JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 90, NO. C4, PAGES 7087-7097, JULY 20, 1985
The Large-ScaleHorizontal Structure of the Antarctic Circumpolar
Current
From
FGGE
Drifters
EILEEN E. HOFMANN
Departmentof Oceano•traphy,
Texas A and M University,Colle•teStation
The last decadeof researchin the SouthernOceanhas shownthat the AntarcticCircumpolarCurrent
(ACC) is a complexsystemcomposedof narrow, high-speedcurrentsseparatedby broad, quiescent
zones.The circumpolarnature of this structurewas examinedusingpositionand velocitydata obtained
from approximately300 surface-drifting
buoysdeployedin the SouthernOcean during the First GARP
Global Experiment(FGGE). The distributionof buoyson 1ø x 1ø squaresshowsthat in someregions,
most notablysouthof Australia,the buoysform three coherentbandsof high buoy densitywhich are
separatedby regions of low buoy density. The latitudes of these bands coincide with those of the
SubtropicalFront, SubantarcticFront, and Polar Front. To further examinethe relationshipbetween
thesefrontsand buoy distribution,locationsof the threefronts,determinedfrom historicalhydrographic
data, were usedto partition the buoysinto zonal bandscorrespondingto front and nonfrontregions.A
meanbuoy densityand mean near-surface
speedwere then computedfor eachzonal band. High buoy
densitieswere associatedwith all three fronts in the region south of Australia.Other regionsalso showed
a tendency,although not as pronounced,for buoys to accumulatein fronts. The mean near-surface
speedssuggestthat the Subantarcticand Polar Fronts are circumpolar.Moreover, the mean near-surface
speedsassociatedwith the three frontal regionsdiffer. Speedswithin the Subantarcticand Polar Front
regionsare approximatelytwice that associatedwith the SubtropicalFront.
1.
INTRODUCTION
The past decade of research in the Southern Ocean has
demonstratedthat the Antarctic Circumpolar Current (ACC)
is a complex systemcomposedof narrow, high-speedcurrents
separatedby broad, quiescentzones.The most intense study
of this zonation of the ACC took place in Drake Passagefrom
1975 to 1980 as part of the International Southern Ocean
Studies program. Hydrographic and current measurements
show that the ACC at this location is composedof two major
fronts, characterizedby large horizontal gradientsof properties and large geostrophicshears,which separate three relatively uniform water masszones [Baker et al., 1977; Nowlin et
al., 1977; Whitworth, 1980; Nowlin and Clifford, 1982]. Ordered from north to south, these fronts and zones are Subantarctic Zone (SAZ), Subantarctic Front (SAF), Polar Frontal
Zone (PFZ), Polar Front (PF), and Antarctic Zone (AAZ). In
Drake Passage,averagewidths [Nowlin and Clifford, 1982] of
the fronts and zones are 50-60 km and 200-300 km, respectively, and mean surfacespeedswithin the frontal regionsare
40-50 cm s-a [Whitworth et al., 1982]. Outside of Drake
Passagethe SAZ is bounded to the north by the Subtropical
Front (STF) and the SubtropicalZone (STZ).
Regional studiesat other locations have also describedthe
meridional zonation of the ACC. Heath [1981] describedthe
thermohaline
structure
south of New
Zealand
and
location
of the STF
and
SAF
ous over this region. Emery [1977], from historical hydrographic and expendable bathythermograph (XBT) data,
showed that the SAF, PFZ, and PF are continuous over the
area extending from south of Australia to east of Drake Pas-
sage. Nowlin and Clifford [1982] examined zonation of the
ACC south of Australia and Africa, in addition to Drake Passage,and found analogousfronts to exist at all three locations.
The first study of the circumpolar nature of zonation of the
ACC was that by Clifford [1982]. The results of this study,
Paper number 5C0212.
0148-0227/85/005C-0212505.00
Southern
Ocean.
