Journalof MarineResearch, 60, 639–676,2002 Journal of MARINE RESEARCH

Volume60, Number 5

Water masses andbaroclinic transports inthe South Atlanticand Southern oceans

byKaren J. Heywood 1 andBrian A. King 2

ABSTRACT Wedescribe the World Ocean Circulation Experiment (WOCE) A23hydrographic section from Antarcticato Brazil,nominally along 35W. The section crossed the center of theWeddell Gyre, the AntarcticCircumpolar Current (ACC) andthe Subtropical Gyre in theArgentine Basin. We precisely deŽne the locations of fronts, the changes in water mass properties across them, and their transports. TheAntarctic Slope Front was crossed above the continental slope of Antarctica,with a baroclinic transportof 4Sv,part of thecyclonic Weddell Gyre circulation of 19 Sv. We repeated a sectionin the WeddellSea occupied in 1973,and saw amarkedwarming of theinowing Warm Deep Water layer bysome0.2° C, butno discernible change in theout owing northern limb of thegyre. An in ow of recentlyventilated water with the same characteristics as Weddell Sea Deep Water (WSDW) was observed owinginto the Weddell Sea from the east. TheWeddell Front was crossed at 61° 7 9Sandthe Southern Boundary (SB) ofthe ACC (often referredto as the Scotia Front) at 58° 38 9S.Betweenthese lay the Weddell-Scotia Con uence, contributing16 Svof eastwardtransport. The Ž rstcrossing of theSouthern ACC Front(SACCF) lay southof SouthGeorgia at ;55°309S.Itthenwrapped anticyclonically around South Georgia and was encounteredat 53° 40 9Sbeforeretro ecting and returning eastward at ;53°309S. Thebaroclinic transportwas ;15Sv at each crossing. In this region the SACCF ismost clearly identiŽ ed by a decreasein thesalinity of thetemperature minimum layer. The core of thePolar Front (PF) layat ;49Swhere the isotherms plunged down sharply to the north, and transported 67 Sv. The PF and SubantarcticFront (SAF) werebarely distinguishable with only one station clearly in the Polar FrontalZone. The SAF, transporting57 Sv, was encountered at ;48°459Swherethe subsurface salinityminimum of AntarcticIntermediate Water (AAIW) began to descend.The Subtropical Front (STF) marksthe boundary between the waters of the subtropical gyre and the colder, fresher

1.School of EnvironmentalSciences, Universityof East Anglia,Norwich NR4 7TJ, United Kingdom. email: [email protected] 2.Southampton Oceanography Centre, Empress Dock,Southampton, SO14 3ZH, United Kingdom. 639 640 Journalof MarineResearch [60, 5 subantarcticwaters to thesouth; its southernmost crossing was at 44 –45Stransporting ;25 Sv. This isseveralhundred kilometers farther south than historical locations of the STF atthislongitude. The BrazilCurrent Front (BCF) wasencountered at ;38Stransporting 43 Sv. Whereas previous observationsfound the STF tobe theprimary means for eastward  owofthewaters of theBrazil Currentafter it hasseparated from the coast, during A23 we Ž ndthatthe BCF carriesthe majority of thistransport. A furtherDeep Front, possibly marking the center of thesubtropical gyre, was crossed at 34°229Sassociatedwith a transportof 2.5 Sv. In the western Vema Channel we encountered a recirculationof theBrazil Current,  owingto thenortheast with a transportof 23 Sv. The section endedprematurely without crossing the Brazil Current. To close the subtropical gyre transport would requirea BrazilCurrent signi Ž cantlyin excess of the historical estimates based on shallowreference levels. Boththe SACCF andSTF exhibitedmeanders probably caused by bathymetry. The STF meander maybe causedby theeastern end of the circulationaround the ZapiolaRise. Both the SB andPF had associatededdies. The SB hadshed a cycloniceddy to the north, which had subsequently been cappedby localwater. It transported8 Svazimuthally. The PF hadshed an anticycloniceddy to the south,which was also capped and circulated at least3 Sv.A smallanticyclonic subsurface lens of AAIW wasobserved in the Vema Channel with a transportof theorder of 1 Sv.In theVema Channel, alevelof nomotionbetween the North Atlantic Deep Water and the Lower Circumpolar Deep Water (LCDW) givesa netnorthward  owof 1.2 Sv WSDW and4 SvLCDW fromthe Argentine to the BrazilBasin.

1.Introduction We describea hydrographicsection from Antarcticato Brazil(Fig. 1), designated A23 intheWorldOcean Circulation Experiment (WOCE) one-timehydrographic program. The sectioncrosses the center of theWeddell Gyre, the Antarctic Circumpolar Current (ACC) ata regionwell away from achokepoint (a locationwhere the ACC narrowsto  ow betweena continentand Antarctica), and thecenterof thesubtropical gyre in theArgentine Basin.We presentfull depth sections of neutraldensity and of alargesuite of chemical tracersand discuss the exchanges of waters betweenpolar, subpolar and subtropical regions.We concentratehere on thewater mass propertiesmarking the various fronts, and thebaroclinic transports of theWeddell Gyre, the ACC andthe fronts of thesubtropical Atlantic. Tsuchiya et al. (1994)undertook a quasi-meridionalsection along ;25Wfrom South Georgiato the equator. During WOCE, sectionA17 skirted the western boundary of the SouthAtlantic (Memery et al., 2000)and section A12 was occupiedat the Greenwich Meridian(Schro ¨derandFahrbach, 1999). A23 lies between A17 and A12, and also east of theDrake Passage repeat occupations SR1 (Cunningham et al., 2003)and of theWeddell Searepeat occupation SR4 (Fahrbach et al., 1994).This paper will discuss the A23 data in thecontext of theseand other studies.

2.Data collection TheA23 data set was collectedduring Cruise 10 of RRS JamesClark Ross, between Marchand May 1995 (Fig. 1) (Heywood and King, 1996). Although the cruise was 2002] Heywood& King:Water masses & baroclinictransports 641

Figure1. Bathymetry of the Southwest Atlantic. Contours are every 1000 m withshade changes every2000 m. Depths greater than 6000 m arewhite; land is shaded black. Station locations from theWOCE A23cruise are marked with selected station numbers. Locations of thefronts crossed areindicated. ASF, AntarcticSlope Front; WF, WeddellFront; SB, SouthernBoundary of the ACC; SACCF, SouthernACC Front;PF, PolarFront; SAF, SubantarcticFront; STF, Subtropical Front;BCF, BrazilCurrent Front; DF, DeepFront; BCR, BrazilCurrent Recirculation. nominallyalong 35W, severe sea ice conditions in the southern Weddell Sea and the latenessin the season prevented access to the region of the Filchner-Ronne Depression (where we hadhoped to encounterrecently formed Weddell Sea Deep and Bottom Water). Thesection began farther east on theAntarctic continental shelf at 16W. Hydrographicstations were undertakenusing a NeilBrown CTD with24 bottlerosette.

Moststations approached the sea bed within 5 –10m.A totalof 128CTDO 2 smallvolume stationswere occupied,of which 3 layoff theA23 section (stations 1, 2 and12), two were 642 Journalof MarineResearch [60, 5 abandonedsoon after deployment because of winchproblems (stations 83 and85),and one collectedno water samples(station 45). Station spacing was close(5 –10km) overregions ofsteep bathymetry and across fronts, and wide ( ;110km) across the central Weddell Gyre andcentral Argentine Basin. The mean station spacing for thewhole section was 50 km.Accuracies were 0.001 °Cfor temperatureand 0.002 in salinity(Heywood and King, 1996)using standard sea water batch P125. Precision and accuracy of chemical measure- mentsare discussed in the cruise report. Mean precision determined from duplicate measurementswere 0.4%(nitrate), 0.65% (phosphate), 0.35% (silicate) and 0.26% (oxygen)over the cruise as awhole. As we approachedthe Polar Front (station 71), a medicalevacuation forced us to leave thearea, and we returnedone week later. Some stations were repeatedand provide an interestingstudy of the behavior of the front that we discusslater. Figures of the whole sectionwill use the second crossing, which has complete near-synoptic coverage of the Polarand Subantarctic Fronts. Thus, the section is constructed from stationnumbers up to 64andfrom 73onward. Unfortunately we didnot have permission to undertakemeasure- mentsin Brazilian waters, so the continent-to-contin entsection was notcompleted and CTDstationsceased in theVema Channelwithout crossing the Brazil Current.

