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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. C5, PAGES 9005-9017, MAY 15, 2001 Export of Weddell Sea Deep and Bottom Water Arnold L. Gordon, Martin Visbeck, and Bruce Huber Lamont-DohertyEarth Observatory,ColumbiaUniversity,Palisades,New York Abstract. An extensiveset of conductivity-temperature-depth (CTD)/lowered acoustic Doppler currentprofiler (LADCP) data obtainedwithin the northwesternWeddell Sea in August 1997 characterizesthe densewater outflow from the Weddell Sea and overflow into the ScotiaSea. Along the outer rim of the Weddell Gyre, there is a stream of relativelylow salinity,high oxygenWeddell Sea Deep Water (definedas water between0ø and -0.7øC), constitutinga more ventilatedform of this water massthan that found fartherwithin the gyre.Its enhancedventilationis due to injectionof relativelylow salinity shelf water found near the northern extreme of Antarctic Peninsula'sWeddell Sea shelf, shelfwater too buoyantto descendto the deep-seafloor. The more ventilatedform of Weddell Sea Deep Water flowsnorthwardalongthe easternsideof the SouthOrkney Plateau,passinginto the ScotiaSea rather than continuingalongan eastwardpath in the northernWeddell Sea.Weddell Sea Bottom Water alsoexhibitstwo forms:a low-salinity, better oxygenatedcomponentconfinedto the outer rim of the Weddell Gyre, and a more saline,lessoxygenatedcomponentobservedfarther into the gyre.The more saline Weddell Sea BottomWater is derivedfrom the southwestern Weddell Sea,where highsalinityshelf water is abundant.The lesssaline Weddell Sea Bottom Water, like the more ventilatedWeddell Sea Deep Water, is derivedfrom lower-salinityshelfwater at a point farther north alongthe AntarcticPeninsula.Transportsof Weddell Sea Deep and Bottom Watermasses crossing 44øWestimated fromoneLADCP surveyare25 x 106and5 x 106m3 s-1, respectively. Thelow-salinity, betterventilated formsof WeddellSeaDeep and BottomWater flowingalongthe outer rim of the Weddell Gyre havethe positionand depth rangethat would lead to overflowof the topographicconfinesof the Weddell Basin, whereasthe more salineformsmay be forcedto recirculatewithin the Weddell Gyre. 1. Introduction Along the continentalmarginsof the Weddell Sea, Antarctica, from the Filchner Depressionto the tip of Antarctic Peninsula(Figure 1), densewater of varied characteristics descendsinto the deepocean[Gordonet al., 1993;Fahrbachet al., 1994, 1995; Wepperniget al., 1996; Mensch et al., 1996, 1998; Gordon, 1998]. Streamsof newly formed Weddell Sea Deep Water (WSDW, definedas deepwater with potentialtemperaturesbetween0ø and -0.7øC [Fahrbachet al., 1995]) and Weddell Sea Bottom Water (WSBW, definedas water colder than -0.7øC [Carmackand Foster,1975]) are carried by the westernboundarycurrent of the Weddell Sea into the northwest corner of the Weddell Gyre [Deacon,1979; Orsi et al., 1993]. From there these water massesflow eastward,either within the northern limb of the Weddell Gyre or reaching northwardinto the ScotiaSea [Gordon,1966;Locarnini et al., 1993], eventuallycoolingthe lower 2 km of the world oceanas Antarctic Bottom Water. The objectiveof this studyis to describe the characteristics of Weddell Sea Deep Water (WSDW) andWeddellSeaBottomWater (WSBW) withinthe 31 to September8, 1997, in the southernScotia Sea, northern Weddell Sea, and BransfieldStrait (Figure 2), as part of the internationalDeep Ocean Ventilation Through Antarctic Intermediate Layers (DOVETAIL) program [Muench, 1997]. Profiles of the water column from surface to near bottom were obtained at 97 sites using a SeaBird ElectronicsSBE9plus conductivity-temperature-depth (CTD) recorderwith an oxygensensorandwater samplersfor calibrationpurposesand for tracer chemistry.A lowered acousticDoppler current profiler (LADCP) wasattachedto the CTD. The ScotiaSea and Weddell Sea stationscharacterizethe thermohalineand oxygen concentration fields of the dense water outflow from the Wed- dell Sea and its overflow into the Scotia Sea. The LADCP data providea snapshotof the velocityfield at the time of the CTD stations.The BransfieldStrait data set is dealt with in a separate paper [Gordonet al., 2000]. 2.1. 