The objectivesof this study were threefold. The first was to
examine the meridional structure of the ACC using position
and speeddata obtained from the FGGE drifters in conjunction with the front locationsdeterminedfrom historicalhydrographic data. The secondstudy objective was to determine if
the FGGE buoys were entrained into the high-speedcurrents
associatedwith the frontal regions.A study of drifters in the
eastern North Pacific [Kirwan et al., 1978b] suggeststhat
buoyshave an affinity for regionsof strongcurrents.The third
objective was to determine the mean near-surface speeds
within the fronts and zones over the ACC.
and found these fronts to be continu-
Copyright 1985 by the AmericanGeophysicalUnion.
which are based on historical hydrographic and XBT data,
suggestthat the STF, SAF, and PF are continuous over the
Southern Ocean and that they exist in the austral summerand
winter. A unique opportunity to further examine the circumpolar nature of the fronts and zones associatedwith the ACC
is provided by approximately 300 surface-drifting buoys
(Figure 1) deployed in the Southern Ocean between 20øS and
65øS during the First GARP Global Experiment (FGGE).
These buoys provided measurementsof position, sea surface
temperature,and sea level barometric pressurefor a period of
approximately 26 months,November 22, 1978, to January 13,
1981. Although the FGGE was primarily a meteorological
experiment, oceanographersacquired a valuable data set that
provides a synoptic view of the near-surfacecirculation of the
Section2 describesthe data usedin this study. Discussions
of the meridional distributions of the FGGE buoys and the
relationship between buoy distributions and front locations
are given in section3. This sectionalso presentsa discussion
of the mean near-surfacespeedswithin the front and nonfront
regions.Section4 is a summary.
2.
D^T^
FGGE Drifters
The original drifting buoy data set consistsof nonuniform
time seriesof position,reported to the nearesthundredth of a
degree of latitude and longitude, sea level pressure,and sea
surface temperature. Discussions of the accuracy of these
7O87
7088
HOFMANN.' LARGE-SCALE HORIZONTAL STRUCTURE OF THE ANTARCTIC CIRCUMPOLAR CURRENT
0 o
20 øW
40 øW
20 øE
lO
øs
•
40
øE
60
60 øW
E
80 øE
80 øW
100 øE
lOO
120 øE
120ø•
140øE
140 øW
160 øW
160 øE
180 ø
Fig. 1. Compositemap of all the FGGE buoytrajectories
in the SouthernHemisphere
oceans.
easternNorth Pacific [McNally, 1981]. Moreover, there is no
reliable method for correctingundroguedbuoys for windage
effects[Kitwan et al., 1978a;Peterson,1985]. In this study,no
distinction has been made between drogued and undrogued
buoys.Also, buoy velocitieswere not correctedfor windage
of positiondatawerethensmoothed
andinterpolated
usinga effects.For time scalesof the order of days, wind effectson
cubicsplinesmoothing
routine[Reinsch,1967].The resulting buoy velocity can be significant[Peterson,1985]. However,
smootheddata were resampledto constructa uniform time whenconsideringthe large-scalemean circulation,the FGGE
of the interiorflow [Patseriesof positions
fromwhichhourlyvelocities
werecomputed buoysprovidea goodrepresentation
measurements
and the buoy data collectionsystemare given
by Garrett [1980a, c].
Beforecomputingvelocitytime series,it was necessaryto
edit the originalpositionseriesto removespikesassociated
with randomnoiseand to fill data gaps.The editedtime series
usinga 2-hour centereddifferencescheme.A detaileddis-
terson,1985].
cussionof the buoy data reductionand processing
techniques
Front Locations
is givenby Patterson[1985].