3.Results anddiscussion Figure2 presentssections of water propertiesalong A23. Locatio nsof fronts deducedfrom theseproperti esaremarked on Figure1. Watermasses and fronts will be discussedinturnfrom southto north, with the former beingde Ž nedin termsof their neutraldensity gn (Jackettand McDougal l,1997) (Fig. 2c). Table 1 listsfor conve- nienceall acronyms of water masses,currents and fronts used in thispaper. Table 2 summarizesthewater mass boundariesand gives equival entboundari esbased on 2 3 traditionalisopycna lssuchas s4.Theunits of density(kg m )are omittedhencefort h. Transportsinwater mass bandsassociat edwith various fronts, currents and gyres are givenin Table3 andFigure 3. a.Bathymetry of thesection Continuouslyrecorded echo sounder depths are superimposed on thesections in Figure 2.Thesection commences on thecontinental shelf of Antarctica(200 m depth)and crosses thesteep continental slope to the abyssal Weddell Basin, relatively smooth and  at at ;5000m. The rugged bathymetry of the South Scotia Ridge is apparent at ;60S. The ScotiaSea also has rough bathymetry and is relativelyshallow at ;3500m. Stations were occupiedprogressing both up and down the steep continental slope of South Georgia (55S).The Georgia Basin deepens sharply toward the north. The topographic feature at 50Sis theNortheast Georgia Rise, the western end of whichis traversedby A23.The A23 sectionpassed meridionally through the gap in the Falkland Ridge at 48S so this bathymetricobstacle is not apparent on thesections. It is important,however, for itsstrong 2002] Heywood& King:Water masses & baroclinictransports 643

Figure2(a). Potential temperature along A23. Values between 0 and2 °Careshaded. Contour intervalsare 22, 21, 20.75, 20.5, 20.25,0, 0.5,1, 2,3 °C(lowerpanel) and 22,0, 0.5, 1, 2, 3, and every 2°C from 4 to 26°C(middlepanel). The neutral density surfaces and water mass boundariesas shownin Figure 2(c) are overlaid as dottedlines. Station locations are marked at the loweraxis. Stations included are 3 –11, 14– 64, 73–83, 85, 87–128.Front locations are marked on theupper axis (see Figure 1 orTable1 foracronym de Ž nitions).Surface temperature is plottedin theuppermost panel. in uenceon thelocation of thePolar and Subantarctic Fronts. The deep Argentine Basin reachedmaximum depths of ;6000m alongA23, but was typically5000 m deep.The sectionprogressed both up and down the steep slope of the Rio Grande Rise ( ;31S), beforeturning northwest toward the continental slope of South America. The Vema Channelwas crosseddownstream of itssill, where Antarctic Bottom Water can spill into theBrazil Basin to thenorth. b.The Weddell Gyre Circulation TheWeddell Gyre isidenti Ž edby the doming of the isolines indicating cyclonic circulationrelative to a deepreference level, between the eastward  owingACC tothe north,and the Antarctic continent to thesouth. In the southern Weddell Sea, much of the 644 Journalof MarineResearch [60, 5

Figure2(b). Salinity along A23. Values greater than 34.7 are shaded. Contour intervals in the lower panelare every 0.1 with 34.65 and 34.67 contours shown dashed. Contour intervals in themiddle panelare the same with the addition of 33.5, 34, 35, 35.5,36, 36.5,37. Surface salinity is plottedin theuppermost panel. Otherwise as Figure 2(a). westwardtransport of the Weddell Gyre isassociated with the Antarctic Slope Front (ASF), whichhugs the continental slope of Antarctica.Heywood et al. (1998)discussed theA23 observations on thecontinental shelf and slope. Relative to thedeepest common level(DCL) betweenstation pairs, a baroclinictransport of 4Svwas deduced(Table 3), butusing shipborne acoustic Doppler current pro Ž ler(ADCP) datato reference the geostrophicshear yielded 14 ( 63)Sv to the southwest. Schro ¨derand Fahrbach (1999) measured22 Sv at the Greenwich Meridian, and suggested that this is consistent with a recirculationbetween the two sections. The temperature and salinity properties of the warm deepwater in the coastal regime south of the ASF arevery similar on the two sections (;34.68, 0.7°C, Fig. 4b). Themost voluminous water masses in the Weddell Sea are a warm mid-depthwater mass from theACC (Lower CircumpolarDeep Water, LCDW, usuallyreferred tointhe WeddellSea as Warm DeepWater, WDW), anda colddense water mass formedin Antarcticregions (Weddell Sea Deep Water, WSDW). WSDW isable to escape the 2002] Heywood& King:Water masses & baroclinictransports 645

Figure2(c). Selected neutral density surfaces de Ž ningwater mass boundaries. WSBW, WeddellSea BottomWater; WSDW, WeddellSea Deep Water; WDW, WarmDeep Water; LCDW, Lower CircumpolarDeep Water; NADW, NorthAtlantic Deep Water; UCDW, UpperCircumpolar Deep Water;AAIW, AntarcticIntermediate water; SW, surfacewater. Station locations are marked at thelower axis. Front locations are marked on theupper axis (see Figure 1 orTable1 foracronym deŽ nitions).Dashed vertical lines at the SB andSAF denotethe southern limits of the relevant watermasses.

WeddellSea and to permeate the world ocean as Antarctic Bottom Water. Beneath WSDW, WeddellSea Bottom Water (WSBW) occupiesthe bottom of theWeddell Sea. It is identiŽ edby potentialtemperatures less than 20.7°C(Carmackand Foster,1975). Using theneutraldensity along A23, the local de Ž nitionof thisboundary is gn 5 28.4.WSBWis toodense to over  owthe topography that surrounds the Weddell Sea, and can only in uenceAntarctic Bottom Water by mixingwith the WSDW aboveit. WSDW isusually de Ž nedby therangeof potential temperatures 20.7°C to 0°C (Reid et al., 1977).Although Orsi et al. (1999)used the densest class of water passingthrough DrakePassage ( gn 5 28.27)todistinguishWSDW from LCDW, NaveiraGarabato et al. (2002a)argued that some WSDW ispresentat the sill of DrakePassage and therefore a slightlyless dense boundary is moreappropriate. We hereuse the less dense boundary of gn 5 28.26.Inpractice this distinction is insigni Ž cantaway from DrakePassage. Meredith et al. (2000)presented A23 chloro  uorocarbon(CFC) datato argue that there existsa signi Ž cant in owfrom theeast of cold, dense, recently ventilated water inthe 646 Journalof MarineResearch [60, 5

Figure2(d). Dissolved oxygen ( mmol kg21 )alongA23. Values less than 220 mmol kg21 are shaded. Contourintervals are every 20 mmol kg21 .Surfaceoxygen concentration is plotted in the uppermostpanel. Otherwise as Figure2(a).

densityclass of WSDW. Thiswater is clearly seen in Figure 2g as the region of higher CFC11 (.0.2pmol/ kg)at depths of 2000 – 4000m andlatitudes south of 67S.Potential temperature-salinityproperties for thisin  owingwater (stations 9 –20inFig.4c) indicatea straightmixing line toward 34.65, 20.8°C.Thiscore of easternsource water is also seen at theGreenwich Meridian (Klatt et al., 2002).They show a CFC11section taken in 1996, oneyear after A23, at the Greenwich Meridian. The eastern source water clearly mixes betweenthe two sections, since the maximumCFC11 values decrease from 0.6to 0.5 pmol kg21,andthe meridional extent of the .0.5 pmol kg21 coredecreases from ;400 km to ,200km. The core has also apparently descended the slope by some500 m, althoughthis mustbe viewed cautiously since both cores are de Ž nedon the basis of relatively sparse verticalbottle spacing. The out owingdeep and bottom waters inthe northernpart of theWeddellbasin exhibit a kink at ;20.6°Ctowarda fresher,colder end member (34.64, 20.9°C)(stations23 to31 inFig.4c). In the central Weddell Sea, the u-Srelationalso shows this tendency toward the 2002] Heywood& King:Water masses & baroclinictransports 647

Figure2(e). Silicate ( mmol kg2 1 )alongA23. Values greater than 120 mmol kg21 are shaded. Contourintervals are every 20 mmol kg2 1 .Theneutral density surfaces de Ž ningwater masses in Figure2(c) are overlaid as greydashed lines. Bottle locations are marked as greydots. Otherwise asFigure2(a). fresh WSBW closeto the sea bed (stations 19 to 22 in Fig. 4c), and the CFC sections (stations19 –22in Figs. 2g, h) indicaterecently ventilated WSBW withthe eastern source waterabove. Fahrbach et al. (1995)suggest that the kinktoward the fresher bottomwater is dueto the addition of new WSBW formedalong the continental shelves to the east of the AntarcticPeninsula, particularly near the Larsen Ice Shelf. The A23 results con Ž rm that thisis apparent as far eastas 30W.WSBW andWSDW formedin theSouthern Weddell Seanear the Filchner-Ronne Depression tends to circulate within the interior of the WeddellGyre, whereas that formed east of the Antarctic Peninsula takes a pathon the outerrim oftheWeddell Gyre, and is thus able to escapethrough gaps in the South Scotia Ridge. Thedensest water inthe Weddell Sea, the WSBW againstthe southern  ank of the SouthScotia Ridge, exhibits the highest concentration of CFCs.This water does not escape theWeddell Sea. CFC valueshere are higher than those seen in thesame eastward  owing coreat theGreenwich Meridian a yearlater (Klatt et al., 2002),decreasing from .1 to 0.6 pmol kg21 indicatingmixing with surrounding waters. At A23the highest concentrations arefound at the seabed ( ;5000m), butat the Greenwich Meridian they are detached from 648 Journalof MarineResearch [60, 5

Figure2(f ). Phosphate( mmol kg21 )alongA23. Values .2 mmol kg21 areshaded. Contour intervalsare every 0.5 mmol kg2 1 plus2.2 and 2.3 mmol kg21 .Otherwiseas Figure 2(e). theseabed (peakingat 4500m inwater depth ;5300m). TheGreenwichMeridian section isdeeperthan A23, but themaximum densities at the seabed are similar. This suggests that thedensest portion of the core of eastward  owingWSBW hasbeen blocked by bathymetrybetween A23 and the Greenwich Meridian, or that it has recirculated before reachingthe Greenwich Meridian. Abovethe WSDW isfoundthe WDW, characterizedby awarm, salty,low oxygencore 21 (u . 0°C, S . 34.68, O2 , 200 mmol kg ),derivedfrom thewarm saltycore of CircumpolarDeep Water. This salinity maximum layer exhibits pronounced variations acrossthe Weddell Sea (Fig. 4b). The warmest (0.9 °C)andmost saline (approaching 34.7) WDWis,not surprisingly, found in the southern, in  owinglimb of the Weddell Gyre (stations9 to22), closest to theentry of the warmer, saltiercircumpolar water in theeastern WeddellGyre. The water in the eastward-  owingnorthern limb of the Weddell Gyre is fresher (,34.69)and considerably cooler ( ,0.6°C)thanthe in  owingWDW. Thesalinity maximumfound above the Antarctic continental slope (stations 6 – 8)iscooler(0.7 °C) and fresher (34.68)because of anaddition of seaice and/ orcontinentalice. Comparison with sectionsat the Greenwich Meridian presented by Schro ¨derand Fahrbach (1999) shows that,closer to its origin in the ACC tothe north, the in  owingdeep water is warmer (;1.2°C)atthe Greenwich Meridian than at A23 ( ;0.9°C),butthere is little salinity 2002] Heywood& King:Water masses & baroclinictransports 649