44øW Section The approximatelymeridionalsectioncomposedof stations 1-32 along44øW(Figures2 and 3) crossesthe South Orkney Plateau separatingthe ScotiaSea to the north from the Wednorthwest Weddell Sea and their overflow into the southern dell Seato the south.The SouthOrkneyTroughis immediately Scotia Sea. north of the South Orkney Plateau.CircumpolarDeep Water 2. DOVETAIL Data (CDW), markedby a potentialtemperaturemaximumamaxof > 1.6øC,is introducedto the Scotia Sea by the Antarctic CirThe data set usedin this studywas collectedduring cruise cumpolarCurrent (ACC) from the PacificOcean. Accompa97-5 of the polar researchvesselNathanielB. Palmerfrom July nying the CDW amax,at a slightly deeper level, are salinity Copyright2001 by the American GeophysicalUnion. maximumand oxygenminimumcore layers.A streamof CDW entersthe WeddellGyre at the easternmarginof the gyrenear Paper number 2000JC000281. 0148-0227/01/2000JC000281509.00 30øE[Deacon,1979;Orsiet al., 1993].The amaxto the southof 9005 9006 GORDON ET AL.: EXPORT OF WEDDELL SEA DEEP AND BOTTOM WATER 40 50 ,30 Scotia SeaDrako DOVETAIL Passage Weddell Sea Figure 1. Distributionof Deep OceanVentilation ThroughAntarcticIntermediateLayers(DOVETAIL) program[Muench,1997]stationsobtainedfrom the polarresearchvesselNathanielB. Palmercruise97-5from July31 to September8, 1997,shownas dots.The DOVETAIL studyregionis delineatedby the dashedline. Stationsobtainedas part of the 1992Ice StationWeddell program[Gordon,1998],manyof whichare used in this study,are shownas open circles.The bathymetryis a smoothedcompositeof the Smithand Sandwell [1997]bathymetryandETOPO5, joinedat 72øS.The 1, 2, and3 km isobathsare shown.Arrowsdenotedeep and bottom flow pattern. The waters below the CDW and WDW progressivelycool, the South Orkney Plateau definesthe Weddell Deep Water (WDW), which is a modified remnant of CDW. WDW ad- decreasein salinity,and increasein oxygenas the seaflooris vected along the outer rim of the Weddell Gyre is generally approached.Gradientsare much enhancedwithin the lower cooler than 0.6øC in the northwestern Weddell Sea. Within the 500 dbarin the Weddell Sea,a trait whichincreasesin intensity WDW the salinitymaximumand oxygenminimumcore layers in the westernWeddell Sea [Gordon,1998].In the deepocean southof the South Orkney Plateau,bottom temperaturesare nearly coincidewith the WDW 0max. The CDW and WDW cores are weakened as the South as coldas -1.01øC, with salinityof 34.633and oxygenof >6.36 Orkney Plateauis approachedfrom both the Scotiaand Wed- mL/L, 79% of full saturation (station 30). The very cold dell Seas.The 0m• of -"0øCover the plateau marksthe Wed- benthiclayer is composedof WSBW formed alongthe Weddell-ScotiaConfluence[Gordon, 1967; Pattersonand Sievers, dell marginsto the west and south.North of the plateaulies 1980;Whitworthet al., 1994].The confluencezone, emanating the SouthOrkney Trough,with warmer, more salineand less from the northern tip of Antarctic Peninsula,is sandwiched oxygenatedbottomwater relativeto that of the Weddell Sea. betweenthe eastwardflowingACC to the north and the east- This water can be traced to Weddell flow over a 3200 m sill in ward flowing limb of the Weddell Gyre. The Weddell-Scotia the SouthScotiaRidge near 38øW[Gordon,1966;Locarniniet Confluencemay be consideredas a seawardadvectedstream al., 1993]. of continentalmargin waters of the Weddell Gyre, though some deep-reachingconvectionhas been suggested[Deacon 2.2. Potential Temperature Versus Salinity and Oxygen PotentialTemperatureversussalinity(0/S, Figure4a) and and Foster,1977]. Stations21-23 over the South Orkney Plateau (depthsof oxygen(0/O2;Figure4b) plotsdisplaythe variedwater masses 500 to 1500m) reveala cold benthiclayer (-0.4øC) with low of the northwest Weddell Sea and southern Scotia Sea. A salinity(slightly<34.66) and high oxygen(5.4 mL/L), relative near-freezingpoint (within 0.1øC)surfacemixed layer with a to watersof similar depth or densityo-o within the Scotiaand wide rangeof salinitycapsthe water column,with coldest(at Weddell Seas.The benthic layer water masspropertiesindi- the freezingpoint, -1.91øC), mostsaline(34.60)water at the cate that it may be consideredto be a better ventilated com- seaflooradjacentto JoinvilleIsland.The oxygenof the winter surfacelayer varies from 6.9 to 8.2 mL/L, 85 to 100% of ponentof WSDW (see section3). GORDON ET