JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 104, NO. C2, PAGES 3059-3073, FEBRUARY
15, 1999
Location and dynamics of the Antarctic Polar Front
from satellite sea surface temperature data
J. Keith Moore, Mark R. Abbott, and James G. Richman
Collegeof Oceanicand AtmosphericSciences,Oregon State University,Corvallis
Abstract. The locationof the Antarctic Polar Front (PF) was mappedover a 7-year
period (1987-1993) within imagesof satellite-derivedsea surfacetemperature.The mean
path of the PF is stronglysteeredby the topographicfeaturesof the SouthernOcean. The
topographyplacesvorticity constraintson the dynamicsof the PF that stronglyaffect
spatialand temporalvariability.Over the deep oceanbasinsthe surfaceexpressionof the
PF is weakened, and the PF meandersover a wide latitudinal range. Near large
topographicfeatures,width and temperaturechangeacrossthe front increase,and largescalemeanderingis inhibited. Elevated mesoscalevariability is seen within and
downstreamof these areas and may be the result of baroclinic instabilitiesinitiated where
the PF encounterslarge topographicfeatures.The strongcorrelationsbetweentopography
and PF dynamicscan be understoodin the contextof the planetary potential vorticity
(PPV or f/H) field. Mean PPV at the PF variesby more than a factor of 2 alongits
circumpolarpath. However, at the mesoscalethe PF remainswithin a relativelynarrow
range ot m'¾ valuesaround the local mean. Away from large topographicfeatures,the
PF returnsto a preferred
PPVvalueof-25 x 10-9 m-1 s-1 despitelargelatitudinal
shifts.The mean paths of the surfaceand subsurfaceexpressionsof the PF are closely
coupledover much of the SouthernOcean.
1.
Subsurface
Introduction
The AntarcticPolar Front (PF), or Antarctic Convergence,
marks the location where Antarctic surface waters moving
northwardsinkbelow subantarcticwaters [Deacon,1933].The
PF is a regionof elevatedcurrentspeedsand stronghorizontal
gradientsin density,temperature,salinity,and other oceanographic properties [Deacon, 1933, 1937; Mackintosh,1946].
The PF is one of severalstrongjets within the Antarctic CircumpolarCurrent (ACC), which flowseastwardaroundAntarctica[Nowlinand Klinck, 1986].North of the PF is the SubantarcticFront (SAF), and to the southis the southernACC
front (SACCF) [Orsiet al., 1995].The PF marksan important
climate boundaryin terms of both air-sea fluxesand the heat
and salt budgetsof the oceans.The path of the PF exhibits
considerablevariability in the form of mesoscalemeandering,
eddies, and ring formation [Mackintosh,1946; Joyceet al.,
1978].These mesoscaleprocesses
are likely importantin meridional fluxesof heat and salt within the Southern Ocean, yet
little is known about their regional and temporal variability
[Nowlinand Klinck, 1986;Gouretskiand Danilov, 1994].
We haveusedthe stronggradientin seasurfacetemperature
(SST) acrossthe AntarcticPF to map its locationwithin satellite-derivedimagesof SSTovera 7-yearperiod(1987-1993).
Satellite data have been used previously to map the PF
[Legeckis,
1977;Mooreet al., 1997]and other strongfronts[i.e.,
Hansenand Maul, 1970; Olsonet al., 1983; Cornilion, 1986].
The PF has both surfaceand subsurfaceexpressions
whose
locationsdo not necessarilycoincide [Botnikov,1963; Lutjeharmsand l/alentine,1984]. Stronggradientsin SST mark the
surface expression[Deacon, 1933, 1937; Mackintosh, 1946].
Copyright1999 by the American GeophysicalUnion.
Paper number 1998JC900032.
0148-0227/99/1998JC900032509.00
definitions
for
the PF
mark
the location
where
Antarctic surfacewater moving northward descendsrapidly,
such as the point where the minimum potential temperature
layer sinksbelow 200-m depth [Deacon,1933, 1937] (see also
Belkinand Gordon[1996]for a reviewof PF definitions).South
of Africa, the subsurfaceand surfaceexpressions
of the PF are
typicallyseparatedby distances<-50 km, althoughseparations
of up to 300 km havebeennoted[Lutjeharms
and l/alentine,1984;
Lutjeharms,1985].Sparrowet al. [1996]concludethat there is a
wide separationbetweenthe surfaceand subsurface
PF expressionsin the vicinityof the KerguelenPlateau.A similarsurface/
subsurfacesplit may occur at Ewing Bank [Mooreet al., 1997].
Several
recent
studies have examined
the historical
data set
in an attemptto map the large-scalemeanlocationof Southern
Ocean fronts, includingthe Antarctic PF [Lutjeharms,1985;
Belkin, 1993; Orsi et al., 1995;Belkin and Gordon, 1996; Sparrow et al., 1996].In addition,Gille [1994]usedGeosataltimeter
data and a meanderingjet model to map the mean location of
the PF and the SAF. The large changesin sea surfaceheight
detectedby the two-jet model would likely be associatedwith
the subsurfaceexpressionof the PF [Gille, 1994]. Comparison
of our results with
those based on altimeter
and in situ sub-
surface data can provide insightsinto relationshipsbetween
the surfaceand subsurfaceexpressionsof the PF.
Strong topographicinfluenceon the ACC and the PF has
been noted in numerous
studies of the Southern
Ocean includ-
ing analysisof data from ships[Gordonet al., 1978;Lutjeharms
and Baker, 1980], moorings[Inoue, 1985], satellite altimeters
[Cheltonet al., 1990; Gille, 1994], drifting buoys [Hofmann,
1985; Patterson,1985], and models [Johnsonand Hill, 1975;
Killworth, 1992;Boyeret al., 1993;Hughesand Killworth, 1995;
Gille, 1997].Deacon [1937] arguesthat the locationof the PF
is determined by the movementsof deep and bottom waters,
which are expectedto be stronglyinfluencedby topography.
3059
3060
MOORE
ET AL.: SATELLITE
MAPPING
Barotropicflow in the oceansover smoothlyvaryingtopographytendsto conserveangularmomentumby followinglines
of constantpotentialvorticity(f + •)/H, wheref is planetary
vorticity,• is relativevorticity,andH is oceandepth.In open
ocean areas the planetary vorticity is much larger than the
relativevorticity,and mean potentialvorticitycan be approximated asf/H. We use the term planetarypotentialvorticity
(PPV) for thisf/H approximation.Moore et al. [1997] found
that the PPV field stronglyinfluenced the dynamicsof the
Antarctic PF in the Drake Passage/Scotia
Searegion.Here we
comparethe path of the PF with the PPV field throughoutthe
Southern
Ocean.
OF THE
ANTARCTIC
POLAR
FRONT
computedmeanpathwassmoothed(in latitudeandlongitude)
with a 5-point movingboxcarfilter.
Two methodswere usedto quantifyvariabilityin the location of the PF. In method1 the spatialdisplacement
of PF path
pointsat right anglesfrom the meanpath wascalculated.The
root-mean-square(rms) of these spatial displacements
is a
measureof meanderingintensity[Lee and Cornilion,1995].
Method 1 describeslarge-scalevariability,suchas that due to
latitudinalshiftsof the PF. In method2 the averagenumberof
pointsalongthe PF was calculatedwithin 1ø longitudinalbins
and then normalizedby the numberof path pointsalongthe
meanpath for eachbin. This methodis a measureof mesoscale
variability,or path curvature,relativeto the meanpathwhere
suchmesoscalevariability has been smoothedout. When calculatingthe averagenumberof path pointsper bin, pathswith
fewer points than the mean path minus 3 were excludedas
incomplete.
To quantify the temperaturegradient acrossthe front, we
began at the polewardedge of the PF and moved up the
temperaturegradientuntil SSTdid not increaseovera 3-pixel
(-20-30 km) distanceor until a missingpixel was reached.
In the SouthernOcean,large changesin oceandepth associated with the mid-oceanridges and Drake Passagedo not
permit the ACC to follow circumpolarlines of constantPPV
[Koblinsky,1990]. In these regionsthe ACC is forced across
isolinesof PPV, causinginputsof relativevorticityto the water
column through the shrinking of vortex lines. This relative
vorticityis likely dissipatedthroughnonlinearprocesses
such
as eddyactionor Rossbywaves[Hughes,1995,1996].Elevated
eddy kinetic energyis seendownstreamof major bathymetric
features(i.e., Drake Passage
andKerguelenPlateau)[Daniault The same method was used to measure the width of the front.
and Mdnard, 1985;Patterson,1985;Sandwelland Zhang, 1989; Only completetransectswere usedto calculatethe meantemCheltonet al., 1990;Morrow et al., 1994].
perature changeand distanceacrossthe PF. We defined the
seasonsas spring(weeks38-50), summer(weeks51, 52, and
1-10), fall (weeks12-24), andwinter (weeks25-37).