Figure2(g). CFC11 (pmolkg 21 )alongA23. Values .0.2 pmol kg2 1 areshaded.Contour intervals areevery 1 pmolkg 21 plus0.1, 0.2, 0.5 pmol kg 21 .Otherwiseas Figure 2(e). change(both ;34.70).The out  owingdeep water observed at the GreenwichMeridian, on theother hand, has similar properties to thaton A23( ;0.5°C, 34.69). Coincidentally,the eastward shift of the A23 track to avoid ice meant that a section occupiedby the 1973 International Weddell Sea Expedition (Carmack and Foster, 1975) was reoccupied.Data downloaded from NODC havebeen compared with A23, and show thatchanges are detectable throughout the water column. The most striking change is a markedwarming of thein  owingWDW layer(Fig. 4d). In 1973 the WDW enteringthe WeddellSea from theeast is nowarmer than0.65 °Candis typically 0.5 °C,withsalinity lessthan 34.69. In 1995, the core of theWDW inthe southern limb of theWeddell Gyre is allwarmer than0.8 °Cwithsalinity more than 34.69 (stations 9 –20).However in the out owinglimb of the Weddell Gyre (Fig.4e), there is no signi Ž cantdifference in the temperatureand salinity maximum between 1973 and 1995. We concludethat the warming observedat the in  owhadnot had suf Ž cienttime to advectaround the Weddell Gyre and reachthe out  owatthe longitude of ourobservations(30W). Inthe surface layer above WDW liesAntarctic Surface Water. Surface waters onthe continentalshelf near Antarctica are close to freezing at about 21.85°Candare relatively fresh dueto ice melt, with salinities ;33.9.In the ASF (stations8 to11) there are local maximain sea surface temperature and sea surface salinity (these can be seenin Fig. 3 of 650 Journalof MarineResearch [60, 5

Figure2(h). Carbon tetrachloride (pmol kg 21 )alongA23. Values . 0.5 pmol kg21 are shaded. Contourintervals are every1 pmolkg 21 plus0.1, 0.2, 0.5 pmolkg 2 1 .Otherwiseas Figure2(e).

Heywood et al., 1998)indicating that the currents associated with the ASF providea sourceof warmer waterfrom theeast. These waters exhibita moresigni Ž cant ACC in uencethan do the waters eitherto the north or the south of the ASF. Surface temperaturesin the Weddell Gyre arein general close to freezing, until they begin to increasenorth of station 23, with a rapidincrease between stations 27 and 30. Surface salinitiesare relatively high ( ;34.1)in the southern part of thegyre, where the waters  ow west.In the northern part of thegyre,  owingeast, surface waters arethe freshest of any observedon A23( ;33.5).This indicates the in  uenceof seaice and glacial ice melt in the southwesternWeddell Sea. Thetemperature minimum at the base of the Antarctic Surface Water layer is the wintertimeextremum of Antarctic Surface Water, Winter Water. A23 was undertakenat theend of summer and the Winter Water is therefore covered by awarmer, fresher layer, in uencedby solar radiation and sea ice melt during recent months. There is aninteresting rangeof WWsalinitiesobserved in theWeddell Sea (Fig. 4a). On the continentalshelf and slopethe Winter Water is relativelyfresh (34.38,stations 3 – 8)duetoseaice and glacial ice melt;these are the properties of Eastern Shelf Water, known to inhibit the formation of WeddellSea Deep and Bottom Water (Fahrbach et al., 1994).To the north of the ASF, WinterWater is at its warmest (stations9 to11), and is increasingly saline across the 2002] Heywood& King:Water masses & baroclinictransports 651

Figure2(i). d3 He (‰)alongA23. Values . 8 ‰ areshaded. Contour intervals are every 2 ‰. Otherwiseas Figure 2(e). southernlimb of the Weddell Gyre. The most saline ( ;34.45)is in the centre of the WeddellGyre (stations19 to22,Fig. 4a). This is wheredense isopycnals associated with highersalinities are found nearest to the surface due to the doming of the gyre. It is thereforeeasier to mix up saltier water from below.This water is signi Ž cantlysaltier than theWinter Water observed further east by Schro ¨derand Fahrbach (1999). There is a sharp decreasein WinterWater salinity between stations 22 and23 (34.45to 34.35)indicating the in uenceof sea ice melt. The freshest umin (;34.35)is in the northern part of the WeddellGyre (stations23 to31). Relativeto the DCL, thetransport of the southern limb of theWeddell Gyre is19Sv, reachedat station23 (Fig. 3; Table 3). The gyre axis at station 23 is markedby the coldest andfreshest bottom water on A23( 20.926°C,34.638).Up to andincluding the Weddell Front,only 6 Svreturns to the east across A23. This is likely to be an underestimate, becausea DCLreferencelayer is clearlyinappropriate in the northern Weddell Gyre where we knowthat there is a bottomlayer of WSBW  owingeast. Use ofaneastward bottom referencevelocity would increase the net eastward  owinthe northern limb. Furthermore, someof thereturn eastward  owofWeddellGyre waters occursthrough incorporation into theWeddell-Scotia Con  uence,discussed in the next section. Naveira Garabato et al. (2002b)address the problem of closing the volume budget for theWeddell Gyre by 652 Journalof MarineResearch [60, 5

Figure2(j). Geostrophic velocity (cm s 21 )acrossA23, relative to the deepest common level for stations3 to100 (south of theDeep Front), relative to gn 5 27.92betweenstations 100 and 110, andrelative to gn 5 28.11inthe Vema Channel. Contours are shown every 20 cm s 2 1 , with additional 62 cm s21 shownin grey. Eastward velocities (positive) greater than 20 cm s 21 are shadedpale grey; westward velocities (negative) greater than 20 cm s 2 1 areshaded dark grey. Surfacevelocity is plotted in theuppermost panel. Otherwise as Figure2(a). combiningthe A23 data set with a hydrographicsection along the South Scotia Ridge in an inversemodel. c.The Weddell Front, Weddell-Scotia Con uence and Southern Boundary of theACC TheWeddell-Scotia Con  uence(WSC) liesbetween the Weddell Front and the SouthernBoundary of the ACC (alsoknown as the Scotia Front). In Figure 2a it is identiŽ edas the region near 60S where the subsurface umax doesnot reach 0.5 °C. Whitworthand Nowlin (1987) showed that traces of theWSC arestill identi Ž able at the GreenwichMeridian by interruptionsin thesubsurface temperature and salinity maxima, andoxygen minimum. The Weddell Front is locatedat station 33 (61 °79S), wherethe sea surfacetemperature increases from ;21.2°C to ;20.7°C(Fig.5). Sea surface and mixed layersalinities (and consequently densities) are higher (up to 33.75) in the core of the 2002] Heywood& King:Water masses & baroclinictransports 653

Table1. Summary of acronymsused in alphabetical order Frontsand currents ACC AntarcticCircumpolar Current ASF AntarcticSlope Front BCF BrazilCurrent Front BCR BrazilCurrent Recirculation DF Deep Front PF Polar Front SACCF SouthernACC Front SAF SubantarcticFront SB SouthernBoundary of theACC STF SubtropicalFront WF WeddellFront WSC Weddell-ScotiaCon  uence Watermasses AAIW AntarcticIntermediate Water LCDW LowerCircumpolar Deep Water NADW NorthAtlantic Deep Water SW SurfaceWater UCDW UpperCircumpolar Deep Water WDW WarmDeep Water WSBW WeddellSea Bottom Water WSDW WeddellSea Deep Water Miscellaneousacronyms ADCP AcousticDoppler Current Pro Ž ler CFC Chloro uorocarbon CTD ConductivityTemperature Depth instrument DCL DeepestCommon Level between CTD stations WOCE WorldOcean Circulation Experiment

WeddellFront (stations 32 –34)with lower near-surface salinities ( ;33.6)both in the WeddellGyre regionto the south (stations 29 to31)and in the WSC regionto thenorth (stations35 to 39, Fig. 5). The geostrophic velocities and baroclinic transport associated withthe Weddell Front are insigni Ž cant,less than 1 Sv(Figs. 2j and3). Thetemperature maximum signature of the WDW changesmarkedly across the WeddellFront (Fig. 5), from warmer, saltiervalues ( ;0.6°C,34.69)to cooler, fresher values (;0.4°C,34.68)at depths of 200 –500dbar. Above this, at about 100 dbar, the temperatureminimum layer (Winter Water) is ;21.5°Cbothnorth and south of thefront, but is ;21.3°Cinthecore of thefront (Fig. 5). The salinity of theWinter Water is also distincteither side of theWeddell Front; to the south the umin hasa salinityof ;34.35, whereasin the WSC thissalinity is ;34.25.Because of both these changes, the stratiŽ cationin the top 200 m ofthe WSC isdecreasedcompared with the Weddell Gyre water tothesouth. Whitworth et al. (1994)showed that this unusual strati Ž cationis dueto 654 Journalof MarineResearch [60, 5

Table2. Water mass boundaries de Ž nedin neutral density space. Equivalent potential density isopycnalsor isothermsare given.Units of densityare omitted.