Most of the hydrographicand XBT data usedby Clifford
The hourly velocitydata were usedto computemean eastwestand north-southspeedin 1ø x 1ø squaresoverthe region [1982] to determinefront locationscamefrom the Southern
30øSto 70øS.One-degreesquareswere chosenbecausethey Ocean Atlas [Gordon and Molinelli, 1982]; however,other
provided sufficientbuoy observations
for calculationof
averagequantitieswhileretainingthe spatialresolution
necessaryto distinguish
fronts.Also,thenumberof buoytracksper
degreeof latitudein the region30øSto 70øSwasdetermined
by countingthe numberof buoysdrifting througheach
1ø x 1ø square.Eachbuoywascountedonlyoncein a partic-
data sourceswere also included. From these data, 148 north-
south transects,which crossedsome or all of the fronts, were
identified. Vertical sectionsof properties were constructed
usinghydrographic
and XBT data from the stationsalongthe
transects.The location and width of individual fronts along
each transect were determined from featuresin the property
distributionsor the horizontal densitygradient.The specific
ular square.
Approximately
39% of the drifterswereequippedinitially criteria usedto defineand locate the fronts are given by Clifwith drogues.No informationis availableas to whetherthe ford [1982].
The circumpolarlocationsof the STF, SAF, and PF were
drogues
remained
attachedfor theentirelifetimeof thebuoys,
althoughit is likely that they detachedFMeincke,1980].A determined for the austral summer and winter seasons. These
comparison
of meandrift speeds
for the maintypesof buoys positionsshowedlittle variabilitywith season,thusthe frontal
[Garrett,1980b,c] deployedin the SouthernOceanshowed locationsare treated as a singledata set in this study.These
of the STF, 79 observations
of
no obviousdifferencebetweendroguedand undroguedbuoys. data provide57 observations
A similar result was obtained for satellite-trackedbuoysin the
the SAF, and 72 observationsof the PF.
HOFMANN:LARGE-SCALEHORIZONTALSTRUCTUREOF THE ANTARCTICCIRCUMPOLARCURRENT
Fig. 2.
7089
Buoy tracksper degreeof latitude over the region30øSto 70øS.Light shadingindicatestwo or more buoys'
darker shadingindicatesfour or more buoys.
3.
DISCUSSION
eastwardto 75øW, which give the impressionof a continuous
feature.
Buoy Distributions
The distribution of buoy tracks on 1ø squares over the
region 30øSto 70øSis shown in Figure 2. In general,few 1ø
squaresin the South Pacific and in the region south of 70øS
were occupiedby more than one buoy. By contrast,the South
Atlantic, north of 35øS,showsa relatively high track count,
reflectingthe large number of buoys in the southernpart of
the SouthAtlantic SubtropicalGyre.
If it is assumedthat the buoys accumulatedin the frontal
regions,then one might expectto seebandedpatternsin the
buoy track distributions,i.e., regionsof high track number
separatedby regionsof low track number.In the regionfrom
120øEto 170øE,south of Australia, such a banded pattern
appears.Here three coherentbandsof high track number are
located at latitudes of 46øSto 49øS,53øSto 56øS,and 57øSto
61øS,roughly the latitudes of the STF, SAF, and PF in this
region [Heath, 1981; Clifford, 1982]. The maximum number of
buoysin the bands,at 150øE,is four, five, and four, respectively;regionsin betweencontainone or no buoys.
After passing New Zealand the northernmost band moves
north and disappearsbetween175øEand 180ø.It reappearsat
about
179øW
and remains
as an identifiable
feature
until
140øW,after which no continuousband is seenextendingeastward toward the South American continent. The middle band
shows a break near 176øE. Beyond 178øE it can be traced
eastward to almost 120øW. The southernmost band ceases to
appear as a continuousfeature beyond 180ø. However, there
are regions of high buoy number south of 65øS extending
In Drake Passage,south of South America, two bands of
high track number appear at latitudes of 56øS to 58øS and
60øSto 62øS.These latitudes correspondto the historical locationsof the SAF and PF [Whitworth, 1980; Clifford, 1982],
respectively.West of Drake Passage,between 95øW and
70øW,there is a region of high track number centeredaround
65øS. Upon entering Drake Passage,this band appears to
merge with the region of high track number near 60øS.Gordon
and Molinelli [1982] show that the 0ø isotherm at 100 m (a
reliable indicator of the PF) movesnorth about 7ø of latitude
(67øS to 60øS) between 90øW and Drake Passage.Thus the
buoysmay be reflectingthe northward migration of the PF.