2.
Materials
and Methods
A high-resolutionpredictedseafloortopography,derived
Our method for mapping the Antarctic PF has been de- from ship and Geosat altimeter data [Smith and Sandwell,
scribedin detailbyMooreet el. [1997],soherewe presentonly 1994],was usedto createmapsof bathymetryand planetary
a brief description.Daily satellite images of SST at 9 km potentialvorticityfor comparison
with PF paths.The topogresolution were obtained from the National Oceanic and Atraphy had a grid spacingof 3 min of longitudeby 1.5 min of
mosphericAdministration (NOAA)/NASA Pathfinder Pro- latitude, which was remappedto the same resolutionas the
gram,whichusesthe advancedveryhighresolutionradiometer SST data. These bathymetrydata were also used to calculate
(AVHRR) sensorson NOAA polar orbiters [Brownet el., the slopeof the oceanfloor acrossthe PF and depthat the PF
1993;Smithet el., 1996].Daily satellitepasses(both ascending over a 90-km wide swath.
We have included some discussion of interactions between
and descending)
were compositeaveragedto produceweekly
imagesof SST. Subsetsof theseweekly imagesshowingonly the PF and the SAF. The SAF is defined as the maximum SST
thoseareaswith stronggradientsin SST (gradientmaps)were gradientin the temperaturerange of 5ø-9øC[Burling,1961].
constructed,and the locationof the polewardedgeof the PF Lutjeharms
and Valentine[1984]foundthe meanSSTrangefor
was subjectivelydigitizedby examinationof the weeklygradi- the SAF between20øWand 40øEto be 5.1ø-9.0øC(see also
ent and SST maps.We defineda stronggradientasa changein Belkinand Gordon[1996]for a reviewof SAF definitions).
temperatureacrossa pixel of _>1.35øCover a distanceof 45-65
km (dependingon latitude and PF orientation)[Mooreet el.,
3.
Results
1997].
The positionof the AntarcticPF was mappedfrom weekly
On occasion,two stronggradientsin the approximatetemperature range and location of the PF were observed.This SST imagesover a 7-yearperiod 1987-1993(Figure 1). It is
occurredprimarilyin two regions,west of Drake Passageand apparent that there are large regionalvariationsin both the
in the southwesternPacific. When two fronts were observed, latitudinalrangeof the PF and in the numberof pathsdigithe path marked was the one with a temperaturestructure tized. We will demonstratethat theseregionalvariationsare
most similarto the PF upstreamand/or downstreamfrom the primarily a function of the underlyingtopographyand the
doublefront area. If no adjacentdata were available,the more corresponding
PPV field.Relativelyfew pathswere digitizedin
poleward front was digitized to better delimit the northward areasabovethe deep oceanbasins(i.e., 120ø-90øW,50ø-70øE,
extent of Antarctic surface water. This double-front structure
and 110ø-140øE;
seeFigure 1).
has been noted previously[Sieversand Nowlin, 1984;Read et
al., 1995].Seasurfacetemperatureis relativelyconstantsouth 3.1. Topography and the Mean Path of the PF
of the PF, and there is typicallynot a SST gradientassociated
Our mean path for the AntarcticPF is shownin Plate 1 over
with the southernACC front [Orsiet al., 1995].
the topographyof the SouthernOcean.The stronginfluenceof
The mean path of the PF wasdeterminedby first construct- topographyon the mean path of the PF is readilyapparent.
ing a referencepath (a subjectiveestimateof the meanpath) The PF followscloselythe Pacific-AntarcticRidge, the Falkand then calculatingthe mean deviationof all path pointsat land Plateau, parts of the Mid-Atlantic and SoutheastIndian
right anglesto this referencepath. This mean deviationfrom Ridges,and the steeplyslopedtopographyassociated
with the
the referencepath defined the mean path. For most areasa Ob'-LenaRise (40ø-45øE)and KerguelenPlateau(75ø-80øE)
line of constantlatitudewas usedas the referencepath. The (Plate 1).
MOORE
ET AL.: SATELLITE
MAPPING
10ow
OF THE
ANTARCTIC
POLAR
FRONT
3061
10OE
30ow
30OE
)OE
71
70OE
90ow
,90OE
11
110OE
131
30OE
150ow
150OE
170ow 45øS 170øE
Figure 1. Displayedare all pathsfor the Antarctic Polar Front digitizedfor the years 1987-1993.
Our mean path follows the Pacific-AntarcticRidge in the
SW Pacific,crossingjust southof the Udintsev Fracture Zone
(UDF) at - 145øW.The SAF wasobservedfrequentlyto merge
with the PF and alsopassthroughthe UDF. At other timesthe
SAF remainedfarther north, crossingthe ridge at the Eltanin
Fracture Zone. These observationsare consistentwith previousstudies[Belkin,1988;Pattersonand Whitworth,1990;Orsiet
al., 1995].After crossingthe ridgethe PF flowsto the southeast
until it encounters
the Antarctic
Peninsula
at ---78øW.
We have previouslydescribedthe mean path of the PF
through the Drake Passage/Scotia
Sea region [Moore et al.,
1997]. The mean path is largely constrainedby topography
through the Drake Passage(especiallyat ---62ø-57øW[see
Moore et al., 1997] and alongthe Falkland Plateau.There is an
area of highvariabilityand intensemeanderingin the northern
Scotia Sea [Gordonet al., 1977; Cheltonet al., 1990;Moore et
al., 1997].
After crossingthe Mid-Atlantic Ridge at ---10ø-9øWthe PF
flowseastwardnear 50øSdippingsouthwardat ---30øE,in general agreementwith previousstudies[Lutjeharmsand Valentine, 1984;Lutjeharms, 1985; Belkin, 1993; Gille, 1994; Orsi et
al., 1995;Belkinand Gordon,1996].Our meanpath dipssouthward from ---5øto 11øE.In this regionthe PF wasoften located
near the Mid-Atlantic Ridge along---53øS(Figure 1 and Plate
1). Lutjeharmsand Valentine[1984] show a number of PF
crossings
at this steeplyslopedtopography(see alsoBelkin,
[1993,Table 4]).
In the area around ---30øEthe PF often displayedintense
meanderingsuchthat a clear path could not be determined,
similar to the high variability area in the northern Scotia Sea
[seeMoore et al., 1997]. Gouretskiand Danilov [1993, 1994]
have shownthat the PF regularlyspawnswarm core rings in
this region.Other studieshavefound elevatededdyvariability
near 30øE [Daniault and M•nard, 1985; Sandwelland Zhang,
1989; Cheltonet al., 1990;Morrow et al., 1994; Gille, 1994].
Dynamicheightcontoursalsopinchclosertogetherhere [Gordonet al., 1978;Gamberoniet al., 1982].Deacon[1937]suggests
the PF is deflectedsouthwardby the ridge systemat ---30ø31øE(seePlate 1). We alsoobservedinteractionwith the SAF
in this area, as hasbeen reportedpreviously[Orsiet al., 1993;
Read and Pollard,1993;Belkinand Gordon,1996].
The SAF appearsto have a bimodal path distributionbeginningat ---30øE.The SAF often turns northwardupon encountering the steep ridge extending southward from the
Crozet Plateau (along ---30ø-31øE;
see Plate 1), turning eastward and converging
with the SouthSubtropicalFront (SSTF)
and, farther east, the Agulhas Front to form the "Crozet
Front" north of Crozet Plateau [Park et al., 1993; Belkin and
Gordon, 1996;Sparrowet al., 1996].Alternatively,the SAF is
forcedsouthof the steepridge near 30øE,forcingit into close
proximity with the PF. On this southern route the SAF flows
along the southernside of the Crozet Plateaubefore turning
northwardjust eastof Crozet Island,joining the Crozet Front
northof CrozetPlateau[Orsiet al., 1993,1995;Gille, 1994].We
observedthe SAF followingboth routesthroughthis area.