Southof SouthernBoundary BetweenSouthern Boundary of ACC ofACC andSubantarctic Northof SubantarcticFront (Stns 3–40) Front(Stns 41 –80) (Stns 81–128)

Surface waters (SW) gn , 28.00, gn , 27.55, gn , 27.13,

s0 , 27.72 s0 , 27.34 s0 , 27.00 Antarctic Intermediate not present not present 27.13 , gn , 27.55,

Water (AAIW) s0 . 27.00, s1 , 32.00 UpperCircumpolar Deep not present 27.55 , gn , 28.00, 27.55 , gn , 27.92,

Water (UCDW) s0 . 27.34, s2 , 36.98 s1 . 32.00, s2 , 36.98 NorthAtlantic Deep Water not present not present 27.92 , gn , 28.11,

(NADW) s2 , 36.98, s4 . 45.87 Lower CircumpolarDeep 28.00 , gn , 28.26 28.00 , gn , 28.26, 28.11 , gn , 28.26,

Water (LCDW) (here calledWarm Deep 36.98 , s2 , 37.16 45.87 , s4 , 46.05 Water)

s0. 27.72, s1 , 32.55,

s2, 37.16 WeddellSea DeepWater 28.26 , gn , 28.40, 28.26 , gn, 28.26 , gn,

(WSDW) 37.16 , s2, s4 , 46.15 46.05 , s4 46.05 , s4 20.78C , u , 08C WeddellSea BottomWater 28.40 , gn, not present not present

(WSBW) s4 . 46.15 u , 20.78C

anadmixtureof shelfwater from thenorthwestern Weddell Sea. The oxygen minimum is lessextreme ( .200 mmol kg21 Fig.2d) in theWSC, and CFCs are higher (Figs. 2g and 2h), conŽ rmingthe admixture of youngerwater. TheSouthern Boundary of theACC (SB) liesat station41 (58 °389S), wheresubstantial interleavingof ACCandWeddell Gyre waters isseen,particularly at intermediate depths intheWDW layer(Fig. 6). In waters in  uencedby theWSC, the salinity maximum and temperaturemaximum of theWDW coincidein depth (at ;34.68, 0.4°C).As theSB is crossed,the salinity maximum becomes more saline ( ;34.7)and occurs deeper in the watercolumn than the temperature maximum ( ;1.2°C).Surface temperatures increase markedlyacross the SB, from ;20.5°C to ;1°C,asdo salinities,from ;33.7 to ;34.0 (Fig.6a). The SB ismostclearly de Ž nedby thedissolved oxygen signature, since it marks thesouthernmost penetration of Upper Circumpolar Deep Water (UCDW) (Orsi et al., 1995).Although this water bearsa saltysignature originally from NorthAtlantic Deep Water,it has spent some time circumnavigating Antarctica and has mixed with fresher Antarcticwaters. It is notrecently ventilated so ischaracterisedby ahighnutrient and low oxygenconcentration at potential densities, s0, of 27.35–27.75(Sievers and Nowlin, 1984).A23 oxygen data (Figs. 2d and 6b) con Ž rm thatstation 41 marksthe southernmost extentof UCDW. Tothenorth, there is aclearlow oxygen core ( ,200 mmol kg21) at gn 21 ;28 (s0 ; 27.7).Tothe south, the oxygen minimum is greater than 200 mmol kg and n occursat greater densities, g ;28.2 (s0 ; 27.85).In the core of the WSC, the baroclinictransport is small,but there is signi Ž cantbaroclinic transport associated with the 2002] Heywood& King:Water masses & baroclinictransports 655

Table3. Transports of watermasses associated with fronts and gyres. Most acronyms are de Ž ned in Table1. S. limband N. limbare parts of theWeddell Gyre. RGR isRio Grande Rise. The  ow in theVema Channel is dividedinto an easternpart and a westernpart identi Ž edastheBCR. Also identiŽ edarethe Subtropical Gyre (STG) andthe Weddell-Scotia Con  uence(WSC). The ACC is thesum of SACCF, PFandSAF. Ablankindicates that the water mass is notpresent for those stations TotalSW AAIW UCDW NADW LCDW WSDW WSBW stations S. limb 219.2 22.5 28.0 28.4 20.4 3–23 ASF 24.0 22.1 21.6 20.3 0.0 3–11 N. limb 5.7 0.3 2.0 2.8 0.7 23–39 SB 16.1 1.2 2.6 10.4 1.8 39–44 SACCF1 16.52.1 6.7 7.5 0.2 46–53 SACCF2 212.9 21.7 25.1 26.1 20.1 53–60 SACCF3 14.21.1 5.00 7.5 0.6 60–63 PF1SAF 123.919.6 16.3 46.9 12.3 25.9 2.9 73–82 PF 66.9 15.4 30.10.0 19.5 1.9 73–79 SAF 57.04.2 16.3 16.8 12.3 6.4 1.0 79–82 STF1 21.55.0 8.3 5.5 2.4 0.5 20.1 86–90 STF2 229.9 24.0 29.1 28.0 26.4 22.4 20.2 90–92 STF3 24.25.0 8.3 6.4 3.9 0.6 0.0 92–94 BCF 42.817.7 9.8 7.4 6.6 1.3 0.0 94–100 DF 2.6 1.6 20.9 20.91.0 1.1 0.7 100–102 DF to RGR 27.9 24.2 21.5 20.8 0.5 21.2 20.7 102–111 Vema 15.310.1 0.9 0.4 21.7 4.5 1.1 111–128 Vema east 27.5 24.5 22.7 22.2 20.8 3.0 20.4 111–122 BCR 22.914.6 3.6 2.6 20.9 1.5 1.5 122–128 WSC 16.4 1.3 2.6 10.6 1.9 32–44 ACC 141.421.6 16.3 54.1 12.3 34.2 3.0 46–82 STG 61.324.9 15.9 10.4 8.4 1.2 0.5 82–102 Total 211.951.5 31.7 66.6 19.5 42.4 0.0 0.3 3–128

SB,16Sv between stations 39 and 44 relative to a DCLreferencelevel (Figs. 2j and 3; Table3). This is more than suf Ž cientto provide the remaining eastward transport of WeddellGyre waters tobalance the southern limb (Table 3). TheSB exhibitsa markedchange in silicate, at depthsbetween the surface and 3000 db (Fig.2e). Silicate is as muchas 30 mmol kg21 lowerat thesurface in theACC in  uenced waters,than the waters ofthe Weddell Sea. At andbelow 3000 dbar, there is nosigni Ž cant gradient.Although it is not obvious from thesection plots, examination of bottle data showsthat in the upper 3000 db, the waters northof the SB arealso depleted in neon, nitrate,phosphate and enriched in d3He. At station45 we encounteredan eddy. At intermediateand deep depths, it containswater from southof theSB. Its deepwater mass characteristics(Fig. 6a) arevery similar to those ofstation41 at the core of theSB. Sinceit occurred at only one station, its width along sectionmust be lessthan 40 km, but we mayhave only grazed the feature so its diameter 656 Journalof MarineResearch [60, 5

Figure3. Cumulative geostrophic transport across A23, in water mass layers as de Ž nedby Figure 2(c),plus the total, all relative to ourpreferredreference level.

maybe greater. In the near surface layer ( ,150db), the potential temperature and salinity areclose to those of the adjacent stations, north of the SB (Fig.6a). This suggests that station45 sampledan eddy, rather than a meander.During the period since the eddy was shedfrom theSB, it has been capped by a layerof northernorigin water. Unfortunately owingto aproblemwith the CTD cableat station45, we haveno water samples from this station.This cyclonic eddy has asigni Ž cantazimuthal baroclinic transport of ;8 Sv, and is clearin geostrophicvelocities just north of themarked SB (Fig.2j). d.Southern ACC Front TheSouthern ACC Front(SACCF) was crossedthree times. The Ž rst crossingof the frontlay south of SouthGeorgia at ;55°309S(betweenstations 49 and 50). Stations 52 and 53were occupiedon thecontinental shelf of theisland. The SACCF wrappedanticycloni- callyaround South Georgia and was observedwith westward velocities at 53 °409S (betweenstations 58 and 59). It then retro  ectedand returned eastward at ;52°459S (betweenstations 60 and 61). The baroclinic transport relative to theDCL was ;15 Sv at eachcrossing (Fig. 3). The section presented by Tsuchiya et al. (1994)also crossed a retro ectingSACCF justnorth of South Georgia, although they do not identify it. Their sectioncrossed A23 at aboutthis region. Their temperature and density sections suggest thattheir station 274 contains water from southof theSACCF, showing the sameincreased oxygenin the oxygen minimum layer and a domingof theisotherms and isopycnals. This 2002] Heywood& King:Water masses & baroclinictransports 657

Figure 4(a)–(c).Water mass characteristics across the Weddell Gyre. Neutral density boundaries betweenwater masses are marked. Stations 3 – 8inblacklines, stations 9 –22in grey dots, stations 23–31inblackdots. The Winter Water layer (a), Warm Deep Water core (b) and deepand bottom waters(c) areshown expanded. locatesthe SACCF near52S althoughtheir station spacing was slightlytoo sparse to locate itaccurately.

The umin layerhas distinct properties on eitherside of theSACCF (Fig.7). To the south (andat stations 59 and60) it iswarmer ( ;0.7°C)andmore saline ( ;34.2),whereas to the northit is cooler (0 to 0.5 °C)andfresher ( ;34.0).To the north of thefrontit liesat a depth 658 Journalof MarineResearch [60, 5

Figure 4(d)–(e).Changes in Warm Deep Water characteristics between 1973 (grey) and 1995 (black).(d) in  owtotheWeddell Gyre, southeast Weddell Sea (e) out  owoftheWeddell Gyre, northernWeddell Sea.

of ;100dbar, while to thesouth the umin layerlies at ;150dbar. The temperature of the umin layerwas acriterionfor thecircumpolar location of theSACCF derivedby Orsi et al.