Eastwardof Drake Passagethereappearsa broad regionof
high track number between 55øS and 60øS which extends to
approximately 50øW. The SAF and PF in this region are in
close proximity [Clifford, 1982], and averaging the buoy
tracks over 1ø squaresmay causethe two fronts to appear as a
singlefeature.
From
50øW to the Greenwich
Meridian
there are no con-
tinuous bands of high track number extending across the
South Atlantic. The few isolated areas of high track number
that exist in this region are centeredabout 50øSand 54øS.At
about 15øE, south of Africa, a banded pattern in the buoy
track distribution is again apparent.The most distinct band
occurs between 38øS and 41øS, approximately the latitudes
spannedby both the Agulhas Return Current and the STF at
this longitude [Lutjeharms,1981]. Moving eastward,this band
remains as a distinct feature over most of the Indian
Ocean.
7090
HOFMANN: LARGE-SCALEHORIZONTAL STRUCTUREOF THE ANTARCTIC CIRCUMPOLARCURRENT
..
._
Fig. 3. Normalized buoy density x 100 over the region 30øSto 70øS.Light shadingindicatesvaluesgreater than 5'
darker shadingindicatesvaluesgreater than 10.
A second region of high track number occurs south of
Africa at about 48øSto 53øS,which are the approximatelatitudes of the SAF and PF i-Clifford, 1982]. Across the south
Indian Ocean, there are no continuous bands south of 45øS,
but isolated regions of high track number, centered around
The two regionsof high buoy densityin Drake Passageare
located near 56øSand 61øSto 62øS,the approximatelatitudes
of the SAF and PF at this location.East of Drake Passage,at
approximately 35øW, two bands of high buoy density, centered at 50øS and 54øS, appear. These bands can be traced
52øS and 56øS, do occur.
across the South
The buoy track distribution gives the impressionthat the
circulation of the ACC has a banded structure. Moreover, the
bands occur,in someregions,at the latitudesof the STF, SAF,
and PF. This would appear to be evidencefor buoy accumulation in frontal regions.However, before concludingthat this
occurs,it is necessaryto remove the effectof speedon buoy
count, i.e., to distinguishbetween buoy count and buoy density. In a flow producing no change in an initially uniform
buoy distribution, high-velocity regions will result in more
buoy tracksin a given area. Thus, high track numberdoesnot
necessarilyindicate an affinity for a particular region.To correct for this effect, the buoy tracks in each 1ø square were
normalized by the mean speedin the 1ø square.This calculation resultsin a buoy densitydistribution rather than a buoy
central Indian Ocean. Immediately south of Africa, at 38øSto
41øS,the buoy densitiesdo not show a band analogousto that
seenin the buoy track distribution. Rather, there are isolated
regions of high buoy density, centered around 40øS, that
track distribution.
The distribution of the normalized buoy density over the
region 30øSto 70øSis shownin Figure 3. Again, in the region
south of Australia, three coherentbands in buoy densityare
evident. As with the buoy tracks, these bands occur at the
approximate latitudes of the STF, SAF, and PF. Eastward of
Australia the bands of high buoy density are no longer continuous;however,thereare numerousisolatedregionsof high
buoy density which give the impressionof continuousbands.
extend eastward
Atlantic
to south of Africa
across the Indian
and across the
Ocean.
One feature appears in the buoy density distribution that
was not apparent in the buoy track distribution. In the South
Atlantic, beginning at approximately 40øW, there is an additional band of high buoy density,centeredaround 60øS,that
extendseastward to approximately 30øE, after which it turns
south and then extends westward
to the Greenwich
Meridian.
This band of high buoy densitycorrespondsapproximatelyto
the historicallocation of the Weddell Gyre boundary [Deacon,
1979]. However, the property distributions used to delineate
the boundaries of the Weddell Gyre indicate that it turns
southward near 23øE [Deacon, 1979], as opposed to 30øE,
which is suggestedby the buoy densitydistribution.