Our mean path turns southward at the Ob'-Lena Rise
(---46øE).A phenomenondescribedby Sparrowet al. [1996],
who concludethat there is a persistentsplit here betweenthe
3062
MOORE
ET AL.:
SATELLITE
MAPPING
OF THE
ANTARCTIC
POLAR
FRONT
Mid-Atlantic
Ridge10øW..
45os. 10øE
oow
Falkland
I
/
30øE
Plateau
Ob'-Lena
Rise
55øS
50øE
Kerguelen
65ø$
Plateau
70øE
70øW
t
90øW
--•
90OE
110øE
110øW
70øS,•
<500
--.,..
1000
13o øE
3000
4000
>5000
•
150ø•
55oS
'"'"r,--
!
•
,
..
':
',
,
150øE
Southeast
Depth
(rn) Pacific-Antarctic
170øW
45øS170øE Campbell
Plateau
Indian
Ridge
Ridge
Plate 1. The calculatedmean path for the Antarctic Polar Front is shown over the topographyof the
SouthernOcean [Smithand Sandwell,1994].
surfaceand subsurfaceexpressions
of the PF, with the subsurface expressionpassingnorth of Kerguelen Island and the
surfaceexpressionmoving southwardto crossthe Kerguelen
Plateau through an area of deeper bathymetryat -56ø-57øS.
We mapped the PF passingthrough this gap at -56ø-57øS,
which is called the 77øE Graben [Schlichet al., 1987], on a
numberof occasions
(compareFigure1 andPlate 1). However,
we alsomappedthe PF over a wide rangein this region(spanning nearly 10ø of latitude), including,at times,north of KerguelenIsland (Figure 1 and Plate 1).
Previousstudieshave placed the surfaceexpressionof the
PF over a wide latitudinal range in the Kerguelen region
[Gamberoniet al., 1982;Deacon, 1983;Klyausov,1990;Park et
al., 1993;Belkinand Gordon,1996;Sparrowet al., 1996].The
path of Sparrowet al. [1996] likely marksthe southernlimit of
the PF surface expression.In addition, few PF paths were
marked in the region between the Ob'-Lena Rise and the
KerguelenPlateau, even under cloud-freeconditions(Figure
1). Thus, in our analysis,there frequentlywas no detectable
surface expressionof the PF between the Ob'-Lena Rise and
Kerguelen Plateau.
East of Kerguelen Plateau, the envelopeof PF paths narrowssharplyby 80øE(Figure l), and the mean path movesto
the south following closely the eastern flank of Kerguelen
Plateau (Figure 1 and Plate 1). The PF turnsto the northeast
at •95øE
and follows
the southern
flank
of the
Southeast
Indian Ridge (•100ø-110øE) before moving southeastward
again at •110øE (Plate 1). The PF movesnorthward again
while crossingthe SoutheastIndian Ridge at -145øE and then
closelyfollowsthe topographyinto the Pacific(Plate 1).
3.2.
Comparisons With Previous Results
In general, our mean path for the PF is in good agreement
with the mean paths of Gille [1994], Orsi et al. [1995], and
Belkin and Gordon [1996], all basedon subsurfacemarkersof
the PF (Plate 2). This indicatesthat while the location of
MOORE
ET AL.: SATELLITE
MAPPING
OF THE
oow -4sos
ANTARCTIC
POLAR
FRONT
3063
oo.
. "::'"..I
...
oo.
55øS
ß
70øW
;'
•. 70øE
90øW
110øW
Gille(1994)
\
'%
\
/
/-z,,,,
I Orsi
eta,.
(,995
•.-
.,.
.. -...__,,
,
•._ m,,,..-
,.-•-,-" •
,,
...'k2'
x"'"""./•
/•
/1.o. no•
' "'"' '"'
55øs
.•
Belk,n
and
Gordon
(1996)
•--------.'•••[••'•'
170øW45øS 170øE
1 This
Study
150øE
Plate 2. Our mean path for the Antarctic Polar Front (black line) comparedwith the mean pathsof Gille
[1994](dottedgreenline), Orsiet al. [1995](red line), andBelkinand Gordon[1996](blue line).
subsurfaceand surfaceexpressionsof the PF may differ over
short timescales,their mean location is closelycoupled over
much of the Southern
Ocean.
Our PF path agreesvery well with the paths of Orsi et al.
[1995] and Belkin and Gordon [1996] in the vicinity of large
bathymetricfeatures,suchasalongthe Pacific-AntarcticRidge
(typicallyseparatedby <1 ø of latitude;Plate 2). The four PF
mean pathsin Plate 2 are in closestagreementin severalareas
where topographicsteeringof the front is particularlystrong
(---140ø-148øW,---65ø-50øW,
40ø-45øE,and from ---76øto 81øE;
comparePlates 1 and 2). The distancesseparatingthe paths
increaseover deep basin areas (115ø-85øW,25ø-40øE,55ø65øE,and 90ø-140øE)and in the vicinityof KerguelenPlateau
and Ewing Bank on the Falkland Plateau,where there may be
frequent surface/subsurface
separations[Sparrowet al., 1996;
Moore et al., 1997].
A surveyof the literature revealed a number of instances
where a ship crossedthe Antarctic PF at approximatelythe
sametime (within2 weeks)as our satellitemappingof the PF
(seeTable 1). Our locationfor the PF is typicallysouthof the
ship-determinedlocations,on average0.7øof latitude. Our PF
location marks the poleward edge of the front, which is
---35-55
km wide. This indicates a small mean southward
dis-
placement,albeitwith considerable
variability(Table 1). Lutjeharms and Valentine[1984] report the surfaceexpressionas
south of the subsurfaceexpression75% of the time in the
region south of Africa.
Petersonand Whitworth[1989] present data from several
crossingsof the subsurfaceexpressionof the PF in the SW
Atlantic for two periods(seeTable 1). The surfaceexpression
of the PF is typicallynot presentin their transects,generally
being located farther south. During both time periods the
distanceseparatingthe surfaceand subsurfaceexpressions
of
the PF is small near 41øW and increases in areas farther
east
(Table 1). This is consistentwith previousresults[Deacon,
1933; Guretskii,1987].
The mean temperature change acrossthe front over the
7-year period was 1.44øCacrossan averagewidth of 43 km.
3064
MOORE
ET AL.: SATELLITE
MAPPING
10øW
OF THE
45øS•
ANTARCTIC
POLAR
FRONT
10OE
õ5ro
$ ß ...
t3o1::
70øW
g0øE
10øE
110øW•
130øW
I
130øE
Spring
Summer
Fall
150øW
150øE
Winter
170øW45øS 170øE
Plate 3. The seasonalmean pathsof the AntarcticPolar Front are displayed.
variabilityin the locationof the PF is highestover deep basin
areas(120ø-90øW,30ø-10øW,30ø-70øE,and 90ø-140øE).Variability is lower near large bathymetricfeatures, where the
ocean floor is steeplysloped (at the mid-oceanridges,near
78øW at the Antarctic Peninsula,within Drake Passage,along
the Falkland Plateau, and east of Kerguelen Island along the
KerguelenPlateau;Plate I and Figure 2).
The regionalvariability apparent in Figures 1 and 2 can be
quantifiedif we examinethe behaviorof the PF averagedover
5ø longitudinalbins (Figure 3). For this statisticalanalysisthe
region 50ø-45øWhas been divided into two bins north and
south of the North Scotia Ridge (this ridge separatestwo
different domains;see Figure 1 and Plates 1 and 2; see also
MeanPPV alongthe meanpathof the PF was32.08x 10-9 Mooreet al. [1997]).All cross-correlation
coefficientsgivenfor
m-• s-•. Therewere largeregionalvariationsin all of these Figure 3 are significantat the 95% confidencelevel by Stuparametersof the PF.
dent'st test, (exceptwhere notedotherwise).
There is a strongpositivecorrelationbetweenthe tempera3.3. Spatial Variability
ture changeacrossthe front and the width of the PF (Figures
coefficientof 0.73). Gille [1994]
The spatialrms displacementof AntarcticPolar Front (PF) 3a and 3b, cross-correlation
path pointsat right anglesfrom the mean path wascalculated reported that the width of the PF varied by -20% over large
(Figure2). ComparingPlate 1 and Figure2, it canbe seenthat spatialscales.We observeda similarvariabilityin frontalwidth
This cross-frontaltemperature changeis somewhatlessthan
previousestimates(1.7øC [Mackb•tosh,1946], 1.8øCsouth of
Africa [Lutjeharms,1985],1.9ø-2.0øC
in the southwest
Atlantic,
[Guretskii,1987],1.6øC(35ø-49øE),1.9øC(97ø-112øE)[Belkin,
1989], and 1.7øCfor the region90ø-20øW[Mooreet al., 1997]).