(1995).They suggested that the umin layershould be ,0°Csouthof theSACCF, but on

A23 the umin layersouth of the front (red curvesin Fig.7) isnot colder than 0 °C. The umin layeris only less than 0 °Csouthof theSB (Fig.6). We concludethat this criterion does not applyin the Georgia Basin. Thereis aclearsignal of theSACCF inseasurface salinity, which decreases from saline southernwaters (S ; 34.0)to fresher northernwaters (S ; 33.8).The reverse pattern is seenin thepotential temperature at the umax layer,which increases northwards across the front from ;1.7 to ;2.1°C(Fig.7). Orsi et al. (1995)suggested umax 5 1.8°Casone ofthe criteriato locatethe (northern boundary of the)front, but as can be seenin Figure 7, thisis nota usefulcriterion at this longitude. Although the criterion is approximately ful Ž lled, thereis no clear distinction in the umax properties.On A23, the change of salinityof the umin isa muchmore distinct and unambiguous de Ž nitionof the SACCF. Waters to the southof the SACCF exhibitincreased silicate and CFCs throughout the water column comparedwith waters tothe north of thefront (Figs. 2e, 2g, 2h). 2002] Heywood& King:Water masses & baroclinictransports 659

Figure5. Water mass characteristics across the Weddell Front. Neutral density boundaries between watermasses are marked.Stations 29 –32,south of theWF, ingrey dots. Station 33, in the WF, in blackline. Stations 33 –39,north of theWF, inblack dots. Surface values south of thefront, circles, andnorth of the front, asterisks. e.The Polar and Subantarctic fronts Whitworthand Nowlin (1987) describe the two fronts bordering the Polar Frontal Zone, theSubantarctic Front (SAF) tothe north and the Polar Front (PF) tothe south. They report,to the south of the PF, thepronounced umin andsalinity minimum of the Winter Water at ;100m, descendingsharply to 300 –400m atthePF. Inthe Polar Frontal Zone, thesalinity minimum is abovethe umin,eitherat the surface or weaklyat ;200–300 m. DuringA23, stations were beingoccupied in the vicinity of the PF whena medical evacuationforced us to break off atstation 71. On returning a weeklater, stations were repeatedsouth of thePolarFront (Fig. 8) sothat a synopticcrossing of boththe PF andSAF was obtained.On both crossings there is an indication of awarm eddyat ;50S of water whoseproperties correspond to thePolar Frontal Zone (stations 67 on the Ž rst crossingand 76onthesecond crossing; Figs. 8 and9). The eddy is notpresentat adjacent stations, but it iscappedby watersimilar to adjacent stations in thesurface layer (Fig. 9). Gouretski and Danilov(1994) document similar warm coreeddies shed by thePF intheeastern Weddell Seabut point out that cold core eddies just north of thePF appearto be morefrequently observed.This anticyclonic eddy transports at least 3 Svazimuthally. Thecore of thePF liesbetween stations 78 and 79 ( ;49S)where the isotherms plunge 660 Journalof MarineResearch [60, 5

Figure6. Water mass characteristics across the Southern Boundary of theACC. Stations39 – 40 in blue,station 41 incyan,stations 42 –43inred.(a) u-Salsoshowing station 45 (green), which has characteristicssouth of thefrontin thelower water column, but characteristics north of thefrontin theupper water column. Neutral density boundaries between water masses are marked. (b) Neutral density-dissolvedoxygen concentration showing southern limit of the low oxygen characteristic of UCDW, markingthe SB. downsharply to thenorth (Fig. 9b). Because the PFandSAF areso close here, station 79 is theonly one clearly within the Polar Frontal Zone. The SAF liesbetween stations 79 and 80 (48°459S;Fig.9d) where the subsurface salinity minimum of Antarctic Intermediate 2002] Heywood& King:Water masses & baroclinictransports 661

Figure7. Water mass characteristics across the Southern ACC Front.Neutral density boundaries betweenwater masses are marked. Stations 42 –44 and 46–49,south of the SACCF, inred. Stations 50–58,around South Georgia and north of theSACCF, inblue. Stations 59 –60, north of SouthGeorgia and in the retro  ectionof the SACCF, ingreen. Stations 61 –64,north of the SACCF, incyan.

Water(AAIW) beginsto descend.The salinity minimum plunges from ;350m atstation 79 (;49S) to ;550m atstation81 ( ;48S),a rateof ;2mperkm. Station 79, which is in thePolarFrontal Zone, was arepeatof station71, whichis clearly south of, but close to, the PF.Inthe seven intervening days, the PF hadmigrated south by ;40 km. Watermass propertiesshow the uniformity of the waters wellto the south of the PF (greydotted pro Ž lesin Fig.10) and to the north of theSAF (blackdotted pro Ž les in Fig. 10).In contrast, the stations within 100 km to the south of the PF (black)show a high degreeof variabilityand interleaving (stations 71, 73 –78).This may indicate the in  uence ofeddies, or lateral exchange across the fronts further upstream. Surface waters can becomedecoupled from deeperlayers and be displacedmeridionally by Ekmantransport. Thusthe sharpest meridional gradient of seasurface temperature need not be collocated withthe subsurface PF (stations74 –75cf.stations 78 –79,a separationof ;125 km). The effectof this confused region of interleavingis tomakethe classical de Ž nitionof thePF lessuseful here. Whitworth and Nowlin (1987) look for thedescent of a umin,butit isnot easywith the A23 data to identify a clearcore layer descending. There is a similarproblem withsimply seeking the northern limit of theWinter Water umin,theapproach adopted by 662 Journalof MarineResearch [60, 5

Figure8. Bathymetry in the region of the Polar and Subantarctic Fronts, showing the repeated stationsbefore and after the medical evacuation, passing through the gap in theFalkland Ridge. The5500, 5000 and 4500 m contoursare plotted. Depths shallower than 5000 m areshaded grey; depthsshallower than 4500 m areshaded dark grey.

Petersonand Whitworth (1989), but station 78 isthenorthern limit of the2 °C isotherm. Gordon et al. (1977)previously noted the complexity of de Ž ningthe PF intheScotia Sea, andthis prompted them to introducethe concept of thePolar Frontal Zone, of whichthe northernboundary is theSAF. Arhan et al. (1999)discuss the deep water masses observed on A23 and A17 in the vicinityof theFalkland Ridge. A23 passed through the gap in the Falkland Ridge at 36W, 48°559S(stations71 or 79 on the two crossings respectively, Fig. 8). Whitworth et al. (1991)placed moorings to monitorthe boundary current of WSDW  owingwest along the northern  ankof the ridge entering the Argentine Basin. They suggest that the ACC fronts meandermeridionally here; when they lie close to the Falkland Ridge, as was thecase whenwe observedthem on A23,the westward WSDW  owisnarrower andweaker than whenthe ACC liesfurther to thenorth. They also speculate that a morenortherly location for theACC allowsa  owofcoldGeorgia Basin water to travelthrough the Falkland Gap, 2002] Heywood& King:Water masses & baroclinictransports 663

Figure9. Sections across the Polar and Subantarctic Fronts. Station numbers are labelled on top axes.(a) Potential temperature [ °C], Ž rstcrossing.(b) Potentialtemperature [ °C],secondcrossing. (c)Salinity, Ž rstcrossing. (d) Salinity, second crossing. whereasa moresoutherly location might allow warmer water from theArgentine Basin to enterthe Georgia Basin. The presence of theeddy of PFwaterobserved during the two crossingsof thisregion implies that this exchange may have occurred recently. Thedeep and bottom waters southof thePF exhibithigher CFCs than those north of the SAF(Fig.11) aswas shownpreviously along the FalklandEscarpment by Whitworth et al. (1991).Station 79, lyingbetween the PF andthe SAF, showsthe highCFCs of the Georgia Basinin the waters above0 °C,butbelow that level, the bottom water is less recently ventilated,more like those of the Argentine Basin. Bottom velocities deduced from shipborneADCP data(Arhan et al., 1999)suggest that the bottom water at stations80 – 82 ismovingwest as acorealong the Falkland Escarpment. CFC valuesin thebottom water implya componentthat has been more recently ventilated than the recirculating WSDW in theArgentine Basin, but the fact that they are lower than in the Georgia Basin implies a signiŽ cantadmixture of thisolder, recirculating water. Tothe north of theSAF, thesalinity and temperature maximum of NorthAtlantic Deep

Water(NADW) isdistinct(S max ;34.8;Fig. 10). Further west of A23,CTD dataacross the PFandSAF atthe Falkland Plateau (Arhan et al., 2002)show little in  uenceof NADW 664 Journalof MarineResearch [60, 5

Figure10. Water mass characteristics across the Polar and Subantarctic Fronts. Neutral density boundariesbetween water masses are marked. Stations 50 –58, 61–66, 68–70,73, southof thePF, ingrey dots. Stations 67, 71,74 –78,in the PF, inblacklines. Stations 79 –81,in the SAF, ingrey lines.Stations 82 –87, 91–92,north of theSAF, inblack dots. southof theSAF. Howeverat A23 there is strongevidence of interleavingand intrusionsof NADW in,and just to thesouth of, the PF (blackand grey lines in Fig. 10). This suggests thatbetween the FalklandPlateau and A23 there has beensubstantial mixing southwards of NADW. Itis this mixing that has caused the ambiguity in de Ž ningthe precise location of thePF, hencethe black pro Ž lesin Figure10 thatdenote a strongin  uenceof waters north ofthePF. Themeridional, cross-frontal exchanges at this depth occur over the same group ofstationsin Figure 10 asthe interleaving discussed earlier. Usingtwo hydrographic sections in the Argentine and Georgia Basins, Peterson and Whitworth(1989) locate the PF at49 –50Sbetween 35 and 40W. They note that at this longitudethe PFandSAF arein closeproximity, something also seen in analtimetricstudy ofthetwo jets (Gille, 1994). The baroclinic transport of thePF ismerged with that of the SAFatA23. Between stations 77 and 81 there is 100.5 Sv of baroclinic transport, over a distanceof only ;150km. The PF carriesa transportof 67Sv, and the SAF 57Sv (Table 3).Read and Pollard(1993) calculated baroclinic transport at 33E for thecombined PF and SAFtobe73 Sv.Their maximum surface geostrophic speed was 73cm s 21 inthe merged frontwhile ours was 49cms 21 (Fig.2j); clearly both speeds will depend critically on the angleat which the front is crossed, and the station spacing, so the difference is not 2002] Heywood& King:Water masses & baroclinictransports 665

