroundingwater. In contrast,ring 2 has characteristics Figure 3. Trajectories of satellite-trackeddrifters (solid lines)launched near the centerof surveyedrings (a) 1, (b) 2, betweendensity surfaces 26.5 and 26.7 close to that of the and (c) 3. Overlappedare the trajectoriesof the ringsfrom surroundingwater, relativelycolder, fresher, and more oxygen altimetry(dotted lines).The locationof the deploymentand rich than typicalAgulhas rings. In section5, further analysisof mostrecent position of the driftersare indicatedby solidcir- altimetryand a studyof the water masscharacteristics of ring cles. 2 are presentedto determineits origin. 20,978 GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT

Table 1. Ring Characteristics This could be an indicationof the ring movingas a cylinderof water. Parameter Ring 1 Ring 2 Ring 3

Depth of 10øCisotherm, m from XBT 630 735 625 from altimeter 600 650 600 4. Geometric and Dynamic Ring Parameters Maximumtangential velocity, cm s-• Fundamentalring geometricand dynamicparameters were from ADCP 50 40 35 calculatedfor the threerings (Table 1). The depthof the upper from LADCP 37 23 25 layer was calculatedfrom the hydrographicobservations and Radius of maximum tangential velocity,km from ADCP 100 90 95 the altimeter-derivedsea height anomalies. The maximumtan- from LADCP 100 110 100 gentialvelocity and the radiusof maximumtangential velocity Mean translationspeed, cm s-• were determinedfrom smoothradial ADCP and LADCP pro- from drifters 5.5 6.5 6.3 files. The ADCP-measured velocitiesare consistentlyhigher from altimeter 9 8 8 than the LADCP ones. The translation direction and the ro- Translation direction 303 276 287 Rotation period, days 13.1 12.5 N/A tation period were determined from satellite-trackedsurface Rossbynumber 0.07 0.06 0.05 drifters(except for ring 3; seeFigure 3). The mean translation speedwas obtainedboth from the satellite-trackeddrifters and Abbreviationsare defined as follows: XBT, expendablebathyther- the altimeter data. mograph;ADCP, acousticDoppler currentprofiler (ADCP); LADCP, lowered ADCP; and N/A, not available. Rossbynumber is defined as The size and intensityof the ringscan be quantifiedin terms Vmax/fg.... where Vmax is the maximumtangential velocity and Rmax of ring volume anomalyrelative to its surroundingwater and is the radius of maximum tangentialvelocity. ring energycontent. Available potentialenergy, volume anomaly, and heat content anomalywere calculatedfor each ring. The volumeanomalies of the ringswere computedfollowing Olsonand Evans [1986] and DuncombeRae et al. [1996]:

