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Water Masses and Baroclinic Transports in the South Atlantic And 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 dene 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 theinowing 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 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 identi 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 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 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 narrows to 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 and the 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) acrossthe 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 batchP125. 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 esare markedon Figure1. Watermasses andfronts will be discussedinturnfrom southto north, with the former beingde nedin terms oftheir 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
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