Figure11. CFC concentrationsin the deep Georgia and Argentine Basins. Stations 73 –78, Georgia Basin(circles); station 79, Falkland Gap (plusses); stations 80 – 87,Argentine Basin (dots).

signiŽ cant.Our combinedPF andSAF transportis 124Sv (Table 3), of whichthe majority (73Sv) is,not surprisingly, UCDW andLCDW. LCDW isthe densest portion of the CircumpolarDeep water that circulates around Antarctica with the ACC. Itis identi Ž ed by itswarm saltycore ( u . 0°C;Fig.2a) beneath the oxygen minimum of UCDW (Fig.2d). Tsuchiya et al. (1994)document crossings of the PF (49.5S)and SAF (45S)at a longitudeof ;25W.Although they do notdiscuss u-Sdiagrams,they identify water mass changesat eachfront. It is surprising that they observe the PF andSAF tobemuchmore widelyseparated than on A23.However their Figure 1 indicatesthe southernmost crossing oftheSAF justone stationnorth of thePF, butthenmeandering further north. Thus the two frontsare collocated on their section also, but must retain their individual characteristics duringsubsequent meandering. We donotobserve a meanderof theSAF asobserved by Tsuchiya et al. f.Subantarctic Zone, Subtropical Gyre, Brazil Current Front and exchange with the BrazilBasin Northof theSAF, we dividethe waters originatingfrom theNADW intothree layers (Fig.2c; Table 2). Beneath the recently descended AAIW liesthe low oxygen signature of UCDW, clearlyvisible beneath the highly oxygenated AAIW (Fig.2d). UCDW isalso clearlyidenti Ž edby itshigh phosphate signature ( .2.25 mmol kg21;Fig.2f) andby high 666 Journalof MarineResearch [60, 5 d3He (Fig.2i). In a general,basin averaged sense, UCDW mustbe moving northward. BeneathUCDW liesNADW, spreadingsouthward overall, identi Ž edby its salinity maximum(Fig. 2b) and oxygen maximum (220 –240 mmol kg21)(Fig.2d). NADW exhibitsa lownutrient and low CFC signature,and minima in helium and d3He (Fig.2i). It marksa minimumin potentialvorticity, seen in widely spaced isotherms between 2 °C and 3°C(Fig.2a). LCDW isdenseenough to movenorthward beneath NADW. Althoughin the WeddellSea this water mass (referred tothere as WDW) ismarked by a temperature maximum,by the time it has moved north into the Argentine Basin it doesnot exhibit a localmaximum in u butit shows a local d3He maximumand oxygen minimum (Figs. 2d and2i). We takeits lower boundary at gn 5 28.26for consistencywith the de Ž nition furthersouth on A23, although it should be recognised that mixing of LCDW withthe WSDW beneathit isinevitable. As thesetwo Antarctic water masses penetrate northwards, theyare often referred tocollectivelyas Antarctic Bottom Water. TheSubtropical Front (STF) marksthe boundary between the waters ofthe subtropical gyreand the colder, fresher subantarcticwaters tothe south. The STF is Ž rst encountered betweenstations 87 and 88 (44 –45S).The STF meanders,because stations 91 and 92 containwater of the southern (subantarctic) type. The front is seen in both near surface temperature(increasing from 12.7 °C to 15.0°Cbetweenstations 87 and 88) and salinity (increasingfrom 34.01to 34.16between stations 87 and88), and in theproperties at the salinityminimum marking the core of theAAIW (Fig.12c). AAIW tothenorthof the STF exhibitsa similartemperature but is moresaline, increasing from ;34.18 to ;34.22. The deepand bottom waters donot show signi Ž cantlycontrasting temperature and salinity acrossthe STF, althoughthere is a widevariety of values of the salinity maximum denoting theNADW core.Between the SAF andthe Brazil Current Front (BCF) thesalinity maximumvaries between 34.78 and 34.88. Roden (1986) Ž ndsthat mixed layer tempera- tureand salinity compensate in density across the STF (whichhe refers toas the SAF), producingweak baroclinic shears of horizontal velocity in the upper ocean. The A23 sectionhowever reveals signi Ž cantbaroclinic shears in eachcrossing of theSTF (Fig.2j). Deacon(1982) noted that the STF canbe identi Ž edby large surface temperature and salinitychanges. Orsi et al. (1995)show properties at 100 m (toreduce the effects of seasonality)to change from 10to12 °Cand34.6 to 35.At thelongitude of A23, Deacon (1982)locates the STF at ;43S,and the transitional band drawn between 41 and43S by Orsi et al. (1995)concurs with this. Tsuchiya et al. (1994) Ž ndthe STF tohave strong baroclinicshear and locate it at41Son their 25W section. Peterson and Stramma (1991; theirFig. 18) show the STF atalatitudeof ;40Sat thelongitude of A23,so our crossings areseveral hundred km furthersouth. On A17, west of A23,in 1994the STF meandered andwas crossed Ž vetimes;its southernmost crossing was at46S(Memery et al., 2000). Wesuggestthat the meander seen on A23 is associated with the eastern end of thedeep circulationaround the Zapiola Rise, drawing the STF roundanticyclonically. Inagreement with Roden (1986) we Ž ndsurface geostrophic velocities in theSTF tobe ;20 cm s21 inthe Argentine basin, smaller than those of thePF, SAForBCF (Fig.2j). 2002] Heywood& King:Water masses & baroclinictransports 667

Relativeto the DCL, theA23 crossings of theSTF givea nettransport of ;15 Sv, with transportsof 20to30Svin eachcrossing (Table 3). These values are substantially smaller thanthe valuesquoted by Petersonand Stramma (1991)of ;30Svfor theSTFtransportin theupper 1000 m relativeto 3000 m. Itis conceivablethat they included transport of the BCFsincethey did not expect this front to appear so far east.Tsuchiya et al. (1994) calculatedSTF transportrelative to theDCL of56 Sv.The transport on A23is primarily contributedby UCDW, NADW andAAIW (Fig.3; Table3). TheBCF isseenbetween stations 96 and97, at ;38S.Isohalines and isotherms in the top1000 db plungesharply (Fig. 2), as noted by Roden(1986) as an indicatorof thisfront. Themap drawn by Petersonand Stramma (1991)implies that the A23 section should not havecrossed the BCF. Tsuchiya et al. (1994)however document a crossingof theBCF duringtheir hydrographic section at 25W, at 34 –36S,and note that this is likely to bethe samefeature identi Ž edintheCape Basin at 10W as theSouth Atlantic Current Front by Gordon et al. (1992).On A17 the BCF isobserved at 39S(Memery et al., 2000),very close toitslatitude on A23(38.5S). It appears that the fronts in theArgentine basin were further southin 1994/1995than climatologies suggest. Peterson and Stramma (1991) suggest that thereis aseasonalityin thepositionof thesubtropicalgyre, ;5° furthernorth in winterdue tothe zero wind stress curl being ;5° furthernorth, so it is possible that this apparent interannualchange is caused by seasonality. Roden (1986) Ž ndsthe BCF tobe a deep reachingfront, identi Ž ableto below 3000 db. However on A23 there is littlesignature of theBCFintheproperties of theAAIW layerand below (Fig. 12c). While the STFhaslittle signaturein the nitrate and phosphate Ž elds,the nutrients in theupper 500 m aregreatly depletednorth of the BCF. Campos et al. (1999)show signi Ž cantincreases in iodide concentrationson A23in tropical waters northof theSTF. Thetransport associated with the BCF is ;28Svrelativeto 2500 m andan impressive 43Svrelative to the DCL betweenstations 96 –98(Fig.3). Across thewhole BCF region (stations94 –100),we obtaina transportof 43 Svagain, with two small recirculations on eitherside of the main BCF jet(Table 3). More than half of this transport is of surface waters,the remainder split ;equallybetween AAIW, UCDW andNADW. Gordon(1989) calculatestransports of theBrazil Current further west than A23, relative to 1500 m, of ;21Sv. Our transportrelative to 1500 m is18 Sv (between stations 96 and 98; 15 Sv betweenstation pair 96 –97alone).Although the sections he usesare zonal (38S) and ours ismeridional, the Brazil Current has turned offshore between the two sections, and we concludethat the transport is remarkably similar on both sections. Relative to deep referencelevels ( ;3000m), southwardtransports of theBCF at37 –38Sof about70 Sv havebeen calculated (Peterson and Stramma, 1991), so the 43 Svon A23is notunusual. Tsuchiya et al. (1994)at 25W determined the transport associated with the BCF tobeonly 16Svrelative to the DCL. Thus whereas they argued that the STF was theprimary means for eastward  owofthewaters oftheBrazil Current after it hasseparated from thecoast, duringA23 we Ž ndthat the BCF carriesthe majority of thistransport. The difference is eitherbecause A23 is fartherwest than their 25W section, or perhapsmore likely, there is 668 Journalof MarineResearch [60, 5