AL.: EXPORT OF WEDDELL SEA DEEP AND BOTTOM 5O WATER 9007 45 55 40 500 m2/s 608 658 Figure 2. Distributionof Palmer 97-5 DOVETAIL (CTD)/lowered acousticDoppler current profiler (LADCP) stations. Integratedvelocitiesderivedfrom LADCP for stationsdeeperthan2000m are shownas vectors.Bathymetryis from Smithand Sandwell[1997]. saturation,respectively.Lower oxygenlevelsoccurwithin the Weddell Sea, where snow-covered sea ice blocksoxygenexchangewith the atmospherewhichwould otherwisemitigate the effect of incorporationof oxygenpoor WDW into the surfacelayer [Gordonand Huber, 1990]. The highestsurface oxygenis found in the ScotiaSea,where the seaice cover is of shorterdurationand generally<100% coverage. The oxygenconcentrationwithin the surfaceand shelfwater adjacentto JoinvilleIsland is 7.56-7.73 mL/L (92% saturation), -0.5 mL/L higher than the surfacelayer to the east. Elevated oxygenlevelssuggestbetter exposureto the atmosphere,most likely throughcoastalpolynyas,with limited ac- tive of a more complex "recipe" than a simple two-endmember mixing process.In both WSDW and WSBW strata, there are two groupings:a higher-salinitybranchand a lowersalinitybranch.These subsetsof WSDW and WSBW are discussed below. 2.3. Bottom Water Distribution Bottom water propertiesfor the northwestWeddell Sea and southernScotiaSea provide a view of bottom water flow patterns at the time of the 1997 DOVETAIL cruise. The 1992 Ice Station Weddell data [Gordon, 1998] are added to provide informationon the upstreambottomwater properties.Locarcess to the offshore WDW. Joinville Island shelf water is less nini et al. [1993] showsbottom 0 for the DOVETAIL region saline than 34.61 and so is consideredas low-salinityshelf and farther north, to 35øS;Gordon [1998] shows0 and salinity water, in comparisonto the more salineshelfwatersfarther to for the Weddell Sea west of 20øW. The bottom 0 distribution the south [Whitworthet al., 1998, Figure 11; Gordon, 1998, (Figure5) revealsa massof waterwith 0 < -1.0øC drawnfrom Figure14].Presumably, thisis a reflectionof greaterwinter sea the continentalmarginsof the western Weddell Sea. Within ice formationin the southwestern Weddell Sea and/or injec- the northwest Weddell Sea the cold bottom water stream shifts tion of more freshwater water from the Antarctic Peninsula from a generallynorthward orientation into an eastwardorialongthe westernWeddell Sea. entation, reflecting the senseof the Weddell Gyre. Bottom The pycnoclinewithin the ScotiaSea is significantly warmer water colder than -0.7øC is traditionallycalled WSBW, as at andlesssalinethan the WeddellSeapycnocline.The Weddell that isothermthe O/S curvemakesan abruptchangein slope Sea pycnoclineis of much lower stabilitythan the ScotiaSea [seeCarmackand Foster,1975] (Figure 4a). The coldestbotpycnocline,as the Weddell winter surfacewater attains rela- tom water along the northern Weddell Sea occursalong the tively high density,forcingthe underlyingpycnoclinetoward 4000 m isobath,with cold water filling the depressionsboth reducedstability. west and east of the Orkney Plateau, including the Powell At depthsgreater than the WDW 0max,mixturesof WDW Basin and the passageleading the primary overflowinto the with watersfrom the continentalmarginsresultin a decrease Scotia Sea. The -0.7øC isotherm intersects the seafloor near the 3200 m in temperatureand salinitywith increasingdepth, accompanied by an increasein oxygenconcentrations.While the O/S isobathof the SouthScotiaRidge'sWeddellflank (Figure 3a). slopeof the water columnbelowthe WDW is approximately Over the interior of the Weddell Sea the -0.7øC isotherm lies linear, there are noticeabledeparturesfrom linearity,indica- between4000 and 4200 dbar [Fahrbachet al., 1994]. Thus the 1410 (a) 0 5OO 1000 15oo 2000 2500 3000 3500 4OOO 45OO 5OOO 55OO 58øS 59 60 61 62 63 64 62 63 64 Latitude 6 (b) 7 81•05 5OO IOO0 15OO • 2000 • 2500 • 3000 J• "0 3500 4OOO 45OO 5OOO 55OO 58øS 59 60 61 Latitude Figure 3. (a) Potentialtemperature(øC),(b) salinity,and (c) oxygen(mL/L) of the DOVETAIL section along44øW.The sectionextendsfromthe southernScotiaSeato the northernWeddellSea.The bottomwater colderthan -0.7øC (shaded)southof the OrkneyPlateau(stations24 to 32) represents the exportof Weddell Sea Bottom Water. Overflowof Weddell