3.3. Ring 3 Ring 3, the smallestof the ringssurveyed, was centerednear VOL=f• [h•(r) - h=]dA 29.8øS,6.1øW on September 17, 1997. This ring was located roughly400 km northwestof ring 2 and on the extensionof the where h•(r) is the depth of the ring signatureinside the area line joining ring 2 and the AgulhasRetroflection region, 2300 A andha is the depth of the thermoclinein a regionoutside of km away. Altimetry analysisshowed that the ring was shed the rings' influence. duringApril 1996, 17 monthsprior to the survey.This indicates Assumingthat the ring crosssection has a Gaussianshape, a meantranslation velocity of 7 km d-•. The 10øCisotherm VOL = 2 •rL 2h0 reached625 m in the ring centeras compared to depthsaround 450 m outsidethe ring. The diameter of the 10øCisotherm at where L is the radius of maximumvelocity and ho = h - 500 m was roughly200 km. is the difference between the maximum depth of the ring, The vertical structureof ring 3 (Plate 7) showedthree re- defined as the region of maximum shear, and gionswith weak vertical gradientsin temperature,salinity, and The available potential energy (APE) for each ring was oxygen.An upper near-surfacelayer betweenthe surfaceand obtainedfollowing Duncombe Rae et al. [1996]: 80 m had a temperatureof 17.6øCand a salinityof 35.83. The oxygenconcentration was 5.35 mL L-•, andthe saturation was APE = 9•7'•rh•L 2/2 1.00. Another near-surface layer occurred between 140 and 195 m depth with a temperature of 15.25øCand a salinityof where p is the mean water densityand •7' is the mean reduced 35.51.The oxygenconcentration was 5.15 mL L -• at 0.91 gravity(•7' = 0.011 m s-a). The heat contentof the ring saturation.Deeper in the water column, a layer between 340 volume anomalieswas estimatedfollowing van Ballegooyenet and 440 m was measuredwith a temperatureof about 14.45øC al. [1994]: and salinity of about 35.45. This layer had an oxygenconcen- Q = 9C'pVOLAT tration of 5.0 mL L- • and saturation of 0.88. It is inferred that these layersformed by convectionduring the winter of 1996. whereC e is the specificheat of the seawater,VOL is the Salinity and temperatureare higher in ring 3 than in the sur- volume anomaly computed above, and AT is the difference rounding water between the 26.4 and 26.6 density surfaces. betweenthe temperaturesinside and outsidethe ring. Volume Oxygenshows little variationalong density surfaces across this and heat transports are the amount of mass and heat trans- ring. ported by the ring anomaliesin a year. Near-surfacevelocities of 25 cms-• wereobserved in ring3 The parametersused to calculatethese quantitieswere de- (station29, Plate 7). A subsurfacevelocity maximum (38 cm rived from the in situ observations. The radius of maximum s-•) wasobserved near 700 m (station29), presumably related velocitywas determinedfrom the direct measurements(Table to the depressionof the thermocline underneath the subsur- 1). The vertical extent of the ring h is obtained from the face well-mixed layers. The averagemean tangentialvelocity LADCP-observed tangential velocity profiles. The depths of profilehas two maxima of 18 cm s-• from0 to 100m andat maximumshear h are 900, 950, and 750 m for rings 1, 2, and 3, 650 m. After 1000 m the velocityindicates a similarbehavior to respectively(Table 2). For the heat contentcalculations, AT rings 1 and 2. This deep mean velocityis in the direction of the was taken as the differencebetween the integratedtempera- translationof the ring and is of the sameorder of magnitudeas ture to the depth of the potential densitylayer corresponding the translationvelocity obtained from the drifters (section4). to the depth of maximum shear and the same quantity in a GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT 20,979

Table 2. HydrographicObservations L, km h, m h•, m rr VOL, X 1012m 3 TvoL,Sv Q, x 1020J Tq,PW APE,x l0is J Ring 1 100 900 400 27.157 31.4 1 0.88 0.003 43 Ring 2 110 950 400 27.079 41.8 1 1.31 0.004 52 Ring 3 100 750 450 26.987 18.8 0.5 0.77 0.002 15

L is the radiusof maximumvelocity, h is the depth of maximumshear, h • is the depth of the unperturbedthermocline outside the ring area, rr is the densityof thewater at the maximumshear depth, VOL is thevolume anomaly of the ring,Tvo L is thevolume transport (1 Sv: 106 m3 s-I), Q isthe heat content anomaly, Tq is the heat transport (1 PW = l0is W), andAPE is the available potential energy.