Our lower estimate is likely due to better spatial resolution
(---9km) than mostshiptransectsandincreasedsamplingover
oceanbasin areaswhere the SST gradient is typicallyweaker.
Our mean PF width of 43 km agrees well with the 44 km
estimateof Gille [1994] and the 40 km estimateof Sciremammanoet al. [1980].The meanspatialrmsdisplacement
was136
km. This is higherthan the valueof 100 km reportedpreviously
for the Drake Passage/Scotia
Sea region [Mooreet al., 1997].
MOORE
ET AL.:
SATELLITE
MAPPING
OF THE
ANTARCTIC
POLAR
FRONT
3065
Table 1. A Comparisonof the Positionof the AntarcticPolar Front (PF) from shipTransects(Subsurfaceand Surface
Location)and With Our Satellite-DerivedSurfacePositionat Approximatelythe SameTime (Within 2 Weeks)
Reference(PF Crossing)
Read et al. [1995, Figures4-7]
(Nov. 14, 1992)
(Dec. 11, 1992)
Petersonand Whitworth[1989, Figure 11]
(March 22 to April 5, 1997)
(April 5-16, 1987)
Ikeda et al. [1989, Figure 4]
(March 12-20, 1987)
Tsuchiyaet al. [1994, Figure 2]
(Feb. 13, 1989)
Laubscheret al. [1993, Figure 5]
(first half Dec. 1990)
(mid-Feb. 1991)
Robertsonand Watson[1995,Figure 5]
(Feb. 11-12, 1993)
Longitude
Subsurface
-56øW
-88øW
57.8øS
61.5øS
Surface
58.2øS
61.5øS
-41.2øW
49.3øS
...
•39øW
49.3øS
...
Satellite(week)
58.7øS(Nov. 5-11 and Nov. 12-18)
63.9øS(Dec. 17-23)
•41øW
49.3øS
...
•39øW
•38øW
49.3øS
49.0øS
...
...
49.6øS(March 26 to April 1)
49.8øS,50.8øS(March 19-25 and March 26 to
April 1)
49.7øS(April 2-8)
50.3øS(April 2-8)
50.7øS(April 2-8)
-54øW
56.4øS
...
56.5øS,56.4øS(March 5-11 and 11-17)
-33øW
49.5øS
•49.0ø-50.0øS
50.6øS(Feb. 12-18)
•4øW
•27øW
....
....
48.8ø-49.5øS 50.9øS,52.3øS(Dec. 10-16 and 17-23)
49.3ø-50.3øS 53.4øS,52.7øS(Jan.29 to Feb. 4 and Feb. 12-18)
•20øE
...
51.0ø-51.6øS 51.0øS(Feb. 12-18)
Week of satelliteobservation
givenasyear (firsttwo digits)andweek (secondtwo digits).Where possible,the dateof the in situPF crossing
was obtained
from the authors of the listed references.
(Figure 3b). Both temperaturechangeand width of the PF
were inverselycorrelated with meanderingintensity as measuredby the spatialrms displacement(Figures3a, 3b, and 3c;
cross-correlation
coefficientsof -0.65 and -0.54, respective-
ly). Both temperaturechangeandwidth of the front were also
inverselycorrelatedwith oceandepth (Figures3a, 3b, and 3e;
cross-correlation
coefficientsof -0.19 (not significantat the
95% confidencelevel) and -0.46, respectively).
1oow 45os_._ 10øE
30OE
;OOE
70OE
90ow --
90OE
11
10øE
130øW
150øw
1'50"E
170øW45øS 170øE
Figure 2. The mean path of the Antarctic Polar Front plus or minusthe root mean squareof the spatial
displacements
of AntarcticPolar Front path pointsat right anglesto the mean path is shown.
3066
MOORE
ET AL.'
SATELLITE
MAPPING
OF THE
ANTARCTIC
POLAR
Pacific-Antarctic
Ridge
FRONT
Southeast Indian
DrakePassage
Kerguelen
Plateau Ridge
•..
2.0
a ...............
•o
1.8
•"
1.6
•_cE•
1.4
1.0
55
50
45
40
,..-,
35
'•o
½J•
•'"
•
d
-
-
-
I
-
.
-
I
-
-
-
I
-
.
ß
1.6
• >,
1.4
1.2
1.0
1000
F:
2000
•
3000
ß
4000
5000
......
!
'
-
-
-
i
'-I
80
40
20
60
40
20
g
g
g
g
g
g
S
'"
'"
'"
'"
'"
'"
Figure 3. Displaysare somestatisticalpropertiesof the AntarcticPolar Front averagedover5ølongitudinal
bins.Temperaturechangeacrossthe (a) front,(b) frontalwidth,(c) spatialrmsdisplacement,
(d) mesoscale
variability(relativeunits,seetext for details),(e) oceandepth,(f) percentageof weekssomeportionof the
PF wasmapped,and (g) percentageof pixelsbetween45øand65øSwith no datain ourweeklyimagesbecause
of persistentcloud cover are shown.
The PF is intensified (increasedwidth and temperature
changeacrossthe front) at majorbathymetricfeatures,including the Pacific-AntarcticRidge, Drake Passage,the Kerguelen
Plateau, and crossingthe SoutheastIndian Ridge (Plate 1,
Figures 3a, 3b, and 3e). An exceptionwas the Mid-Atlantic
Ridge,where therewasno intensification(Figures3a, 3b, and
3e). Dynamicheightcontourstypicallypinchtogetherin these
regions[Gordonet al., 1978]. Gille [1994] found that the total
height difference acrossthe PF and SAF increasedwhere the
ACC crossedtopographicfeatures.This relationshipbetween
topographyand temperaturechangeacrossthe front is apparent in the data of Mackintosh[1946], with weaker gradients
overlyingthe oceanbasinareas.Emery [1977]notedthat ACC
fronts were broad and diffuse in the southeastPacific, where
no large topographyacts to concentratethe ACC flow. The
strengthof the SST gradient(temperaturechangedividedby
frontal width) also increasedin these areas associatedwith
largetopographicfeatures.Gradientstrengthaveragedover5ø
longitudinalbins ranged from 2.6ø to 4.1øC/100km and was
strongestjust downstreamof KerguelenPlateau.
The intensificationof the PF associatedwith major bathymetric featurespersistsfor somedistancedownstream.In fact,
three parameters(width, temperaturechangeacrossthe front,
and the percentageof weeksthe front wasmapped;Figures3a,
3b, and 3f) had highercross-correlation
coefficients
with ocean
depth(Figure3e) whentheylaggeddepthby one longitudinal
bin. The crosscorrelationsfor Figures3a, 3b, and 3f lagging
Figure 3e by 1 were -0.27, -0.50, and -0.54 (all significant
and all higherthan with no lag). Thus increasedcross-frontal
widthsand temperaturegradientsare seendownstreamof the
Pacific-Antarctic Ridge (---135ø-128øW), Drake Passage
(--•55ø-45øW),KerguelenPlateau(--•79ø-86øE),andthe SoutheastIndian Ridge (--•150ø-155øE;
seePlate 1 and Figure3).
It is in theseareasof intensificationassociated
with major
MOORE
ET AL.:
SATELLITE
MAPPING
bathymetricfeaturesthat we seeelevatedmesoscalevariability
at the PF (Figure 3d). Mesoscalevariabilitywas stronglycorrelatedwith cross-frontaltemperaturechangeandwidth (cross
correlationsfor Figure 3d with Figure 3a and 3b were 0.61 and
0.40, respectively).Mesoscalevariabilitywas also significantly
correlatedwith depth when it laggeddepth by one bin (cross
correlation for Figure 3d lagging Figure 3e by one bin of
-0.20). We alsoobservedthe formationof warm and coldcore
ringsat the PF mostfrequentlyin theseareasjust downstream
of large topographicfeatures(ring formation at the PF will be
discussedin detail elsewhere).Altimeter, hydrographic,and
drifter studiesof the Southern Ocean have also found high
eddy variability in these areas [Lutjeharmsand Baker, 1980;
OF THE
ANTARCTIC
POLAR
FRONT
3067
the fall, a pattern observedby Mackintosh[1946]. The mean
seasonalcycle at the poleward edge of the PF varies from
---2.8øCduringthe summerto ---0.4øCin the winter (Figure4a).