water into the ScotiaSea coolsthe bottomwater in the deep trough northof the OrkneyPlateau(stations7-16) to <0øC,colderthanwaterderivedfromthe Drake Passage. The <0øC bottom water over the southernslope of the Orkney Plateau (stations21-23) representsa wellventilatedlayer derivedfrom the Weddell-ScotiaConfluence.The warm deepwater stratumnear 500 m is weakenedover the Orkney Plateau,which alsomarksthe influenceof the Weddell-ScotiaConfluence.The coldwinter surfacelayerwascoveredby seaice southof station5. Bathymetryis derivedfrom underwayecho soundingsobtained during the cruise. GORDON ET AL.: EXPORT OF WEDDELL SEA DEEP AND BOTTOM WATER 9009 (c) 0 5OO 1000 / 15OO 5.4 ..• 2000 ß 2500 • 3000 3500 4000 45OO 5OOO 55OO 58øS 59 6O 61 62 63 64 Latitude Figure 3. (continued) WSBW shallowsover SouthOrkney Ridge. Overflow of Wed- of that feature do not displaythe fresheningobservedin the dell Sea water VWSDW into the Scotia Sea occurs at a 3200 m sill [Gordon,1966],the OrkneyPassage, just eastof SouthOrkney described here. The VWSDW is most pronouncedat DOVETAIL stations Plateau. Overflow of -0.6øC renews the bottom water of the in the Weddell Sea with bottom depths between 1500 and SouthOrkneyTroughof the ScotiaSea(Figures3a and 5). As 2000 m: stations69 and 70 over the Joinville Ridge, 86 at the the overflowwater is slightlywarmer than -0.7øC, it is desig- westernboundaryof Powell Basin,56 at the northern boundnated asWSDW, but in view of the proximityto the sill depth ary of the Powell Basin, and 20-22 over the South Orkney and the expectedmixing at the sill, componentsof WSBW Plateau. The VWSDW overrideswhat may be considered probablycontribute to the overflow.The Weddell overflow somewhatlesswell ventilatedWSDW (more saline,lessoxyinto the Scotia Sea flows westward into the southern Drake genated)near the -0.4øC isotherm,between2000 and 2500 Passage[Gordon, 1966;Nowlin and Zenk, 1988], then curls dbar at stations 23-25 over the southern flank of the South back to the northeast,passingthroughthe deep passages east Orkney Plateau and at stations57-62 along 48øW. It is not of SouthGeorgia Island into the westernSouthAtlantic [Lo- found at the 40øW section,indicatingthat VWSDW exitsthe carniniet al., 1993]. Weddell Sea to enter the Scotia Sea before reaching 40øW. Over the Joinville Ridge at stations70 and 71 and along the westernboundaryof Powell Basin,station86, an extremeform 3. Discussion of VWSDW (muchlowerin salinityand elevatedin oxygen)is 3.1. Weddell Sea Deep Water observed(Figure 6). Presumably,an admixtureof this deep The WSDW is definedasthe deepwater between0.0øCand water with WSDW forms the VWSDW observed downstream. At station 55, VWSDW characteristicsextend to the seafloor -0.7øC [Fahrbachet al., 1994; Carmack and Foster, 1975]. at 5300 m. It is proposedthat station55 fallswithin an isolated Within this temperaturerangemany DOVETAIL stationsreveal a relatively low salinity, high oxygenform of WSDW deep basinwithin the easternend of the HesperidesTrough. (Figure 6). We refer to thiswater massas a better ventilated The O/S relationshipand potential temperature-salinityproform of WSDW, which we designateas VWSDW, meaningit files indicate the source of the Hesperides Trough bottom is WSDW that has been subjectedto greater influx of high water is VWSDW, with a controllingsill at -1800 m. Stations 17 and 54, both within the Scotia Sea along the oxygensurfacewater. It is most distinctfrom the more widespreadWSDW between the -0.3øC and -0.4øC isotherms. southernwall of the South Orkney Trough, reveal traces of Meredith et al. [2000] describea CFC-11 maximum found VWSDW. Station 51 (northern side of Pirie Bank) showsa within the WSDW stratum near -0.45øC in the southeastern slightly lower salinity in the temperature range of the Weddell Sea,which they attribute to a WSDW sourceoutside VWSDW. It is suggested that VWSDW (seeinseton Figure6), the Weddell Gyre to the east.However,the O/S characteristics guidedby isobaths,entersthe ScotiaSea followinga counter- 2,0 (a) ..CDW .•+ 1.5 +.•'• • +5" ß 1.0 ß" ß ß :ee ß ., ... ,•.'; • • ß ß•e• I e •e_ •' ß ße ? .':". ' _._ _•,.-%• •. '••.•' .•' • .... ++*+ +• ß'ß ee + e ee • "'"' 0.5 ee + + ß ß ½e e .• + ½e +++ + ++ + + E +++ • 0.0 + ++ + + (D -0.5 0 // /'/ -1.0 Slope change marking top ' E. Bransfield of / -1.5 / •.,•/ / ,C.Br•ansfieid ;•-','?