region outside the area of influence of the ring. Results are layer thicknessdepths at the time of the hydrographicobser- shown in Table 2. vationswere 605,655, and 600 for rings 1, 2, and 3, respectively According to Table 2, the amount of mass entering the (Table 1). These estimatesare slightlysmaller than the ob- Atlantic and carried by the ringsis 1, 2, and 0.5 Sv for rings 1, servedXBT valuessince the altimeter ground trackswere not 2, and 3, respectively.On the basisof previousestimates of the probably crossingthe center of the rings during the XBT ob- numberof ringsshed at the retroflection(four to sixper year) servations.The volume transport anomaliesof each ring en- and a mean volume transportof 0.8 Sv per ring, the contribu- tering the South Atlantic were estimatedby dividing the vol- tion of the rings' transportto the interoceanexchange of water ume anomalyby the number of secondsin a year, with values rangesfrom 3 to 5 Sv. The heat transportedby rings 1, 2, and of 0.4, 0.4, and 0.3 Sv for rings 1-3, respectively.The available 3 is 0.003, 0.004, and 0.002 PW, respectively.These transport potential energyof the ringswas computedfrom the altimeter- estimates are to be considered a lower limit because of the derivedsea height anomaly data [Gogi et al., 1997] to be 10, 17, locationat which the ringswere surveyedand the time elapsed and 3 x 10•s J for rings!, 2, and 3, respectively.The heat after they were shed from the retroflection, more than 1 year contentwith respectto the 10øCisotherm, Q, was computed in the last two cases.A rough estimatefrom Byrneet al. [1995] [Shayet al., 1999]by consideringAT as the differencebetween determinedthat at least50% of the seasurface height anomaly the 10øCisotherm and temperatureof the surroundingwaters, has disappearedat thesedistances from the sheddingarea. In taken here as 15øC,to comparewith previousestimates of van addition,after more than a year, most of the heat transported Ballegooyenet al. [1994]. In this case,h is the difference be- by the ringsmay have been lost. As much as twice thesevalues tween the depth of the 10øC and 15øC isotherms.The heat (6 to 10 Sv and 0.006 PW per year) are more reasonable transportvalues calculated for the three rings are 0.005, 0.006, estimatesof the contributionsof the rings to the interocean and 0.003 PW for rings 1, 2, and 3, respectively.It is interesting exchangeof massand heat. to note that the calculations from the satellite underestimate The availablepotential energyof the rings (Table 2) is 43, 52, and15 x 10•s J for rings1, 2, and3, respectively.Previous the volume transportand APE by half and that the heat content is larger. The latestis due to the method usedthat assumes estimatesof rings in the region [van Balegooyenet al., 1994; DuncombeRae et al., 1996;Gogi et al., 1997] rangedfrom 1 to alwaysa differenceof temperatureof 5øCbetween the interior 71 x 10•s J with an averageof 24 x 10•s J. of the rings and the surroundingwaters. The estimatesof all It is not alwayspossible to calculatethese parametersfrom the altimeter-derivedparameters are shownin Table 3. in situ observations.The altimeter data provide an indepen- The radial distributionsof the 10øCisotherm depth and the dent way in which the sea height anomaly along with climato- tangential(swirl) velocity were determinedfrom the shipboard logicaldata can providea goodproxy to estimatethese param- XBT and ADCP surveys(Figures 5 and 6). The scatterin the eters. In what follows the same variables were calculated from individualdepth and velocityobservations is due in part to ring the altimeter-derivedsea height anomalyfield, whichwas used motion (both translationand rotation) during the 36- to 72- to estimate the depth of the 10øC isotherm. Here h o is ob- hour ring surveys.Noncircular ring geometry(particularly ev- tained as the differencebetween the maximum depth of the ident in ring 1) also contributedto the observedscatter. It is upper layer (in this caserepresented by the depth of the 10øC interestingto note (Figure5) that while the maximumdepth of isotherm)in the ring and in a regionoutside the ring influence the 10øCisotherm is greatestby almost100 m in ring 2, the net (h•) at the time of the hydrographicobservations. L is the changein isotherm depth acrosseach ring is similar (about radiusof maximumvelocity, which is givenby the length scale 200 m). The decreasein maximumswirl velocity from ring 1 to of the Gaussianfit of the altimeter-derivedupper layer thick- ring 3 (Figure 6) is consistentwith the time and distanceeach nesssection of the ring. The maximumaltimeter-derived upper ring has traveled from the Agulhas Retroflection.

Table 3. Altimeter Observations

L, km hi, m h•, m VOL, X 1012m 3 Tvor,Sv Q, X 102o J Tq,PW APE,X l0is J Ring 1 100 605 400 13 0.4 1.5 0.005 10 Ring 2 90 655 400 13 0.4 1.7 0.006 17 Ring 3 95 600 500 11 0.3 0.9 0.003 3

L is the length scaleof the ring obtainedfrom a Gaussianfit of the 10øCisotherm; h i is the maximumdepth of the 10øCisotherm in the center of the ring;h• is depthfar fromthe ring;VOL, Tvo•, Q, Tq, andAPE are aslisted in Table2. 20,980 GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT

3O0

4OO i.

•::•5:f-•*•:'•' :...... ':•..•'•::•'::•:•'i•.d:::•-•':?;':•:'d•;.:'½ ':. 500

600

700

800 0 50 • 00 • 50 0 50 • 00 • 50 0 50 • 00 • 50 Distance(km) Distance(km) Distance(km) Figure5. Depthof the 10øCisotherm determined from the shipboard XBT andADCP surveysas a function of the distancefrom the centerof the rings.