Houtman [1964] calculateda mean summer/wintertemperature differenceof 2.1øCfor a constantlatitude.Lutjeharmsand
Valentine[1984] gave a mean poleward edge temperature of
2.5øCfrom mainly summertimecrossingsof the PF south of
Africa.
topographyin regionsof reducedambient vorticity gradients
[Witterand Chelton,1998]. The increasein frontal width associatedwith crossingmajor bathymetricfeaturesresemblesthe
"supercritical to subcritical" transition described by Pratt
[1989].
Note the strong correlations between the percentage of
weeksthat someportionof the PF wasmappedand the change
in temperature acrossthe PF, the width of the PF, and ocean
depth (Figures3f, 3a, 3b, and 3e; cross-correlation
coefficients
of 0.74, 0.76, -0.42, respectively).The PF was mapped most
frequently in areas where the PF was intensified.The SST
gradientwas easierto detectin theseareas.Fewer pathswere
mapped over the deep oceanbasins(Figures 3e and 3f). In
theseareasthe PF SST gradientis weakened(Figures3a and
3b), often making it difficult to distinguishfrom the backgroundmeridionaltemperaturegradient.Figure 3g showsthe
mean percentageof pixels (45ø-65øS)with no data in our
weekly imagesbecauseof persistentcloud cover. Drake Passageconsistentlyhad the least cloud-maskedareas (17.5%),
Weakest seasonalitywas seen where the PF was at lower
latitudes(Figure 4b). Seasonalitywasgreater at high latitudes
(Figure 4c). Lowestwinter temperatureswere seenjust westof
Drake Passage(80ø-65øW), where SST at the PF at times
approachedthe freezing point of seawater(Figure 4d). The
cold temperaturesobservedin this area likely reflect substantial amountsof sea ice enteringthe PF. The maximumextent
of the seasonalice sheetcan reachthe PF during australwinter
in this region [Gloersenet al., 1992]. SST was consistently
higher (--•iøC) in the region just to the east (65ø-45øW,
Figure 4e).
Seasonalvariations in the properties of the PF were relatively small comparedwith the spatialvariability(Figure 5).
The PF mean width and temperature gradient were highest
during the spring(44.4 km and 1.53øC;Figures5a and 5b).
Weakest seasonalgradientsand narrowestfrontal widthswere
observedduringthe fall (1.37øCand 42.6 km). Mean latitude
was farthest south during the winter at 57.0øSand at lowest
latitudes during the fall at 55.9øS(Figure 5d). Spatial rms
displacements
were higher during the summerand fall (140
and 142 km, respectively).Meandering intensitywas lower
duringwinter and spring(spatialrms displacementof 130 km
for both seasons).There is an inverserelationshipbetween
spatial rms displacementsand both width and temperature
changeacrossthe front. Fewer PF pathswere mapped during
fall and winter comparedwith the spring-summerperiod (Figure 5f).
In areaswhere the Antarctic PF wasmapped alongthe same
longitudinalline in successive
weeks an averageweekly latitudinal shift of 22 km was calculated(n - 174). A maximum
shift of 177 km wasobservedin the northern ScotiaSea [Moore
et al., 1997].Note that temporalcoverageof the PF wasbetter
in low-variabilityareas(Figure 3c and 3f); thusour value of 22
km may be an underestimate.
Seasonalvariability in the location of the PF was qualitatively similar to the spatial variability describedabove (Plate
3). Seasonalvariabilitywas low where topographicsteeringof
the PF is strong,and it was higher over the deep oceanbasins
(Plates 1 and 3). Some seasonalitywas observedin the area
from ---175øEto 170øW;the PF moved southwardduring the
summerandfarthestnorth duringwinter. Theseseasonalshifts
maybe due to the influenceof the seasonalice sheetextending
while a maximum
from the Ross Sea. A southward
Daniault and Mdnard, 1985; Cheltonet al., 1990; Morrow et al.,
1990; Gouretskiand Danilov, 1994; Gille, 1994].
The intensificationof the PF may lead to baroclinic instabilities and eddy/ringformation in these regions.Mesoscale
variabili_tywas also highly correlatedwith the strengthof the
SST gradientacrossthe PF when it laggedgradientstrengthby
one longitudinal bin (cross correlation of 0.52). Gradient
strengthis a measureof verticalshearwithin the PF. Baroclinic
instabilitiesare more likely in areas of increasedshear. Mesoscalevariability was also significantlycorrelatedwith PPV
when laggingPPV by one longitudinalbin (crosscorrelationof
0.22). This suggests
that the relativevorticityput into the water
columnwhere the topographyforceschangesin PPV is dissipated downstreamby eddy and ring formation. Witter and
Chelton[1998] studiedthe effectsof zonal-varyingtopography
on a meanderingjet with a quasi-geostrophic
channel model.
Eddy kinetic energy and the growth rate of instabilities
reached
their
maximum
downstream
of zonal
variations
in
in cloud cover of 59.2% was seen 85ø-90øE.
Our PF coveragewas correlatedwith this measureof cloudiness(Figures3f and 3g; crosscorrelationof -0.43). This correlation is significantat the 95% confidencelevel, but it is
weaker than the correlationswith frontal intensity.Thus, despite persistentlycloudy conditionsover the Southern Ocean
our temporal coverage in most areas was limited by the
strengthof the SST gradient.
shift of the surface PF relative
to the subsurfaceexpressionhas been documentedsouth of
Africa (0ø-30øE)during australsummer[Lutjeharmsand McQuaid, 1986; Lutjeharms and Foldvik, 1986]. This may not
alwaysbe the case,however,asour seasonalmean pathsdo not
show a mean southwardshift during summer in this region
(Plate 3). Our meanpath is southof thoseOrsiet al. [1995]and
BelkinandGordon
[1996]in thisregion,
perhaps
indicating
that the surfaceexpressionis frequently south of the subsurface position(Plate 2).
Mean temperature at the poleward edge of the PF for sevVariability at interannualtimescaleswas alsostronglyinflueral regions is displayedin Figure 4. In general, springtime enced by the underlyingtopography(Plate 4). The annual
warmingis relativelyrapid,with a more gradualcoolingduring mean paths are in closeagreementwhere topographiceffects
3.4.
Temporal Variability
3068
MOORE ET AL.' SATELLITE MAPPING OF THE ANTARCTIC POLAR FRONT
The dynamicsdrivingmuchof the observedspatialand
temporalvariabilityin the locationof the PF canbe understoodif we examinethe meanpath of the PF in the contextof
2
the PPV field. In Plate 5 the mean path of the PF is shown
overlainon a map of PPV. The meanPPV alongthe PF has
beencalculated
withina moving2øof longitudewindow.Along
eachlineof longitudea relativelynarrowrangeof PPV values
0
-1
-2
.........
, .........
, .........
4•
, .........
,.,
(10øW-30ø[)
bi
2
-•
, .........
-2
::::::::::::::::::::::::::::::::
ß•
.
• 4••1••,(90ø-80øW'
•
E
2
•
-1
I
o
46
B -2 ...........
m
•
, .................
4
E
44
•
42
b
40
3
•:
2
38
36
•I
I•
•I
•I
i
i
i
I
I
I
• 145
E
-1
-2
:::::::::::::::::::::::::::::::::::::::::::::::::::
(so-soow)
$135
• 125
1
o
03 115
'1
.
3.0-
-2
0
10
20
30
40
-
50
Week
Figure 4. The seasonalcycleof sea surfacetemperature
(SST) at the polewardedgeof the AntarcticPolarFrontis
shownfor selectedregions:(a) the circumpolar
mean,(b)
10øW-30øE,
(c) 90ø-80øW,(d) 80ø-65øW,
and(e) from 65ø50øW. Error bars show _+1 standard deviation.
•
o
o
2.5-
2.0-
d
.
.
•_ 1.503 1.0
0.5.
o
onthePF arestrongest
(---148ø-150øE,
---145ø-140øW,
through
muchof the Drake Passage/Scotia
Searegion,---15ø-20øE,
and
---75ø-85øE;
Plates1 and 4). Interannualvariabilitywasalso
elevatedabovethe deepoceanbasins(Plate4). Cheltonet al.