•J•invill'e Isla•dShe!f • .... -2.1 34.3 34,4 34.5 Tfreeze 34.6 34,7 Salinity 2.0 ( ) 1.5 ' , CDW 1.0 0.5 •:.'.• •}:. -0.5 -1.0 -1.5 -2.0 4 ......... . .............. 5 , 6 frae 7 8 DissolvedOxygen(ml/I) Figure 4. Potentialtemperature (øC)versus(a) salinity(CTD data)and(b) oxygen(mL/L) (oxygenbottle data)forallNathaniel B.Palmercruise97-5CTD/rosette stations (shaded circles). Datafromthe44øWsection are highlightedas solidsymbols. GORDON ET AL.' EXPORT OF WEDDELL SEA DEEP AND BOTTOM WATER 55W 50 9011 45 655 Figure 5. BottomWater potentialtemperatureof the lower 10 dbar of the CTD cast,if within30 dbar of theseafloor (a station wherethelastscanis30dbarfromthebottom wouldhavea valuethatistheaverage of data40-30 m fromthe bottom).Average velocity withinthe lower120dbarderivedfrom(bottom referenced) LADCPdataisshown asvectors. Themajority of DOVETAILstations approached within10m of thebottom. Temperature datafromthe1992IceStation Weddelldatasetareincluded [Gordon, 1998]as shownby the opencircleswith no velocityvectors. clockwise path aroundSouthOrkneyPlateau,then clockwise tween -0.4 and -0.7øC may be consideredVWSDW, but in aroundthe SouthOrkneyTrough,to enter into the interiorof view of the separationin O/S spacefrom the main massof the Scotia Sea. Exit from the Scotia Sea into the South Atlantic VWSDW discussed above,it ismorelikelyproducedat a more is expected[Locarniniet al., 1993]. remote site to the south, with a strongercomponentof The mostprobablesourceof the surfacewatercomponent WSDW. of VWSDW is freezingpointshelfwater.Near-freezing point surfaceand shelfwater overJoinvilleIslandRidge exhibitsa wide rangeof salinity(though<34.61, whichwouldbe dense enoughto directlyreachthe deep-sea floor) andoxygen(Figure4, stations 72-75).It maybe thoughtof aslow-salinity shelf water, as it is not denseenoughto descendto the deep-sea floor [Gordon,1998].It is proposedthat VWSDW is formed from low-salinity shelf water from Antarctic Peninsula which descends to the 1500-2000 dbar level somewhere south of 63øS,where it mixes with the resident WSDW of the Weddell Gyre. 3.2. Weddell The stations overthe seafloordeeperthan3800m (30-33) fall withina moresaline,lower-oxygen WSBW. Stations29 and 34-36, alongthe southernslopeof the EnduranceRidge,fall within the deeperwater groupand may representmore of a transitionbetweenbranches.On proceedingfrom -0.55 ø to -0.82øC, they displayan increasing componentof VWSBW with increasingdepth; below -0.82øC they fall within the VWSBW O/Sspace.NathanielB. Palmercruise92-2 (the Ice StationWeddellrecoverycruisein June1992[Gordon,1998]) showsan abruptseparationbetweenthe two branchesbetween stations 9 (bottomdepth4000m) and10(3000m), near63.5øS Sea Bottom Water and 46øW, west of 97-5 station 29. Powell Basin bottom water is similar in characteristics to the Below2000dbarsignificant spatialvariabilityis exhibited by low-salinity,high-oxygenVWSBW group at 44øW, though potentialtemperature,salinity,and oxygenprofiles(Figures lowerin salinityand higherin oxygen.This suggests a similar 6-8). Overshallower waters(bottomdepth<3800 m, stations upstream source from Antarctic Peninsula. 24-28 and38-41), thereisa relatively lowsalinity, highoxygen It is proposedthat the more saline,lower-oxygen WSBW is groupingfor water colderthan -0.4øC. We refer to thiswater, derivedfromshelfwaterdescending intothedeepoceanin the that componentcolderthan -0.7øC, as a more ventilatedform southwest Weddell Sea.The highersalinityof this WSBW is of WSBW, which we designateas VWSBW. The water be- due to injection of high-salinityshelf water characteristicof 0.5 '• 0.0 (1>-o.5 71 o xxx 3'• -1 .o (a) 34.60 34.65 34.70 Salinity i i • i i i i •-0.5 .,,o(b) 80% 4.5 5.0 5.5 6.0 6.5 Dissolved Oxygen(ml/I) Fi•ur• 6. Potential temperature ('C) versus (a) salini•(CT• data)and(b) o•cn (mL/L)(o•cn bottle databelowthe•) •orselect stations c•ibifin• ventilated WS•W. Theinsertstation mapdisplays •hcvaried symbols usedto denoterc•ions.The arrowson the mapinsertshowthe advective pattern•or the more ventilated •ormso• WeddellSea•ccp Water(VWS•W) andWeddellSeaBottomWater GORDON ET AL.: EXPORT OF WEDDELL SEA DEEP AND BOTTOM WATER 2000 2000 CTD 24-41 CTD 24-41 25OO 25OO 3OOO 03 9013 3OOO 3500 3500 4000 4000 45OO 45OO (a) (b) 30 -0.7oc 31 5OOO -1.0 -0.8 -0.6 -0.4 -0.2 Potential Temperature 500304.62 34. 