5. Origin of the Rings are highand oxygenis low relativeto the surroundingSouth Atlantic Water. Ring 1 showedthe typicalprofile of a youngAgulhas ret- roflectionring duringthe firstyear after shedding[Duncombe The hydrographyof ring2, however,appears to be atypical. Rae, 1991].There is a ,.reasonablywell definedupper thermo- The structurallyanomalous portion of the water columnof this stad,and the mainthermocline is unbrokenby additionalther- ring lies aboutthe 26.61 isopycnal.At this densitylevel the mostads.Similarly, the water masscharacteristics of ring 3 temperatureand salinityare not noticeablydistinct from the suggestthat its structurecan be explainedin termsof succes- surroundingwater. Only the oxygenconcentration (5.4 mL sive atmosphere-drivenheating/mixing and cooling/mixing L-•) atthis level appears different from the surrounding water eventsover the 17 monthssince leaving the vicinityof the andindicates recent contact with the atmosphere(0 2 satura- retroflection.The propertycharacteristics of rings1 and 3 also tion ---0.9).The 26.61isopycnal approaches the surfacein the appearto be usualfor Agulhasrings. Salinity and temperature SubtropicalFront around 40øSin the South Atlantic Ocean

• 50 ." .•;:::'•i . ::'?-•.•::.--:•?;•:•.:?;•:•:•:.4-:)•;•?•:::•', ' -. o • 25

0 so • 00 • so 0 so • 00 • 50 0 50 • 00 • 50 Distance(km) Distance(km) Distance(km) Figure6. Tangential(swirl) velocity determined from the shipboardXBT andADCP surveysas a function of the distancefrom the centerof the rings. GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT 20,981

Temperature Salinity Oxygen Velocity 12 10 8 ß 12 10 8 ß 12 10 8 ß 12 10 8 ß 11 9 7 I 9 11 9 11 9 7 ,• ,

500 500

1000 lOOO

1500 1500

200 100 0 200 100 0 200 100 0 200 100 0 Distance from ring centre (km) Plate 5. Verticalsections of temperature(øC), salinity (psu), oxygen (mL L-•), andcross-section velocity versuspressure for ring 1 derivedfrom conductivity-temperature-oxygen/loweredADCP (CTDO/LADCP) casts.The radial sectionis northwestwardfrom the center of the ring. The center is indicatedwith an arrowhead.Approximate distance from ring center is indicatedon the horizontalaxis.

(Plate 8). The 26.61 isopycnalis closeto the densityof Sub- been observedin the field in at least two prior studiesby antarcticMode Water [McCartney,1977]. Although there is Cresswell[1982] and SchultzTokos et al. [1994]. It has also some evidenceof water on this densitylevel with oxygensat- been modeled in the laboratory,under various conditionsof urationvalues near 0.9 in the Indian Ocean,the densitysurface eddydensities, vertical extent, and horizontalspacing [Nor and generallyhas oxygenconcentrations below 5 mL L -• in the Simmons,1987; Griffithsand Hopfinger,1987]. Indian Ocean. With the exception of the retroflection, the 3. The ring may not be of Agulhasorigin. Given that prior AgulhasCurrent and recirculationare particularlydepleted in studieshave suggesteda Brazil Current origin for somerings oxygen.However, high saturationand concentrationsof oxy- encounteredin the southeasternAtlantic [Smythe-Wrightet al., gen do occuron this densitysurface in the SouthAtlantic. This 1996;Duncombe Rae e! al., 1996], it is possiblethat ring 2 observationaccomplishes two objectives:it rules out an Agul- describedhere may have been shed from the Brazil Malvinas has Current or Indian Ocean origin of the anomalousthermo- Confluence. stad and indicatesa South Atlantic origin. The origin of the water in this ring must remain undeter- The followingthree scenarioscan be envisagedthat account mined, with coalescencewith an eddy from the STF the more for the structureof ring 2: likely source,without totally discounting the othersas possible 1. The ring was formed completelyat the Agulhas Ret- explanations.Ring 2 was readily trackedback to the Agulhas roflection with the anomalous thermostad in some manner Retroflectionin the altimetry and was observedto spend an createdby the incorporationof SubantarcticWater duringthe appreciableamount of time southof the STF, allowingample processof eddy sheddingwhen water with low salinity, low opportunity for incorporationof an eddy from the STF and temperatureand highoxygen was drawn between the retroflec- suggestingthat scenario3 may be discarded. tion and the newly formed eddy [Lutjeharmsand van Balle- Arhan et al. [this issue]observed, in one of the three rings gooyen,1988]. that theyanalyzed, a thermostadof 11.6øC(their ring 1). Itwas 2. The ring formed normally at the retroflection and later also associatedwith somewhatanomalous water properties, incorporateda lensof waterfrom the SubtropicalFront (STF) lower in salinity and higher in O2, comparedto more usual by coalescingwith another eddy. Coalescenceof eddies has Agulhas rings. Their ring was also inferred to have formed Temperature Salinity Oxygen Velocity 22 20 • • 2O 22 20 • 23 21 1' 23 21 19 0

