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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. R., The Weddellgyre,DeepSeaRes.,26, 981-995, 1979.
Deacon, G. E. R., and T. D. Foster,The boundaryregionbetweenthe
Weddell Sea and Drake Passagecurrents,Deep SeaRes.,24, 505510, 1977.
SEA DEEP
AND
BOTTOM
WATER
9017
Bayer, Transienttracer observationsfrom the westernWeddell Sea
duringthe drift and recoveryof ice stationWeddell, in Oceans,Ice
and Atmosphere:Interactionsat the Antarctic ContinentalMargin,
Antarct.Res. Ser.,vol. 75, edited by S.S. Jacobsand R. Weiss,pp.
241-256, AGU, Washington,D.C., 1998.
Meredith, M.P., R. A. Locarnini, K. A. Van Scoy,A. J. Watson, K. J.
Heywood,and B. A. King, On the sourcesof Weddell Gyre Antarctic Bottom Water, J. Geophys.Res., 105, 1093-1104, 2000.
Muench, R. D., Deep OceanventilationthroughAntarcticintermediate layers,Antarct.J., 32, 65-67, 1997.
Nowlin, W. D., Jr., and W. Zenk, Westwardbottomcurrentsalongthe
margin of the South ShetlandIsland Arc, Deep SeaRes.,35, 269301, 1988.
Orsi, A. H., W. D. Nowlin, and T. Whitworth, On the circulation and
stratificationof the Weddell Gyre,DeepSeaRes.,40, 169-203, 1993.
Fahrbach,E., G. Rohardt, M. Schroder,and V. Strass,Transport and
Patterson, S. L., and H. A. Sievers,The Weddell-Scotia Confluence, J.
structureof the Weddell gyre,Ann. Geophys.,12, 840-855, 1994.
Phys.Oceanogr.,10, 1584-1610, 1980.
Fahrbach, E., G. Rohardt, N. Scheele,M. Schroder, V. Strass,and A.
Robertson,R., L. Padman,and G. Egbert, Tides in the Weddell Sea,
Wisotzki,Formation and dischargeof deep and bottom water in the
in Ocean,Ice andAtmosphere:Interactionsat theAntarctic ContinennorthwesternWeddell Sea,J. Mar. Res.,53(4), 515-538, 1995.
tal Margin,Antarct.Res. Ser.,vol. 75, edited by S.S. Jacobsand R.
Fischer,J., andM. Visbeck,Deep velocityprofilingwith self-contained
Weiss,pp. 341-369, AGU, Washington,D.C., 1998.
ADCP's, J. Atmos. Oceanic Technol.,10, 764-773, 1993.
Smith,W. H. F., andD. T. Sandwell,Global seafloor topographyfrom
Gordon,A. L., Potentialtemperature,oxygenand circulationof botsatellite altimetry and ship depth soundings,Science,277, 1956tom water in the Southern Ocean, Deep Sea Res., 13, 1125-1138,
1966.
Gordon, A. L., Structure of Antarctic waters between 20øW and
170øW,Antarct. Map Folio Ser., folio 6, 1967.
Gordon, A. L., Western Weddell Sea thermohaline stratification, in
1962, 1997.
Weppernig,R., P. Schlosser,S. Khatiwala,and R. G. Fairbanks,Isotope data from Ice Station Weddell: Implicationsfor deep water
formation in the Weddell Sea,J. Geophys.Res.,101, 25,723-25,740,
1996.
Ocean,Ice, andAtmosphere:
Interactionsat theAntarcticContinental
Whitworth, T., III, W. Nowlin, A. Orsi, R. Locarnini, and S. Smith,
Margin, Antarct. Res. Ser., vol. 75, edited by S.S. Jacobsand R.
Weddell Sea shelf water in the Bransfield Strait and the WeddellWeiss,pp. 215-240, AGU, Washington,D.C., 1998.
Scotia Confluence,Deep Sea Res., 41, 629-641, 1994.
Gordon, A. L., and B. A. Huber, Southern Ocean winter mixed layer,
Whitworth, T., III, A. H. Orsi, S.-J. Kim, W. D. Nowlin Jr., and R. A.
J. Geophys.Res.,95, 11,655-11,672,1990.
Locarnini,Water massesandmixingnear the AntarcticSlopeFront,
Gordon, A., B. Huber, H. Hellmer, and A. Ffield, Deep and bottom
in Ocean,Ice andAtmosphere:Interactionsat theAntarctic Continenwater of the Weddell Sea's Western Rim, Science,262, 95-97, 1993.
tal Margin,Antarct.Res.Ser.,vol. 75, edited by S.S. Jacobsand R. F.
Gordon,A., M. Mensch,Z. Dong, W. SmethieJr., and J. de BettenWeiss,pp. 1-29, AGU, Washington,D.C., 1998.
court,Deep and bottomwater of the BransfieldStrait easternand
central basins,J. Geophys.Res., 105, 11,337-11,347,2000.
A. L. Gordon, B. Huber, and M. Visbeck, Lamont-Doherty Earth
Locarnini,R. A., T. Whitworth, and W. D. Nowlin, The importanceof
Observatory,ColumbiaUniversity,Route 9W, Palisades,NY 10964.
the ScotiaSea on the outflow of Weddell Sea Deep Water, J. Mar.
([email protected]
ia.edu)
Res., 51, 135-153, 1993.
Mensch,M., R. Bayer, J. Bullister,P. Schlosser,and R. Weiss,The
distributionof Tritium and CFCs in the Weddell Sea during the
mid-1980s,Prog.Oceanogr.,38, 377-415, 1996.
Mensch, M., W. Smethie Jr., P. Schlosser,R. Weppernig, and R.
(ReceivedFebruary28, 2000;revisedJanuary22, 2001;
acceptedJanuary23, 2001.)