4 ' • 34.66 Salinity 2000 CTD 24-41 25OO 3OOO t 39 40 03 3500 4000 45OO 29 (c) 30 31 32 5OOO 5.5 6.0 6.5 DissolvedOxygen(ml/I) Figure7. (a)Potential temperature, (b)salinity, and(c)oxygen versus depth(pressure, dbar)profiles below 2000dbarforstations 24-41.Shaded dotsonFigures 7band7cindicate salinity andoxygen at a potential temperatureof -0.7øC, takenas the upperlimit of WSBW. 9014 GORDON ET AL.: EXPORT OF WEDDELL SEA DEEP AND BOTTOM WATER -0.2 -0.4 o:1-o.6 E c: -0.8 o -1.0 e I • I $linit¾ Figure8. Potential temperature versus salinity andversus oxygen forstations 24-41and58-71(Powell Basin).Transition stations 29 and34-36 arehighlighted. thisregion[Gordon,1998].The VWSBWwithlowersalinity rr units between waters at the seafloor and waters above the andenrichedin oxygen is derivedfromtheshelfwaterfarther benthiclayeris 0.0128.Uponmixingthe watercolumnfrom thedensitydifference decreases to 0.0098for a 40 northalongAntarcticPeninsula's WeddellSeashelf.Fahrbach theseafloor, dbar mixed layer, 0.0070 for an 80 dbar mixed layer,and0.0048 et al. [1995]propose thatlow-salinity bottomwateris formed near the Larsen Ice Shelf. The transition from WSBW to for a 160 dbar mixedlayer.Further mixingbeyond240 dbar causes a slightincrease in mixedlayerstability.Thus,under forcing,onemayexpectthe layerto increase rapidly explained asfollows. TheVWSBWbranchpassing through the constant in thickness to at least 200 dbar. Thermobaric-assisted mixing PowellBasinissplitintotwostreams, onecontinuing eastward overthe shallowplainnorthof the Endurance Ridgeandthe maybe an importantfactorin upwardmixingof WSBWas overthe confining topograotherdescending intodeeperwateralongthesouthernescarp- requiredfor suchwaterto escape VWSBWwithincreasing depth(stations 29 and34-36) maybe ment of the Endurance Ridge. The WSBW is unstablein rro, as is characteristic of the phy of the WeddellBasin. benthiclayerin the westernand southwestern WeddellSea 3.3. Transport of WSBW and WSDW Full oceandepthvelocitymeasurements weremadefor the [Gordon, 1998].Thermobaric effectsareneededto produce in situ stable stratification.The benthic layer mixesas it flows first time in the WeddellSea gyreand ScotiaSea.The techtwo acoustic Doppler alongtheseafloor, andtheresultant homogeneous layerwarms nologyusedis fairlynewandemploys asit incorporates warmerwaterfromabove. Thisdecreases the currentprofilers(ADCP) mountedon the CTD frame.The measurethevelocityshearovera combinedrange thermobariceffect and leadsto an interestingresult:As the instruments makingit bottomhomogeneous layerthickens, its susceptibility to fur- of 300m depthat a rateof oneprofileper second, to obtainbottom-referenced flowvelocities within ther mixingincreases, inducinga positivefeedbackto mixing. possible shearprofile As an example,station32 (a thinstratifiedbenthiclayer;bot- 300m rangeof thebottom.The depth-averaged integrated togivea fulloceandepthvelocity profile tom4764dbar)is inspected. Initially,thedensity difference in isvertically GORDON ET AL.: EXPORT OF WEDDELL SEA DEEP AND BOTTOM WATER o 22 9015 30 0.10 0.08 •,•0.06 :"":"•i••] 0'04 iY,. ---...--•-..-,•-.. 02 O.00 -0.02 -62.5 -63.0 -63.5 -64.0 rns-1 Latitude lOO• 2ooi 3ool 4ool 500( o 2 4 6 8 Transport (m3s'1)x 106 lO o lO 20 30 40 Transport (m3s'1)x 106 Figure9. Currents andtransports across the44øWsection (stations 22-32)estimated fromtheLADCP data.(a)LADCP-derived velocity computed approximately normal tothesection withpotential temperature isotherms superimposed. (b) Cumulative transport ofventilated (thinsolidline)andunventilated (dashed line)varieties of deepandbottom waterasa function of temperature. (c)Totalcumulative transport asa function of depth, witherrorenvelope estimated assuming a worst-case barotropic velocity errorof 1 cms throughout. with an unknown barotropic meanflow[Fischer and I/isbeck, deepwatersouthandeastof theOrkneyPlateau,tidalcurrents 1993].However, in combination withaccurate ship'sposition- are <3 cms.In northernandwesternpartsof PowellBasinthe ing(GPS)at thebeginning andendof eachcast,the unknown tidalcurrents reach5 cms, andovershallower topography of barotropic meanflowcanbe determined