500- ß It'

lOOO

1500 1500

200 100 0 200 100 0 200 100 0 200 100 0 Distancefrom ringcentre (kin) Plate 6. Sameas Plate 5, but for ring2. The radialsection is westward from the centerof the ring.The center is indicatedwith an arrowhead.Approximate distance from ring centeris indicatedon the horizontalaxis. Temperature Salinity Oxygen Velocity J- "2 3130292 • 3231 292•' ' i ø

5OO 5O0

1000 o c1000•

1500 1500

Iø 200 100 0 200 100 0 200 100 0 200 100 0 Distancefrom ring centre (km) Plate 7. Sameas Plate 5, but for ring3. The radialsection is westward from the centerof the ring.The center is indicatedwith an arrowhead.Approximate distance from ring centeris indicatedon the horizontalaxis GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT 20,983

Depthof IsopycnalOo=26.61

60'W 40'W 20'W 0 ø 20øE 40øE 60'E 80øE

20'S 20'S

40'S

60'W 40'W 20'W O' 20'E 40'E 60'E 80'E

OxygenConcentration (Oo=26.61)

60'W 40'W 20'W O' 20'E 40'E 60'E 80'E

OxygenSaturation (Oo=26.61)

20'S ...... •

40'S •

60'W 40'W 20'W O' 20'E 40'E 60'E 80'E Plate 8. Distributionof propertieson the 26.61kg m-3 densitysurface in the SouthIndian and South Atlantic Oceans.Data from NODC databaseinclude (top) depth of the densitysurface, (middle) oxygen concentration(mL L -1) on the densitysurface, and (bottom)oxygen saturation on the densitysurface.

from the Agulhas Retroflection.Arhan et al. suggestthat the concludedthat both extreme cooling and much higher than 11.6øCthermostad in their ring 1 was formed by intense con- usualprecipitation throughout the period sinceshedding were vective modification of Agulhas ring water during a winter required. The additional cappingof the cold thermostadand spent south of 42øS.The surfacewater above the core was the translation of ring 2 acrossthe Walvis Ridge, where we explainedas being colder, fresher, and more oxygenatedSub- found it, would require even greatercooling and precipitation antarcticSurface Water (SAASW) that had becomeentrained rates. The presenceof water in the nearby SubtropicalFront by the ring as it transited through the SubantarcticZone and and SubantarcticZone with the requiredcharacteristics of low subductedbeneath the SAASW. We attempted to model the temperature and low salinity led us to seek a mechanismthat formation of the 12.2øC cold thermostad in our ring 2 but could incorporatethis water into the ring core. We suggestthe 20,984 GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT

merging of ring 2 and an eddy of SubtropicalFront water paper,which alsodescribes three Agulhasrings. Gail Derr and Veta accomplishedthis without the need for extremecooling and Green prepared the manuscriptfor publication.This programwas funded by a grant from the National ScienceFoundation, OCE95- precipitation.That two out of the sixobserved rings contained 28574, and receivedadditional supportfrom NOAA/AOML/PhOD. these cold thermostadssuggests that they are more common than previouslyrealized. It mustbe left for futurework to docu- ment the mechanism(s)that causedthese cold thermostads. References Arhan, M., H. Mercier, and J. R. E. Lutjeharms,The disparateevo- 6. Summary and Discussion lution of three Agulhasrings in the SouthAtlantic Ocean,J. Geo- phys.Res., this issue. Rings shedfrom the Agulhas Current and carried into the Boebel,O., C. M. DuncombeRae, S. L. Garzoli,J. R. E. Lutjeharms, Atlantic as transientcomponents of the BenguelaCurrent are P. L. Richardson,T. Rossby,C. Schmid, and W. Zenk, KAPEX: importantcontributors to interoceanheat, salt, and mass trans- Observingthe intermediateflow at the tip of Africa, Eos Trans. ports.The three ringssampled during September 1997 as part AGU, 79, 1, 7, and 8, 1998. Broecker,W. S., The great oceanconveyor, Oceanography, 4(2), 79- of the BenguelaCurrent Experimentadd new insightto the 89, 1991. originsand physicalproperties of theseenergetic features. The Byrne, D. A., A. L. Gordon, and W. F. Haxby, Agulhaseddies: A ringswere measuredby a combinationof techniquesand were synopticview using GEOSAT ERM data, J. Phys. Oceanogr.,25, backtrackedto their formation region with altimetry. The 902-917, 1995. Chapman,P., C. M. Duncombe Rae, and B. R. Allanson, Nutrients, three rings share some commoncharacteristics but also have chlorophyll,and oxygenrelationships in the surfacelayers at the some significantstructural differences. Within each ring the AgulhasRetroflection, Deep Sea Res., Part A, 34(8), 1399-1416, maximum swirl velocity was located at a radius of approxi- 1987. mately 100 km. The magnitudeof this maximumvelocity ap- Cresswell, L. G. R., The coalescenceof two east Australian Current pearedto decreaseroughly proportional to the inferredages of warm-core eddies, Science,215, 161-164, 1982. DuncombeRae, C. M., AgulhasRetroflection rings in the SouthAt- the rings. lantic Ocean:An overview,S. Aft. J. Mar. Sci., 11,327-344, 1991. Ring 1 had characteristicstypical of Agulhasrings in their DuncombeRae, C. M., S. L. Garzoli, andA. L. Gordon,The eddyfield first year after shedding[Duncombe Rae, 1991]. The vertical of the southeast Atlantic Ocean: A statistical census from the Ben- structure of ring 3 can be explained in terms of successive guelaSources and TransportsProject, J. Geophys.Res., 101, 11,949- 11,964, 1996. atmosphere-drivenheating/mixing and cooling/mixingcycles Garzoli, S. L., and A. L. Gordon, Originsand variabilityof the Ben- during the ring's 17-monthlife. guela Current, J. Geophys.Res., 101,987-906, 1996. Ring 2 had embeddedinto its main core at depthsfrom 440 Gofii, G. J., S. L. Garzoli, A. J. Roubicek, D. B. Olson, and O. B. to 580 m a thermostadof low-temperature(12.2øC), low- Brown, Agulhasring dynamicsfrom TOPEX/POSEIDON satellite salinity(35.07 to 35.10), and high-oxygenconcentration. Ther- altimeterdata, J. Mar. Res.,55(5), 861-883, 1997. Gordon, A. L., Indian-Atlantic transfer of thermocline water at the mostadsof temperaturesbetween 13ø and 14øC have been AgulhasRetroflection, Sciences N.Y., 227(4690), 1030-1033, 1985. previouslyobserved in these rings. Rings with thermostads Gordon, A. L., and W. F. Haxby, Agulhas eddiesinvade the South between13 ø and 14øCand saltierthan Agulhasrings have been Atlantic: Evidencefrom Geosataltimeter and shipboardconductiv- previouslyidentified as Brazil rings[Smythe-Wright et al., 1996; ity-temperature-depthsurvey, J. Geophys.Res., 95, 3117-3125, 1990. Gordon,A. L., J. R. E. Lutjeharms,and M. L. Grundlingh,Stratifica- DuncombeRae et al., 1996]. The oxygenconcentration and tion andcirculation at the AgulhasRetroflection, Deep Sea Res., Part saturationwithin the anomalousthermostad of ring 2 are also A, 