Figure12. Summary of potentialtemperature —salinitycharacteristics for fronts and zones crossed onA23.(a) South of thePolar Front. Stations 13 –32in blue,34 –40ingreen, 42 –44 and 46– 49 in cyan, 50–58 and 61– 66in red. (b) North of theSouthern ACC Front.Stations 50 –58 and 61–66 in red, 76–79in magenta, 82 –87 and 91–92in blue, 88 –90 and 93–96in green, 97 –100 in cyan, 102–123in yellow and 124 –128in black. (c) As bbutexpandedscales for clarity. 2002] Heywood& King:Water masses & baroclinictransports 669 temporalvariability of the relative strength of the two fronts. It is clear from comparing Figure2 withTsuchiya et al.’ssectionsthat isopycnal slopes are greater at theSTF thanat theBCF ontheirsection, and the other way round on A23in 1995. Afurtherfront, named the Deep Front (DF) byMcDonagh et al. (2002),is located at station101 (34 °229S). Its signatureis anincreasein thesalinity of theAAIW core(from 34.23to 34.30), and an increasein thesalinity of theNADW salinitymaximum. On A23 it ismarkedby thesouthern limit of thedeep34.9 isohaline. Although it was notdiscussedby Tsuchiya et al. (1994)it isapparent in theirsection also at 31S.Tsuchiya et al. attributethe changesin AAIW propertiesat theDF tothe BCF. AlthoughMcDonagh et al. (2002) name ittheDeep Front, there is alsoa signaturein surfacetemperature, which increases sharply from ;19°C to ;22°C.Tothe north of the DF, theproperties lie close to alinebetween NADW andWSDW, whereasto the south, the deep u-Sisrather fresher dueto the inclusionof ahigherproportion of LCDW(replacingNADW), addedduring the passage of thedeep western boundary current around the Argentine Basin (Fig. 12c). Thegeostrophic shear associated with the DF (Figs.2j and 3) isconsiderably weaker thanthat of theBCF. It represents only 2.5 Sv of transport(Table 3). It may represent the axisof theSubtropical Gyre. Tsuchiya et al. (1994)refer tothecentre of thesubtropical gyrein this basin at ;32S.Thus it maymark the boundary between the westward  owing surfacewaters tothenorth and the eastward  owingsurface and intermediate waters tothe south(Boebel et al., 1997,their Fig. 19). AAIW  owingeast is closer to its formation region,has a morepronounced salinity minimum, is higher in CFCs(Figs. 2g and2h) and isthereforemore recently ventilated. AAIW  owingwestward, north of theDF, ismore salineand older. Onthe southern  ankof theRio Grande Rise we observeNADW thathas  owed south throughthe Vema Channeland turned eastward following the topography. It is identi Ž able particularlyin oxygen, salinity and silicate (Fig. 2). Thus far, we havehad no basis to departfrom usingthe DCL asourreferencelevel for geostrophictransport, since the shear isgenerallyof onesign throughout the water column. At theDF howeverwe changeour assumedlevel of no motion from theDCL tothe boundary ( gn 5 27.92)betweenthe NADW, assumedto be movingeast, and the UCDW, assumedto bemovingwest with the surfaceand intermediate waters ofthesubtropical gyre (Boebel et al., 1997).We continue tousethe DCL whenthat surface is not available. Nearlyidentical water masses are encountered either side of the Rio Grande Rise (stations102 –110 and 111–123;Fig. 12). The AAIW inthe Vema Channelis slightlymore saline(34.32) than that to the south of theRio Grande Rise (34.30). Although the surface layertemperature and salinity begin to increase from station120, there is nodeep water frontuntil between stations 123 and 124. There the temperature of thesalinity maximum (andits salinity) increases markedly to form asubsurfacetemperature maximum denoting acoreof NADW (Fig.12). Note the discontinuity in the slope of the u-Scurveat 34.9, 2.3°Ctowardthe warmer NADW, for thestations in thewestern Vema Channel.This is alsoseen on A17(Memery et al., 2000). 670 Journalof MarineResearch [60, 5

Thefront in theVema Channel(marked BCR) isthesame feature as thatseen at 20 –22S by Tsuchiya et al. (1994),identi Ž edparticularlyby a plungingof the3 °Cisothermfrom ;1300 m to ;2300m. They identify it as the boundary between the anticyclonic subtropicalgyre and the cyclonic subequatorial gyre, and call it the Subtropical- SubequatorialFront. It is associated on both sections with a sharpincrease in thesalinityof

NADW, andshallowing of S max.Innear surface waters (aboveand including AAIW), A23 showslower dissolved oxygen concentrations north of thefront (Fig. 2d), as observedby Tsuchiya et al. We thereforeargue that the westward current associated with the Subtropical-SubequatorialFront later turns south at the coast (as partof the subtropical gyre)to becomethe Brazil Current travelling southwards. Our fronthowever is associated withisopycnals sloping the other way, so denotes a northeastwardcurrent. We arguethat thisis arecirculationcell of theBrazil Current, which we hadnot yet crossed when the A23 sectionended. The WOCE sectionA10 data at 30S plotted by Boebel et al. (1997) also showthis front associated with a recirculationof theBrazil Current. It can be identi Ž ed at 43Win the plunging of the 3 °Cisothermand 34.8 isohaline (their Fig. 3). Their geostrophicvelocities show southward  owatdepthsof NADW/AAIW andnorthward at thesurface (like ours) with the southward  owingBrazil Current farther inshore (their Fig. 11).The Brazil Current is con Ž nedto thecontinental shelf (Peterson and Stramma, 1991; theirFig. 18) closeto the2000m depthcontour and we clearlyhad notyetencountered it at theend of A23. TheAAIW inthe Vema Channelexhibits a smallsubsurface anticyclonic lens or vortex atstations 117 and 118 visible in both temperature and salinity centered on a depthof 1000m. It is detectablebetween 500 and 1500 m (Fig.2b). The core of thelens is marked bysalinityminima that are fresher (by ;0.02)than those at the surrounding stations, and aremore akin to those south of the Rio Grande Rise. We speculatethat this is a ‘SubmesoscaleCoherent Vortex ’ (McWilliams,1985), of which Meddies are the most commonexample. It is seen at only two stations in water mass properties,although its in uenceon thevelocity Ž eldextends over about 5 stations.It has a transportof 0.6Sv to thenortheast between depths of 700 –1500m atstations 114 –116, and 21.6 Sv to the southwestat stations 116 –118.These vortices are usually anticyclonic, with a velocity maximumin the deep ocean, and a strati Ž cationminimum in the centre. McWilliams arguesthat they are formed by diapycnal mixing events following geostrophic adjustment. Alikelysource of mechanicalmixing in thiscase is thefrictional boundary layer around theRio Grande Rise. We speculatethat the lens formed on the eastern  ankof the Rio GrandeRise and circulated anticlockwise around the Rise with the AAIW  ow (Boebel et al., 1997). Thetransport of water throughthe Vema Channelmeasured on A23is discussedin detailby McDonag h et al. (2002).They conclude that care should be taken in referencingthe  owusing an intermed iatelevel of nomotionde Ž nedby water mass boundaries,because there are complex recircula tionson the  anksof the channel. In theVema Channelwe chosea levelof nomotionbetween the NADW andthe LCDW, 2002] Heywood& King:Water masses & baroclinictransports 671 gn 5 28.11,since on average we expectthe NADW tomovesouth and the LCDW to movenorth. This is close to a referencelevel taken at u 5 2°C(Fig.2a) often used. Thisimplies a northwardnet  owof WSDW of1.2 Sv (stations 120 –122, in the deepestpart of theA23 crossing) ,takinginto account a smallnorthward recircula tion of0.3Sv in theeastern Vema Channel(station s120 –121).The net northward  ow of LCDWthroughthe Vema Channelis 4Sv,so the total transport of AntarcticBottom Water(WSDW plusLCDW) is5.2Sv. g.Transports across the whole section Cumulativebaroclinic transports along A23 were calculatedrelative to a varietyof referencelevels. Our preferredresults use the DCL southof theDF (station101), and the watermass boundariesdiscussed above for theregions between the DFandtheRioGrande Rise (gn 5 27.92)andfor theVema Channel( gn 5 28.11).Using this preferred velocity Ž eld,transports of individual water masses, de Ž nedusing the isopycnal boundaries of Figure2c, are shown in Figure3 andTable 3. We havetaken account of missing bottom trianglesby simply extrapolating the velocity at the deepest common level downwards (obviouslythis is onlyrelevant where we donothave a levelof nomotionat thisdepth). Thetotal transport through the sectionis 212 Sv. Without the intended section across the BrazilCurrent, we cannotclose the transport of thesubtropical gyre. Indeed, in the Vema Channelthe geostrophic transport at the end of thesection is showingnortheastward  ow associatedwith the recirculation cell of the Brazil Current. Relative to 1500 m, ourBCR hasa transportof 23Sv.This is surprisingly large compared with the transports of ;10 Sv intheBrazil Current going south listed in Petersonand Stramma ’sreview(their Table 2). Thistransport is almost entirely composed of surfacewaters. If we sumall the eastward  owof thesubtropical gyre including the STF, BCFandDF, uptoandincluding the DF, we obtain61 Sv. This is therow labelledSTG inTable3. Thismust also equal the return  owof thesubtropical gyre, between the DF andthe Brazilian coast. From theDF tothe endof thesection, there is a neteastward  owof 7Sv,which leaves a surprisinglylarge residualof 68Svfor theBrazil Current. The residual can be reduced by 19 Svby usinga referencelevel of 3000m ratherthan the DCL inthe Argentine Basin between stations 86 and111 (although there is noevidenceto supportsuch a change)but that still leaves 49 Sv. A signiŽ cantreduction of 30Svcan be achievedby using the DCL intheVema Channel. However,this does not seem acceptable because there would then be no AntarcticBottom Water  owingnorth. McDonagh et al. (2002)provide a moresophisticated analysis of the deepand bottom  owintheVema Channel.We thereforeconclude that a BrazilCurrent of 10Svis notconsistent with this dataset using any likely reference level. We alsoconclude thatthe reference level calculations summarised by Petersonand Stramma (1991) may be underestimates.Further in situ current observations in this region would be bene Ž cial. Thenet transport of WSDW acrossA23 is lessthan 0.1 Sv, so is essentiallyzero (Table 3).About 8.5 Svis travellingwestward in the southernlimb of theWeddell Gyre, but this is compensatedby returneastward  ows inthenorthern Weddell Gyre ( ;3Sv),in theWSC 672 Journalof MarineResearch [60, 5