[1990] found that mesoscale
variabilityin the ACC was
strongly
influenced
by topography
andshowed
littletemporal
variabilityover seasonalor interannualtimescales.
3.5. Planetary Potential Vorticity at the PF
PPV doesnot remainconstantalongthe meanpath of the
PF (Figure6a). It canbe seenin Figure6 thatdespitelarge
latitudinalshiftsalongthe meanpathof thePF, PPV islargely
a functionof the underlyingtopography(Figures6a and 6c;
cross-correlation
coefficient
of -0.75). PPV increases
andthe
PF movesequatorward
eachtime it is forcedinto areasof
decreasing
oceandepth(Figures6a,6b,and6c).Aftercrossing
Figure 5. Seasonal
meanvaluesfor severalproperties
of the
eachtopographic
featurethe frontturnspoleward,andPPV AntarcticPolarFront are shown.Displayedvaluesincludethe
returns
to a relatively
constant
valueof---25x 10-9 m-• s-•
despitelargelatitudinalshifts(Figure6). The bottomslope
across
the PF wassignificantly
correlatedwiththe temperature
changeacross
thefront,frontalwidth,andmesoscale
variability (crosscorrelations
betweenFigure6d andFigures3a, 3b,
and 3d of 0.31, 0.45, and 0.35, respectively).
temperature
changeacross
(a) the front,(b) thewidthof the
front,(c) themeanspatialrmsdisplacement
at rightangles
to
the meanpath,(d) themeanseasurface
temperature,
(e) the
mean latitude,and (f) the numberof path pointsdigitized.
Error bars indicate _+1 standarderror assumingeachweekly
imageis an independent
sample(n = 364).
MOORE
ET AL.' SATELLITE
MAPPING
OF THE ANTARCTIC
Pacific-Antarctic
•
Ridge
POLAR
Southeast
Drake Passage
FRONT
3069
Indian
KerguelenPlateau Ridge
•, --'- 6o
• •
40
o..•.
30
•
20
•-
10
O
•--
152
--
45
50 G'
55 ©
60 •
65
lOOO
2000
3000
4000
5000
15
10•E
•
v
o
0
Figure 6. Displayedare somestatisticalpropertiesof the AntarcticPolar Front averagedover 5ø longitudinalbins.The averageplanetarypotentialvorticityalongthe meanpath of (a) the PF, (b) the latitude,(c)
the oceandepth,and (d) the slopeof the oceanfloor acrossthe PF at right anglesto the flow directionare
shown.
aroundthis runningmeanvalueis shownin white. This range 4.
consists
of themeanvalueplusor minustwounitsof 10-9 m-1
s-l'. Thiscorresponds
to themeanvalue_+-7%.Thisrangeof
Conclusions
Variability in the location and dynamicsof the Antarctic
PolarFront are largelya functionof the underlyingtopography
valuesshownin white canbe considereda localplain of quasiand correspondingPPV field. Over the deep ocean basins,
constantPPV [seeMoore et al., 1997].
where
broad plainsof quasi-constant
PPV are found, the surComparing Figure 1 and Plate 5, it can be seen that the
face expressionof the PF is weakened,and the PF meanders
envelopeof PF pathscorresponds
with the size and shapeof
this plain of quasi-constant
PPV. The broad plains in PPV over large latitudinal ranges.Near large bathymetricfeatures
associatedwith the ocean basin areas (-130ø-80øW, 25ø- (with large gradientsin PPV) the PF is intensified,and mean10øW,52ø-65øE,and 85ø-140øE)are preciselythe areaswhere dering is inhibited.
In regionswhere the PF is forcedacrossisolinesof PPV by
the envelopeof PF pathswidens(Figure 1 and Plate 5). Variability in the locationof the PF was highestat all temporal the topography,relativevorticityis input to the water column
of vortex lines. This relative
scalesover thesebroad PPV plains(Figures2 and 3, Plates3 through the shrinking/stretching
vorticity
is
likely
dissipated
through
nonlinearprocesses,
such
and 4). In theseareasthe PF canmeandersubstantially
while
as
eddy
actions.
Both
width
and
temperature
change
across
the
maintaininga relativelyconstantPPV. In contrast,the areasof
low variability associatedwith large bathymetric features PF increasein these areas.Large-scalemeanderingis inhibvariabilityincreases(Figures3c and 3d).
(alongthe mid-oceanridges,the Falkland and KerguelenPla- ited, but mesoscale
The intensificationof the PF initiatedat large topographic
teaus, -78øW where the PF encounters the Antarctic Peninsula, and throughmuch of Drake Passage[seeMoore et al., featurespersistsfor somedistancedownstream.Thus elevated
1997])havestronggradientsin PPV and thusrelativelynarrow mesoscalevariability and increasedwidth and temperature
PPV plains(comparePlates1 and 5). Even thoughmeanPPV change acrossthe PF are seen downstreamof the Pacificchangesdrasticallyalongthe circumpolarpath of the PF (Fig- Antarctic Ridge, Drake Passage,Kerguelen Plateau, and the
ure 6a), locallythe PF tendsto maintaina relativelyconstant SoutheastIndian Ridge. The intensificationof the PF in these
PPV. Thus the envelopeof PF pathsis highlycorrelatedwith areas may lead to baroclinicinstabilitiesand increasededdy
the size and shapeof the local plain of quasi-constantPPV activity.It is here that we primarily observedthe formation of
(compareFigure 1 and Plate 5).
warm and cold core rings along the PF.
3070
MOORE
ET AL.: SATELLITE
MAPPING
OF THE
ANTARCTIC
POLAR
FRONT
70øW
70øE
90øW
90øE
110øW
110øE
1987 1
1988 1
1989
1990
1991
1992
1993
Plate 4. The mean annual path for each of the years 1987-1993 is shown.
Deacon[1937]arguesthat the locationof the Antarctic PF is
determinedby the movementsof Circumpolar Deep and Bottom waters.Specifically,the PF is located at the point where
density isotherms associatedwith warm Circumpolar Deep
Water bend sharply toward the surface as they move southward overridingAntarctic Bottom Water [Deacon, 1937].The
stronginfluenceof topographyon surfaceflow apparentin our
resultssupportsthis hypothesis.Surfaceflowsin the Southern
Ocean can respondto mesoscalebathymetricfeaturesdue to
weak densitystratificationand relatively small RossbyRadii.
The crucialrole of the topography/PPVfield in constraining
the dynamicsof the PF implies that ocean circulationmodels
mustincorporaterealistictopographyat mesoscalespatialresolution before they will be able to reproducethe behavior of
the PF. The closecorrelationsbetweenthe dynamicsof the PF
and the underlyingtopographyalsosuggestthat hydrographic
measurements
made at severallocations(includingover ocean
basinsand in topographicallycontrolled areas) might be extrapolatedto make circumpolarcalculations(i.e., of poleward
heat flux, etc.).
The Antarctic PF is a circumpolarfeature in the sensethat
it is seen at all longitudeswithin the Southern Ocean (Figure
1). However, the surfaceexpressionof the PF is a dynamic,
regionallyvarying feature that is intensifiednear large topographic features and weakensover deep basin areas (to the
point that it wasfrequentlynot detectablein our analysis,even
in unobstructed,cloud-freeimages).Erne/y [1977] noted this
weakeningof ACC fronts over areaswithout large topographic
obstructions.
The mean pathsof the surfaceand subsurfaceexpressions
of
the PF are closelycoupledover much of the SouthernOcean.
When surface/subsurfaceseparationsoccur, the surface expression typically lies south of the subsurface expression
[Lutjeharms and Valentine, 1984; Spa/row et al., 1996; this
study].This likely reflectsthe fact that the thermodynamicsof
the two water
antarctic
waters
masses makes
to override
it easier for the less dense Subdenser Antarctic
surface waters.
The postulated frequent surface/subsurface
separationsat
EwingBank [Mooreet al., 1997],at the Ob'-Lena Rise [Sparrow
et al., 1996], and at the Udintsev Fracture Zone (this study)
MOORE ET AL.: SATELLITE
MAPPING
OF THE ANTARCTIC
POLAR FRONT
3071
10ow
30oW
50øW
50øE
,,,
-65oS
70øE
90øE
90øW
110øE
70øS
, -.,
t::•' 130
ß 130OE
,,,,
• ,,a. '•
75
50
• •
20
170øW 45øS 170øE
Plate 5. Meanpathof thePolarFrontoverlainon a mapof planetary
potentialvorticity(f/H). The mean
planetary
potential
vorticity
alongthePFwascalculated
at eachlongitude
frompoints
withina moving
2øof
longitude
window.