withan accuracy of ridgesandshelves, tidal speedsare quitelarge,over 10 cm s. •--1cm s. As this error is random,the error in estimating In the northwestern WeddellSeawe find a cyclonicflow transportacrossa seriesof stationsis expectedto be small.In throughthe Powellbasinwith a transportobtainedfrom stathe transports presented below,a worst-case error is given, tions57-65 of 18 _+6 Sv (1 Sv = 106 m3/s).Southof the whichassumes a 1 cmsbarotropic velocityerrorin eachof the OrkneyPlateautheflowis directed eastward, clearlymarking profiles used.Obviously, a single LADCPsurvey canonlymea- thenorthernlimbof theWeddellgyre,witha totaltransport of surethesynoptic velocity field,whichneednotto be represen- 40 _+5 Sv across44øW(stations22-32, Figure9). There the tativeof the long-termmean. flowhastwocoresseparated bytheEndurance Ridgeat 63øS. For moststations thevelocity profileisstrongly barotropic, Fromwatermassproperties andthe measured transports we with significant verticalshearsonlyin the vicinityof the sea- expectthat abouthalf of that transportacross44øWis derived floor.Figure2 depictsthedepth-integrated flowat all stations fromthe PowellBasingyre,the restpassed eastward alonga deeperthan 2000 m where the tidal contributionis assumedto pathsouthof the PowellBasin(Figure1). be small.Robertson et al. [1998,theirFigure3] showthat in Fartherto the norththe flowaroundPirie Bank(59øS)is 9016 GORDON ET AL.: EXPORT OF WEDDELL SEA DEEP AND BOTTOM WATER anticylonic,as expectedfrom vorticity dynamics.The largest Depressionoutflowwith Weddell gyre interior bottom water. flow wasobservedjust north of the South OrkneyPlateauwith Farther westis the secondplume of colder,lower-salinitybotwestwardcurrentsexceeding25 cm s at a depth of 4000 m. This tom water derived from outflow of Ronne Ice Shelf water boundary current is one of the pathwaysby which WSDW exitingthe shelf in the region of General BelgranoBank. The enters into the Scotia Sea. We have estimated a westward third plume is a cold, high-salinityvariety of bottom water transportof -10 Sv north of the South Orkney Islands.The formedashigh-salinityshelfwater dropsinto the deepseanear bottom-referencedvelocities,averagedover the lower 120 m of 70øS.It is likely that some mix of the central and western the profiles, show a qualitativelysimilar circulationpattern plumescomprisethe WSBW (stations30-33) observedby the DOVETAIL data southof South Orkney Ridge as the saline, (Figure 5). The near-bottom flow seems to have two cores, one followlower-oxygenvariety of WSBW. The DOVETAIL data reveal ing the slopesouthof the South Orkney bank and the other a fourth more ventilatedplume huggingthe edgeof the northjust southof the EnduranceRidge (Figure2). The flowvectors west Weddell Sea. This plume with lower salinityand higher is derivedfrom the northern end of the in the vicinityof the ridgesare stronglyinfluencedby the local oxygencharacteristics topography.At station29 our observations corroboratethe 10 Weddell Sea. The water mass distribution indicates that the VWSDW is cm s eastwardflow through a gap in the ridge as recorded earlier by a year-long current meter mooring [Barber and advected eastward between 1500 and 2000 dbar in contact with Crane, 1995]. the seafloor;in adjacentdeeperwater it overridescolderlayers The LADCP-derivedvelocitiesacrossthe 44øWsection(Fig- of WSDW. The VWSDW is the sourceof the relatively cold ure 9a) allowan estimateof volumetransportsfor WSDW and bottom water over the southernSouth Orkney Plateau. The WSBW and their more ventilatedcomponents.The total east- VWSDW passesnorthwardinto the ScotiaSea alongthe eastward transportfor waterscolderthan -0.7øC is 5 Sv (Figure ern edgeof the plateau.None is observedto passeastwardof 9b) with about equal contributions from the ventilated that longitudewithin the Weddell Sea. The VWSDW ventiVWSBW north of 63.5øS and the saltier forms of WSBW. The latesmiddepthsof the ScotiaSea.Bottom Water in the southtransport of WSDW across44øW is 25 Sv, which with the ern Scotia Sea is derived from overflow of WSDW with warmer WSBW, yields-30 Sv of eastwardtransportfor water column elements of VWSBW. The VWSBW and VWSDW, being shallower,would have below roughly1500 m (Figure 9c), the approximatedepth of the 0øCisothermmarkingthe