Partitionin# of Buoy Density and SpeedInto Front and
Nonfront Re#ions
The buoy distributionspresentedin Figures 2 and 3 show
that the FGGE buoys were distributed in zonal bands.To test
the correspondencebetweenthesebands and the locationsof
HOFMANN: LARGE-SCALE HORIZONTAL STRUCTUREOF THE ANTARCTIC CIRCUMPOLAR CURRENT
7091
4.5
Fig. 4. Sector west of Australia. Line segmentsindicate the locations of the Subtropical Front (heavy lines), Subantarctic Front (light lines),and Polar Front (medium lines)determinedby Clifford [1982]. Dashed lines are the longitudes
along which the front locationswere used to partition the buoys into zonal bands. The front and nonfront regionsare
identified along 105øE. Light shading indicates depths shallower than 3500 m. Dark shading is the southwesttip of
Australia.
the STF, SAF, and PF, the historical frontal locations determined by Clifford [1982] were usedto partition the buoys into
zonal bands that correspond to front and nonfront regions.
This then provides a way to compare buoy density and speed
exceptfor the Drake Passagesectorin which only two of the
FGGE buoy distribution was uniform, i.e., the buoys were not
all deployed in fronts or in the regions between fronts. The
buoy deployment positions (Figure 5) indicate that the buoys
were, initially, more or less uniformly distributed over the
Southern Ocean. An objective of the FGGE was to have a
nearly uniform buoy array that would provide a nominal spatial resolution of 1000 km; no point in the ocean was to be
more than 500 km from a buoy. Buoy deploymenttook place
over a period of approximatelysix months,with the maximum
buoy coverageoccurringin late May 1979. At that time, 80%
fronts exist. The historical
of the ocean from
between zonal bands.
Beginningat the GreenwichMeridian, the regionfrom 35øS
to 70øS was divided into 12 sectors,each spanning 30ø of
longitude.Within each sector,longitudelines along which observations of the three fronts were available were identified,
front locations
were then used to
divide the Southern Ocean at these longitudes into zonal
bands correspondingto front and nonfront regions.Since it
was not possibleto delineatethe northern boundary of the
STZ and the southern boundary of the AAZ, these regions
were taken to extend approximately 30-4ø of latitude north
and south of the STF and PF, respectively.Therefore these
zonesmay not truly representthe STZ and AAZ.
The front locationsrepresentsingleobservationsof features
that exhibit spatial variability. Therefore,where possible,two
or three observations of individual fronts were averaged to
obtain the front location. In all there were 39 lines along
which the fronts could be identified. However, the number of
lines varied from sector to sector.Two sectors(31øE to 60øE
and 119øWto 90øW) have only two lines,whereasothershave
as many as five. In one sector,29øW to 0ø, the historicalfront
locationsprovidedonly a singleobservationof all three fronts.
A representativesectoris shownin Figure 4.
An implicit assumptionin this approachis that the initial
20øS to 65øS was within
500 km
of an
operationalbuoy [Fleming et al., 1979].
Once the buoys were partitioned into zonal bands, the
number of buoy tracks in each zone was computed. Because
the width of a zone may vary from line to line, the buoy track
counts were normalized by the width of the zone to give the
average number of buoy tracks per degree of latitude. This
value was then normalized by the mean speedof the zone to
give a buoy density for that zone. The buoy densitiesand
near-surface speeds thus obtained were averaged to give a
global mean value for each zone and a mean value for each
zone within
individual
sectors. The
results
of these calcula-
tions are discussedin the following section.
Mean Buoy Densitiesand Near-SurfaceSpeeds
The global mean buoy densitiesand near-surfacespeedsfor
individual zones are shown in Table 1. Mean buoy densities
associatedwith the STZ and STF are essentiallyequal and
approximatelytwice the valueof the buoy densitiesassociated
7092
HOFMANN' LARGE-SCALE HORIZONTAL STRUCTUREOF THE ANTARCTIC CIRCUMPOLAR CURRENT
0 o
2O øW
2O øE
40 øW
1o øS
40 øE
*30 øS
60 *W
.. - •, .•
80 øW
ß \:.