A range
ofplanetary
potential
vorticity
values
consisting
ofthemean
value2 x 10-9 m-]
s-• alongeachlineof longitude
isshownin white.Thisrangecorresponds
to themeanvalue+__7%.
Acknowledgments.The authorswould like to thank I. Belkin, S.
Gille, and A. Orsi for providingdigitizedversionsof their Antarctic
PolarFront paths.Specialthanksalsoto I. Belkinand an anonymous
reviewerfor helpfulcomments
andsuggestions.
Thisworkwasfunded
UdintsevFracture Zone the SAF frequentlycomesinto close by a NASA Earth SystemScienceFellowship(J. K. M.), by NASA/
contactwith the PF, at timesmergingto form a singlefront EOS grantNAGW-4596(M. R. A.), andby NSF grantOCE-9204040
have some features in common. In each location the subsur-
face expression
of the PF maybe lockedin placeby the topography(seePlate 1). In addition,at EwingBank and the
[Peterson
and Whitworth,
1989;thisstudy].
(J. G. •.).
Similar SAF/PF interactions occur west of the Ob'-Lena
Risenear30øE[Orsiet al., 1993,1995;ReadandPollard,1993; References
Belkinand Gordon,1996;thisstudy].Gille [ 1994]notesthat the Belkin, I. M., Alteration of the front distributionsin the Southern
PF and SAF convergeat -33øE. It may be that the surface/
Ocean near the Crozet Plateau, DoM. Acad. Sci. USSR, Earth Sci.
subsurface
separationdescribedby Sparrowet al. [1996] is
Ser.,Engl. Transl.,308, 265-268, 1989.
Belkin, I. M., Front structureof the SouthAtlantic, in Pelagicheskie
YuzhnogoOkeana(PelagicEcosystems
of the Southern
the SAF. While our mean path (and likely the subsurface Ekosistemy
Ocean),(in Russian),editedbyN.M. Voronina,pp.40-53, Nauka,
initiated west of the Ob'oLena Rise where the PF interacts with
expression)
lies north of the Ob'-Lena Rise, we mappeda
numberof PF pathsover and southof the rise (Figure 1 and
Plate 1).
Moscow, 1993.
Belkin,I. M., Main hydrological
featuresof the centralSouthPacific
(in Russian),in PacificSubantarctic
Ecosystems,
editedby M. E.
3072
MOORE
ET AL.:
SATELLITE
MAPPING
Vinogradov and M. V. Flint, pp. 21-28, Nauka, Moscow, 1988.
(English translation,pp. 12-17, N. Z. Transl. Cent., Wellington,
N. Z., 1997.)
Belkin, I. M., A. L. Gordon, Southern Ocean fronts from the Greenwich meridian to Tasmania,J. Geophys.Res., 101, 3675-3696, 1996.
OF THE
ANTARCTIC
POLAR
FRONT
Ikeda, Y., G. Siedler, and M. Zwierz, On the variabilityof Southern
Ocean
front
locations
between
Southern
Brazil
and the Antarctic
Peninsula,J. Geophys.Res., 94, 4757-4762, 1989.
Inoue, M., Modal decompositionof the low-frequencycurrentsand
baroclinicinstabilityat Drake Passage,
J. Phys.Oceanogr.,15, 1157-
1181, 1985.
Botnikov,V. N., Geographicpositionof the Antarctic Convergence
Johnson, J. A., and R. B. Hill, A three-dimensional model of the
Zone in the SouthernOcean(in Russian),Soy.Antarct.Exped.Inf
Bull., 41, 19-24, 1963. (Soy.Antarct.Exped.Inf Bull., Engl. Transl.,
SouthernOceanwith bottom topography,Deep SeaRes.,22, 745751, 1975.
4, 324-327, 1963.)
Boyer, D. L., R. R. Chen, L. Tao, and P. A. Davies,Physicalmodel of Joyce,T. M., W. Zenk, and J. M. Toole, The anatomyof the Antarctic
bathymetriceffectson the Antarctic CircumpolarCurrent,J. GeoPolar Front in the Drake Passage,
J. Geophys.Res.,83, 6093-6113,
1978.
phys.Res., 98, 2587-2608, 1993.
Brown, J. W., O. B. Brown, and R. H. Evans, Calibration of advanced
Killworth, P. D., An equivalent-barotropic
mode in the Fine Resolution Antarctic Model, J. Phys.Oceanogr.,22, 1379-1387, 1992.
very high resolutionradiometer infrared channels:A new approach
to nonlinear correction,J. Geophys.Res., 98, 18,257-18,268, 1993.
Klyausov,A. V., Positionof the SouthPolar Front near Kerguelenand
Heard Islandsin the autumnof 1987, Oceanology,
Engl Transl.,30,
Burling,R. W., Hydrologyof circumpolarwaterssouthof New Zeal142-148, 1990.
and, N. Z. Dep. Sci. Ind. Res.Bull., 143, 66, 1961.
Chelton, D. B., M. G. Schlax, D. L. Witter, and J. G. Richman, Geosat
Koblinsky,C. J., The global distributionof f/H and the barotropic
altimeter observations of the surface circulation of the Southern
responseof the ocean,J. Geophys.Res., 95, 3213-3218, 1990.
Ocean, J. Geophys.Res., 95, 17,877-17,903, 1990.
Laubscher,R. K., R. Perisinotto,and C. D. Mcquaid,Phytoplankton
Cornilion, P., The effect of the New England seamountson Gulf
productionandbiomassat frontalzonesin the Atlantic sectorof the
Southern Ocean, Polar Biol., 13, 471-481, 1993.
Stream meanderingas observedfrom satellite IR imagery,J. Phys.
Oceanogr.,16, 386-389, 1986.
Lee, T., and P. Cornilion,Temporalvariationof meanderingintensity
Daniault, N., and Y. Mdnard, Eddy kinetic energydistributionin the
and domain-widelateral oscillationsof the Gulf Stream,J. Geophys.
Res., 100, 13,603-13,613, 1995.
Southern Ocean from altimetry and FGGE drifting buoys,J. Geophys.Res., 90, 11,877-11,889, 1985.
Legeckis, R., Oceanic Polar Front in the Drake Passage:Satellite
Deacon, G. E. R., A general accountof the hydrologyof the South
observationsduring 1976,Deep Sea Res.,24, 701-704, 1977.
Atlantic Ocean, DiscoveryRep., VII, 177-238, 1933.
Lutjeharms,J. R. E., Locationof frontal systemsbetweenAfrica and
Deacon, G. E. R., The hydrologyof the Southern Ocean, Discovery
Antarctica: Some preliminary results,Deep Sea Res., Part A, 32,
Rep., XV, 1-124, 1937.
Deacon, G. E. R., Kerguelen, antarctic and subantarctic,Deep Sea
Res., 30, 77-81, 1983.
Emery, W. J., Antarctic Polar Frontal Zone from Australia to the
Drake Passage,J. Phys.Oceanogr.,7, 811-822, 1977.
Gamberoni, L., J. Geronimi, P. F. Jeannin,and J. F. Murail, Studyof
frontal zones in the Crozet-Kerguelen region, Oceanol.Acta, 5,
289-299, 1982.
1499-1509, 1985.
Lutjeharms,J. R. E., and D. J. Baker Jr., A statisticalanalysisof the
meso-scaledynamicsof the SouthernOcean,Deep SeaRes.,Part A,
27, 145-159, 1980.
Lutjeharms,J. R. E., and A. Foldvik, The thermal structureof the
upperoceanlayersbetweenAfrica and Antarcticduringthe period
December 1978 to March 1979, S. Aft. J. Antarct. Res., 16, 13-20,
1986.
Gille, S. T., Mean sea surfaceheight of the Antarctic Circumpolar Lutjeharms,J. R. E., and L. H. McQuaid, Changesin the structureof
Current from Geosat data: Method and application,J. Geophys.
thermaloceanfrontssouthof Africa over a three-monthperiod,S.
Res., 99, 18,255-18,273, 1994.
Aft*.J. Sci., 82, 470-476, 1986.