34, 565-599, 1987. different from those observedin Agulhas rings.In this case, Gordon, A. L., R. F. Weiss, W. M. Smethie, and M. J. Warner, three possiblesources for the thermostadwere examined:vor- Thermocline and Intermediate Water communication between the SouthAtlantic and Indian Oceans,J. Geophys.Res., 97, 7223-7240, tex stretchingduring shedding, merging with water from south 1992. of the STF, or incorporationof bottomwater from the nearby Griffiths,R. W., and E. J. Hopfinger,Coalescing of geostrophicvor- AgulhasBank. The originwas not resolvedconclusively. High tices,J. Fluid Mech., 178, 73-97, 1987. saturationand concentrationsof oxygensuch as found in ring Lutjeharms,J. R. E., and R. C. van Ballegooyen,The retroflectionof 2 are observedin the South Atlantic. The analysispresented the AgulhasCurrent, J. Phys.Oceanogr., 18, 1570-1583, 1988. McCartney,M. S., SubantarcticMode Water, inA Voyageof Discovery, leadsto the conclusionthat ring 2 is of Agulhasorigin and later GeorgeDeacon: 70th Anniversary Volume, edited by M. Angel, pp. incorporateda lens of water from the SubtropicalFront by 103-119, Pergamon,Tarrytown, N.Y., 1977. coalescingwith another eddy. Further understandingof how Nof, D., and L. Simmons,Laboratory experiments in the mergingof theserings translate into the interior and their ultimate role in nonlinearanticyclonic eddies, J. Phys.Oceanogr., 17, 343-357, 1987. the meridionaloverturning circulation will be possibleonce the Olson, D. B., Rings in the ocean,Annu. Rev. Earth Planet. Sci., 19, 133-183, 1991. RAFOS floats deployedin their coresend their missionand Olson,D. B., and R. H. Evans,Rings of the AgulhasCurrent, Deep Sea provide further data. Res., Part A, 33, 27-42, 1986. Peterson,R. G., and L. Stramma, Upper-circulationin the South Atlantic Ocean,Prog. Oceanogr.,26, 1-73, 1991. Acknowledgments. The scientificparty wishesto acknowledgethe Roubicek, A. J., S. L. Garzoli, P. L. Richardson, C. M. Duncombe Rae, invaluablesupport of Captain Vince Seiler and the crew of the R/V and D. M. Fratantoni,Benguela Current Experiment,R/V Seward SewardJohnson, who helped make the cruisea success.The authors JohnsonCruise SJ9705, NOAA Data Rep.,ERL AOML-33, 215 pp., wish to acknowledgethe scientificcollaboration of the membersof Atlantic Oceanogr.and Meteorol. Lab., Miami, Fla., 1998. KAPEX, O. Boebel,J. Lutjeharms,T. Rossby,and W. Zenk, and their Saunders,P.M., and B. A. King, Oceanicfluxes on the WOCE All helpful commentsduring the planningstage of this experiment.We section,J. Phys.Oceanogr., 25, 1942-1958, 1995. especiallythank O. Boebeland W. Zenk for deployingone of the U.S. SchultzTokos, K. L., H.-H. Hinrichsen,and W. Zenk, Merging and sound sources,which enabled us to spend more time on the ring migrationof two eddies,J. Phys.Oceanogr., 24, 2129-2141, 1994. survey.We are verygrateful to Doug Wilson and Ryan Smith(NOAA Shay, L. K., G. J. Goni, and P. G. Black, Effects of a warm oceanic AOML) for their help in reprocessingthe LADCP data and to Warren feature on Hurricane Opal, Mon. WeatherRev., in press,1999. Krug for his supportwith the XBT software.Near-real-time altimeter Smythe-Wright,D., A. L. Gordon, P. Chapman, and M. S. Jones, datawere providedby Robert Cheney(NOAA/NODC). M. Arhan, H. CAC-113shows Brazil eddycrossing the SouthAtlantic to the Agul- Mercier, andJ. R. E. Lutjeharmsgenerously gave us a preprintof their has Retroflectionregion, J. Geophys.Res., 101,885-895, 1996. GARZOLI ET AL.: AGULHAS RINGS IN BENGUELA CURRENT 20,985 van Ballegooyen,R. C., M. L. Grundlingh,and J. R. E. Lutjeharms, Institution, Woods Hole, MA 02543. Eddy fluxesof heat and salt from the southwestIndian Ocean into S. L. Garzoli, G. J. Gofii, and A. J. Roubicek, Atlantic Oceano- the southeastAtlantic Ocean: A case study,J. Geophys.Res., 99, graphic and MeteorologicalLaboratory, NOAA, 4301 Rickenbacher 14,053-14,070, 1994. Causeway,Miami, FL 33149. ([email protected])

C. M. Duncombe Rae, Sea FisheriesResearch Institute, P. Bag X2, Rogge Bay, 8012 Cape Town, South Africa. (ReceivedApril 17, 1998;revised February 1, 1999; D. M. Fratantoniand P. L. Richardson,Woods Hole Oceanographic acceptedFebruary 25, 1999.)