ß\
8øø
':t'.-/
lOO
ß
•
ß
ß
_
ß ß ß
ß * •. '%..'
.
120
--<
.
ee
ß
-/-
•
ß
/
I
//
;
/J
/•
_ •
•
..
.
%
/••,
•,•3•ø•
140
160*W
160*E
180 *
Fig. 5. Deployment positionsof the FGGE drifting buoys.
with the other zones. Pairwise comparison of the buoy densities (Duncan's multiple range test, •- .05) shows that the
buoy densities of these two zones are statistically different
from those of the remaining zones.The higher buoy densities
associatedwith the STZ and STF probably reflect the longer
lifetime of the buoys at these latitudes [Patterson, 1985]. Of
the remaining zones the highest buoy density is associated
with the PF. However, the buoy density associatedwith the
PF is not statisticallysignificantly(• - .05) different from that
that the speedsof the SAF and PF are statisticallydifferent
(•t- .05) from those of the nonfront regions to either side.
Mean speedsof the four nonfront regions are not different
from one another or from that associated with the STF.
Mean buoy densitiesfor each zone in the individual sectors
are given in Table 2. In general,the buoy densitiesassociated
with the STZ and STF are higher than those associatedwith
the other zones.All but four sectorsshow higher buoy den-
sitiesassociatedwith the STF than with the nonfrontregions
to either side. The highest buoy density associatedwith the
The highest mean near-surfacespeedsare associatedwith SAF is found in sector4. In fact, over the region 61øEto 180ø,
the SAF and PF regions.The mean speedwithin thesefrontal buoy densitiesof the SAF are higher than those of the zones
zonesis approximately40 cm s-•, whichis almosttwicethat to either side.The highestbuoy densitiesassociatedwith the
associatedwith the STF. The mean near-surfacespeedsob- PF are found in sectors1, 5, and 10. Only one sector,sector6,
tained for the SAF and PF regions are in agreement with showshigher buoy densitiesassociatedwith all three fronts.
The mean near-surfacespeedsassociatedwith the zonesin
measuredspeedsof the currentsassociatedwith thesefronts in
Drake Passage[Whirworth et al., 1982]. Mean speedsof the the individual sectors(Table 3) indicate that in general the
nonfrontregionsrangefrom23 to 35 cm s-•, with the highest speedof the STF region is lower than those associatedwith
speed associatedwith the PFZ. Pairwise comparison shows the SAF and PF regions.The highestspeeds(relative to the
of the PFZ
and AAZ.
TABLE 1. Mean Buoy Density x 100 and Near-Surface Speed
Computed for Individual Zones From the Entire Data Set
STZ
Buoy density
Near-surface
7.3
STF
8.3
SAZ
4.4
SAF
4.7
PFZ
3.9
PF
5.9
AAZ
3.6
23.2
25.5
27.0
42.7
34.7
40.1
28.2
35
35
39
39
39
39
39
speed
Number of
observations
The number of observationsusedfor each zone is shown.Speed
valuesare centimetersper second.
global mean) associatedwith the STF occur in the sectors
south of Africa (sectors 1 and 2) and in the sector east of
South America (sector 11). The lowest speedsoccur in the
central Pacific sectors(sectors7 to 9). The highestspeedsassociatedwith the SAF and PF are found in sectors2, 3, and 6.
Across the central Pacific the mean speedswithin these two
frontal regionsare lower than their globalmeans.