Gille, S. T., The Southern Ocean momentum balance: Evidence for
Lutjeharms,J. R. E., and H. R. Valentine, Southern Ocean thermal
fronts southof Africa, Deep SeaRes.,Part A, 31, 1461-1475, 1984.
topographiceffects from numerical model output and altimeter
data, J. Phys.Oceanogr.,27, 2219-2232, 1997.
Mackintosh,N. A., The Antarctic Convergenceand the distributionof
Gloersen, P., W. J. Campbell, D. J. Cavalieri, J. C. Comiso, C. L.
surfacetemperaturesin Antarctic waters, DiscoveryRep., XXIII,
177-212, 1946.
Parkinson,and H. J. Zwally,ArcticandAntarcticSeaIce, 1978-1987:
SatellitePassive-Microwave
Observationsand Analysis,Natl. AeroMoore, J. K., M. R. Abbott, and J. G. Richman, Variability in the
naut. and SpaceAdmin., Washington,D.C., 1992.
locationof the AntarcticPolar Front (90ø-20øW)from satellitesea
Gordon, A. L., D. T. Georgi, and H. W. Taylor, Antarctic Polar Front
surfacetemperaturedata,J. Geophys.
Res.,102, 27,825-27,833,1997.
Zone in the western Scotia Sea: Summer 1975,J. Phys.Oceanogr.,7, Morrow, R., R. Coleman, J. Bhurch, and D. Chelton, Surfaceeddy
309-328, 1977.
momentumflux and velocityvariancesin the SouthernOcean from
Gordon, A. L., E. Molinelli, and T. Baker, Large-scalerelative dyGeosat altimetry,J. Phys.Oceanogr.,24, 2050-2071, 1994.
namic topographyof the Southern Ocean, J. Geophys.Res., 83, Nowlin, W. D., Jr., and J. M. Klinck, The physicsof the Antarctic
3023-3032, 1978.
CircumpolarCurrent,Rev. Geophys.,
24, 469-491, 1986.
Gouretski, V. V., and A. I. Danilov, Weddell Gyre: Structureof the Olson, D. B., O. B. Brown, and S. R. Emmerson, Gulf Stream frontal
eastern boundary,Deep Sea Res., Part I, 40, 561-582, 1993.
statistics
from Florida Straitsto CapeHatterasderivedfrom satellite
Gouretski, V. V., and A. I. Danilov, Characteristicsof warm rings in
and historicaldata, J. Geophys.Res.,88, 4569-4577, 1983.
the African sectorof the AntarcticCircumpolarCurrent,Deep Sea Orsi, A. H., W. D. Nowlin Jr., T. Whitworth III, On the circulation and
Res., Part I, 41, 1131-1157, 1994.
stratificationof the Weddell Gyre, Deep Sea Res.,Part I, 40, 169Guretskii, V. V., Surface thermal fronts in the Atlantic sector of the
SouthernOcean,Soy.Meteorol.Hydrol.,Engl. Transl.,8, 67-73, 1987.
Hansen, D. V., and G. A. Maul, A note on the use of sea surface
203, 1993.
Orsi, A. H., T. Whitworth III, and W. D. Nowlin Jr., On the meridional
temperaturefor observingoceancurrents,RemoteSens.Environ.,1,
Res., Part I, 42, 641-673, 1995.
Park, Y. H., L. Gamberoni, and E. Charrlaud, Frontal structure, water
161-164, 1970.
extentand fronts of the Antarctic CircumpolarCurrent,Deep Sea
Hofmann, E. E., The large-scalehorizontal structureof the Antarctic
Circumpolar Current from FGGE drifters, J. Geophys.Res., 90,
masses,and circulation in the Crozet Basin, J. Geophys.Res., 98,
12,361-12,385, 1993.
7087-7097, 1985.
Patterson,S. L., Surfacecirculationand kinetic energydistributionin
Houtman, T. J., Surfacetemperaturegradientsat the Antarctic Conthe Southern Hemisphere oceansfrom FGGE drifting buoys,J.
Phys.Oceanogr.,15, 865-884, 1985.
vergence,N. Z. J. Geol. Geophys.,7, 245-270, 1964.
Hughes,C. W., Rossbywavesin the SouthernOcean:A comparisonof Patterson,S. L., and T. Whitworth,PhysicalOceanography,in AntarcTOPEX/POSEIDON altimetrywith model predictions,J. Geophys.
tic Sectorof thePacific,editedby G. P. Glasby,pp. 55-93, Elsevier,
Res., 100, 15,933-15,950, 1995.
New York, 1990.
Hughes,C. W., The AntarcticCircumpolarCurrentasa waveguidefor Peterson, R. G., and T. Whitworth III, The Subantarctic and Polar
Rossbywaves,J. Phys. Oceanogr.,26, 1375-1387, 1996.
Front in relation to deep water massesthrough the southwestern
Atlantic, J. Geophys.Res.,94, 10,817-10,838,1989.
Hughes,C. W., and P. D. Killworth, Effects of bottom topographyin
the large-scalecirculationof the SouthernOcean,J. Phys.Oceanogr., Pratt, L. J., Criticalcontrolof zonaljets by bottomtopography,
J. Mar.
25, 2485-2497, 1995.
Res., 47, 111-130, 1989.
MOORE
ET AL.'
SATELLITE
MAPPING
OF THE
ANTARCTIC
POLAR
FRONT
3073
Read, J. F., and R. T. Pollard, Structureand transportof the Antarctic
CircumpolarCurrent and AgulhasReturn Current at 40øE,J. Geophys.Res.,98, 12,281-12,295,1993.
Read, J. F., R. T. Pollard, A. I. Morrison, and C. Symon, On the
southerlyextentof the AntarcticCircumpolarCurrent in the southeast Pacific,Deep SeaRes.,Part H, 42, 933-954, 1995.
Smith, W. H. F., and D. T. Sandwell,Bathymetricprediction from
dense satellite altimetry and sparseshipboardbathymetry,J. Geophys.Res., 99, 21,803-21,824, 1994.
Sparrow,M.D., K. J. Heywood, J. Brown, and D. P. Stevens,Current
structureof the south Indian Ocean, J. Geophys.Res., 101, 6377-
Robertson, J. E., and A. J. Watson, A summer-time sink for atmo-
Tsuchiya,M., L. D. Talley, and M. S. McCartney, Water-massdistributionsin the westernSouthAtlantic:A sectionfrom SouthGeorgia
Island (54S) northwardacrossthe equator,J. Mar. Res.,52, 55-81,
sphericcarbondioxidein the SouthernOcean between88øW and
80øE,Deep Sea Res.,Part H, 42, 1081-1091, 1995.
Sandwell,D. T., and B. Zhang, Global mesoscalevariability from the
Geosatexactrepeat mission:Correlationwith oceandepth,J. Geophys.Res., 94, 17,971-17,984,1989.
Schlich,R., M. F. Coffin,M. Munschy,H. M. J. Stagg,Z. Ci.L•, anclK.
Revill, BathymetricChart of theKerguelenPlateau,Inst. de Phys.du
Globe, Strasbourg,France, 1987.
Sciremammano,F. Jr., R. D. Pillsbury,W. D. Nowlin Jr., and T.
Whitworth III, Spatial scalesof temperature and flow in Drake
Passage,J. Geophys.Res.,85, 4015-4028, 1980.
Sievers, H. A., and W. D. Nowlin Jr., The stratification and water
massesat Drake Passage,J. Geophys.Res.,89, 10,489-10,514, 1984.
Smith, E., J. Vasquez,A. Tran, and R. Sumagaysay,
Satellite-derived
seasurfacetemperaturedata availablefrom the NOAA/NASS Pathfinder Program,Eos Trans.AGU Electron.Suppl.,1996. (Available
as http://www.agu.org/eos_elec/95274e.html)
6391, 1996.
1994.
Witter, D. L., and D. B. Chelton, Eddy-meanflow interactionsinduced
by modulationsof zonal ridge topographyin a Quasi-Geostrophic
Channel Model, J. Frays.Oceanogr.,28, 2019-2039, 1998.
M. R. Abbott, J. K. Moore, and J. G. Richman,College of Oceanic
and AtmosphericSciences,Oregon State University, 104 Ocean Administrative Building, Corvallis, OR 97331-5503. (mabbott@oce.
orst.edu;jmoore@oce.orst.edu;richman@oce.orst.edu)
(ReceivedApril 15, 1998;revisedSeptember21, 1998;
acceptedSeptember23, 1998.)