warm upper boundof WSDW. better accessto escaperoutesinto the ScotiaSea and South The LADCP presentsa snapshotview of the velocityfield at Atlantic, leaving the deeper, more saline versionsof WSDW higher spatial resolutionthan offered by mooring arraysand and WSBW to recirculatewithin the Weddell Gyre. Therefore thus better definesthe horizontal and vertical scalesof high- the cold, lower-salinityforms of Weddell Sea watersforming speedcoresand their relationshipto the bottom bathymetry. near the northern end of the Weddell Sea's Antarctic PeninCognizant of the lack of temporal resolutionoffered by the sulamay be more activein ventilatingthe Atlantic Ocean than LADCP section, we compare the LADCP results with the the densestcomponentsformedfarther south.The more saline mooringdata of Fahrbachet al. [1994],whichoffer more of a WSBW andWSDW maytake a moreindirect(diffusive)route climaticmean.For the period September1989to January1993 into the Atlantic Ocean. their measurementsoff JoinvilleIsland yield 2.6 Sv of WSBW The VWSBW and VWSDW effectively extract freshwater and 1.2 Sv of WSDW. The DOVETAIL measurementsalong from the Weddell Gyre. This may explainwhy the apparent 44øWsuggest eithertemporalvariability(whichno doubtexists cold end-memberof the central Weddell Gyre is more saline to someextent)or, more likely, that the DOVETAIL section than the averageWSBW formed in the southwesternandwestincorporatesmore of the WSDW transportwithin the Weddell ern Weddell Sea [Gordon,1998].This is similarto the conclugyre which would not be captured by observationsclose to sionof Fahrbachet al. [1995],who saythe FilchnerDepression Joinville Island. outflow dominatesthe central Weddell Gyre, while lowersalinitybottom water formed in the westernWeddell Sea "influencesthe abyssalwatersof the open ocean." 4. Conclusions A simplepattern emerges(seearrowson Figure 1): Upon Within the northwestWeddell Sea, there are two typesof proceedingclockwisearoundthe rim of the Weddell Sea from deep and of bottom water. There are the more saline, less Cape Norwegiato the tip of AntarcticPeninsula,a sequenceof oxygenatedformsof Weddell Sea Deep Water (WSDW) and freezingpoint shelfwater typesis encountered.This sequence of WeddellSeaBottomWater (WSBW), whichare mostlikely is projected as a "fan" of resultantbottom water spreading derived from the southwestern Weddell Sea. A second form of acrossthe deep western Weddell Sea. The bottom water these water masses,with lower salinity and higher oxygen,is formed at Filchner Depressionenters the deeper interior of observedalong the outer, northwestrim of the Weddell gyre. the Weddell Sea,while the lowest-salinitybottom watersarisEnhancedventilation of thesewater masses,which we desig- ing from shelfwater closerto the tip of Antarctic Peninsula nate as ventilatedWSBW and ventilatedWSDW (VWSBW track along the rim of the Weddell Sea and are the first to and VWSDW), relativeto thoseformsderivedfrom the south- escapethe confinesof the Weddell Sea. Between these two west Weddell Sea is due to influx of lower-salinityshelfwater extremesare a seriesof other bottom water typesthat may descendingalongthe continentalslopefrom the northern end enter the centralWeddell Sea or mix upwardto exit the Wedof Antarctic Peninsula. This extendsa pattern of bottom water flow presentedby Gordon [1998] for the deep interior of the westernWeddell Sea.He describesthree plumesof newlyformedbottomwater. The plume most removedfrom the westernboundaryis relativelywarm and salty,and it representsa mixtureof Filchnet dell Sea. Acknowledgments.The researchis supportedby National Science Foundation Grant OPP 95-28807.The LADCP Componentis supported by a grant/cooperativeagreementfrom the National Oceanic and AtmosphericAdministration,grant UCSIO P.O. 10075411.The GORDON ET AL.: EXPORT OF WEDDELL viewsexpressed hereinare thoseof the authorsand do not necessarily reflectthe viewsof NOAA or any of its subagencies. Lamont-Doherty contribution number 6167. References Barber, M., and D. Crane, Current flow in the north-westWeddell Sea, Antarct. Sci., 7, 39-50, 1995. Carmack, E. C., and T. D. Foster, On the flow of water out of the Weddell Sea,Deep SeaRes.,22, 711-724, 1975. Deacon,G. E. 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