(1.5Sv), in the PF/ SAF (3Sv) andin the Vema Channel(1 Sv). The WSBW calculations implya neteastward  owacross A23 of 0.3 Sv. Assuming zero velocity at the DCL is unlikelyto beappropriatefor theWSBWandWSDW layers,and we expecta neteastward  owof bothto be carriedin thenorthern Weddell Gyre. For boththe UCDW andLCDW, theprimary transport (54 Sv and 34 Sv respectively) is eastward with the ACC, and considerablyless with the Subtropical Gyre. Equal amounts of AAIW aretransported east bytheACC andthe Subtropical Gyre (16Sv). Table3 showsthat the ACC baroclinictransport, accumulated between stations 46 –82, includingthe SACCF, PF andSAF, is141 Sv. Cunningham et al. (2003) Ž ndbetween123 and144 Sv on 6repeatsof theDrake Passage SR1b section. Furthermore, they detect no trendin the Drake Passage transport between ISOS inthe late 1970 ’sandWOCE inthe mid 1990’s. Orsi et al. (1995)examined the historical hydrography in the Southern Oceanand found the shear transport of the ACC above 3000 m tobe ;100 Sv at all longitudes,between the STFandthe SB.Our valuewith this reference level is also100 Sv. Whitworthand Nowlin (1987) measured the ACC transportat theGreenwich Meridian to be162 Sv relative to the DCL including ;20Sv of transportat theACC-Weddell Gyre boundary.They argue that the transport is greater there than in Drake Passage because 15–20Sv of NADW crossesthe section, compensated by a  owback into the South Atlanticby Agulhas eddies. Read and Pollard (1993) at 33E obtain 138 Sv south of and includingthe SAF, withan additional 12 Sv in the Subantarctic zone, and up to an additional5 Svbetween their section and the Antarctic continent. The similarity of these baroclinicestimates implies that the baroclinicity of the ACC changeslittle across the SouthAtlantic. At theGreenwich Meridian, there is 52Svof transport south of thePF (Whitworthand Nowlin,1987), whereas at 33Ethis is increased to 65Sv (Read and Pollard, 1993). Read andPollard suggest that there has been a transferof wateracross the front. If we sumthe transportsof the SB andSACCF onA23, we obtain ;30Sv, which would endorse the gradualincrease of transporteastward. Further investigation is required to determine why thetotalbaroclinity seems to bepreservedeastward across the South Atlantic by increasing thatof the region south of thePF atthe same time as decreasingthat of thePF andSAF. Rintouland Sokolov (2001) report a netbaroclinic transport of 147 6 10Sv(1 standard deviation)from 6occupationsof SR3,the Australian choke point. This must include the totalcircumpolar  owplus the water carried by the Indonesian Through  owreturning from theIndian Ocean to thePaci Ž c.TheSR3 SAF transportwas 105 6 7Svand the PF exhibited2 corestotalling 29 6 8Sv.The division of transport into the two fronts appears verydifferent to A23,but it should be rememberedthat on A23only one station lay within thePolar Frontal Zone so the allocation of transport into the SAF andPF ishighly dependenton A23station location. On SR3 the SACCF exhibited2 corestotalling 29 6 6 Sv,which is considerably larger than on A23,but they do notallocate any transport to the SB.Includingthe A23 SB transportgives a valuein accordwith Rintoul and Sokolov ’s. 2002] Heywood& King:Water masses & baroclinictransports 673

4. Summary Figure12 summarizesthe watermass propertiesalong A23. Clusters of stationsindicate thezones between the fronts, and the fronts themselves are evident as clear gaps between thecurves. Consider Ž rst theupper panel, showing the southern portion of A23,up tothe

PF.Thewaters oftheWeddell Gyre (blue)exhibit the most saline umin (.34.4).Because A23was undertakenin austral autumn, near surface temperatures were closeto freezing andpancake ice was beginningto form. Thewaters ofthe Weddell-Scotia Con  uence

(green),between the WF andthe SB, havea lesssaline umin (;34.3)and a coldercore of WDW. Theseare in  uencesof the addition of shelf water from theAntarctic Peninsula (Whitworth et al., 1994)and of the mixing with waters tothe north and south in the eddy-richCon  uenceregion. The SB marksa sharpchange to ACCwaters,with umin no colderthan 0 °C.Southof theSB, theWDW exhibitsboth salinity and temperature maxima atsimilardepths. North of theSB thelocations of thesalinity and temperature maxima are nolonger coincident (red andcyan), with the subsurface temperature maximum lying abovethe salinitymaximum. At theSACCF (red) thisdistinction increases, and the salinity of the umin decreasesmarkedly (to ,34.0).Surface waters becomewarmer butalsofresher. Toorientate the reader, the redpro Ž lesbetween the SACCF andPF arethe same in each ofthethree panels. Consider now thelowertwo panels, showing the propertiesnorth of the SACCF.North of thePF (magenta)surface waters beginto increase in both temperature andsalinity. Across thePolar Frontal Zone the subsurface temperature minimum disap- pears.Because the PF andSAF arebarely separated on A23, we donot see a clusterof stationswith uniform u-Spropertiesin thePolar Frontal Zone. North of theSAF (blue)the subsurfacetemperature minimum is replaced by the subsurface salinity minimum of AAIW. Thesubsurface temperature maximum of LCDW disappearsand the curves are dominatedby the subsurface salinity maximum of NADW. Betweenthe SAF andthe STF, thetemperature monotonically decreases with depth; this is theonly portion of A23 that hasneithersubsurface temperature maxima nor minima.Across thesubtropical gyre, north oftheSTF, thereare stepwise increases in temperatureand salinity at each front, evident in surfaceproperties, in theAAIW salinityminimum and the NADW salinitymaximum. Figure13 summarizesthe variation of propertieson core layers along the extent of A23. Southof theACC, thesalinity maximum (dots) and oxygen minimum (plusses) are almost coincident,marking the core of the WDW layer.The out  owingWDW corein the northernlimb of theWeddell Gyre (northof station22) is denser than that of thein  owing southernlimb, since it has become colder during its passage through the ice-in  uenced southwestWeddell Sea. North of the SB, thesalinity maximum, locating the core of LCDW, andthe oxygen minimum, locating the core of UCDW, diverge,and are both at lesserdensities than in the Weddell Sea. Both have become warmer, withthe UCDW warmer thanthe LCDW; the UCDW corehas become fresher andthe LCDW coremore saline.Between the SBandthePF, thedensityof theLCDW coreremains constant at gn 5 28.1,butnorth of the SAF thedensity of thesalinity maximum decreases to gn 5 28.0; herethe core identi Ž esNADW. Thisshows the increase of densitybetween the southward 674 Journalof MarineResearch [60, 5

Figure13. Core values along A23. Salinity minimum layer denoting AAIW core(asterisks), salinity maximumlayer denoting NADW (dots),oxygen minimum layer denoting UCDW (plusses).

 owingNADW core,and its derived water mass LCDW thathas travelled around Antarctica,becoming denser in theprocess (colder but less saline, Fig. 12). The NADW in thewestern Vema Channelis less dense than that in the centre of the subtropical gyre, possiblydue to an interaction with warmer waterin the Brazil Current. TheUCDW core,identi Ž edby the oxygen minimum (plusses in Fig. 13), also shows a decreasein densitynorth of theSAF, andthereafter becomes steadily less dense throughout thenorthernhalf of thesection. This is due toasteadydecrease in salinity of thiscore and a veryslight increase in temperature, both suggesting mixing with the descending core of AAIW above,also moving northwards. The salinityminimum core (stars) liesat orcloseto theseasurface south of theSAF, atwhichit descends to form AAIW. At thefront there is a sharpincrease in the density of the salinity minimum, associated with a decreasein temperatureand an increase in salinity. North of the SAF, theAAIW corebecomes graduallydenser due to an increase in salinity and temperature, presumably caused by mixingwith the UCDW belowand surface waters above. Table3 summarizesthe transports of eachof thewater mass orfrontalzones on A23. TheWeddell Gyre hasa baroclinictransport of 19Svon A23. This is in accordance with previousdeterminations, which also noted the presence of a signi Ž cantbarotropic component.The threefronts of theACCtransport141 Sv,of which124 Sv is carriedby the PFandSAF thatare essentially merged at this longitude. These are within the range of valuesobserved at Drake Passage and other locations around the Southern Ocean. The SubtropicalFront transports 20 –25Sv,whereas the Brazil Current Front, just to the north, transports43 Sv;this contrasts with observations made in 1989when the transport of the latterfront was signi Ž cantlysmaller than the former. Thesubtropical gyre transport is 61 2002] Heywood& King:Water masses & baroclinictransports 675

Svinits southern limb. Since A23 did not cross the Brazil Current, we didnot measure the completenorthern limb of the subtropical gyre, but a BrazilCurrent transport signi Ž cantly greaterthan previous estimates of ;10Svwould be requiredto close the transport budget.

Acknowledgments. TheWOCE A23cruise was funded by NaturalEnvironment Research Council grantGST/ 02/575.We wish to thank all participants in the cruise, whose hard work produced the measurementsdiscussed here.

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Received: 22January,2002; revised: 30September,2002.