The speedsgivenin Table 3 can be usedto investigatethe
circumpolarnature of the fronts. The speedswithin the zones
that are circumpolar(all but STZ and STF) show a distinct
meridional zonation, with higher speedsassociatedwith the
frontal regions,in all but three sectors.The banding in the
HOFMANN: LARGE-SCALE HORIZONTAL STRUCTURE OF THE ANTARCTIC CIRCUMPOLAR CURRENT
7093
TABLE 2. Normalized Buoy Density x 100 for Each Zone in the Individual Sectors
Sector
STZ
STF
SAZ
SAF
PFZ
PF
AAZ
Number of
observations
The number
1
1ø_
30øE
2
31 ø_
60øE
3
61 ø_
90øE
4
91 ø_
120øE
5
121 ø150øE
6
151øE 180 ø
7
179 ø150øW
8
149 ø120øW
9
119 ø90øW
10.0
5.3
4.2
2.9
4.1
9.6
2.7
3
4.0
9.0
0.7
1.3
2.4
4.8
3.5
2
3.5
4.2
3.7
5.7
3.1
3.2
1.4
3
8.6
10.3
3.8
7.3
3.5
3.0
3.0
5
9.0
5.4
3.6
5.7
5.6
11.5
5.6
5
6.5
9.4
4.0
6.1
5.2
7.1
0.7
4
10.9
11.5
4.6
4.6
4.1
2.2
1.5
4
13.2
19.6
1.7
3.2
4.6
4.9
4.5
3
4.4
7.3
1.6
3.5
1.3
3.4
5.0
2
of observations
available
10
89 ø60øW
11
59 ø_
30øW
12
29øW _
0
3.3
5.5
6.4
15.6
9.5
4
6.8
3.5
3.7
6.5
2.6
3.5
4.1
3
4.2
5.2
17.4
1.0
3.5
1.7
1.4
1
for each zone in the sectors is shown. Values for sector 12 are
based on one observationand do not representmeans.
speedsis most evident over the region IøE to 180ø. In sectors
7, 9, and 10 the higher speedwithin the PFZ probably results
from discrepanciesbetween the historical locations of the PF
and SAF and the location detectedby the buoys. In particular,
this may be true for sector10, which includesDrake Passage.
The estimatedspeedwithin the PF region in this sectoris 25
cm s-x. Direct measurements
indicatespeedswithin the PF to
be almost twice this value. The difference between the speed
estimated from the buoys and the actual speed may be the
result of the mesoscalevariability associatedwith the PF. The
PF at Drake Passage meanders [Legeckis, 1977] and sheds
cold-core rings into the PFZ •Joyce and Patterson, 1977;
Joyce et al., 1981; Petersonet al., 1982; Hofmann and Whitworth, 1985]. Additionally, the PF undergoesnorth-south migrationsof approximately100 km •Klinck, 1985; Hofmannand
Whitworth, 1985]. The historical front positionsused to determine the boundariesof the PF and PFZ in Drake Passagedo
not account for these processes.Therefore buoys that were
actuallyin the PF may have beencountedin the PFZ, thereby
producing a PF speedthat is too low and a PFZ speedthat is
too high.
Inclusion
of the STZ
and STF
still shows meridional
zo-
nation in speedin sectors1 and 2, which encompassthe area
south of Africa, sector 6, which is the region south of Australia, and sector 11, which is the region eastward of South
America. Sectors 1 and 11 are regions in which the northern
boundary of the ACC comes into contact with the Agulhas
Return Current and the Brazil Current, respectively.In sectors
TABLE 3.
2 and 6 the STF flows over large bathymetric features (discussedin following sections).Elsewhere,the speedwithin the
STF is lower than that of the SAZ. Across the central
there is no sharp distinction between the speedsassociated
with the STF and those of STZ. The lack of a clear indication
of the STF in the speedvaluesin this region may be because
the buoys missedthe front, the historical front positionsdo
not adequately representthe location of the STF, or the STF
is not a well-definedfeature over this region.
Comparisonof Buoy Tracks and Front Locations
The buoy densities and near-surface speeds presented in
Tables
2 and 3 indicate
that
differences
between
Mean Near-Surface Speed in Centimeters per Second for Each Zone in the Individual
Sector
1
1 ø30øE
2
31 ø60øE
3
61 ø90øE
4
91 ø120øE
5
121 ø150øE
6
151øE 180 ø
7
179 ø150øW
8
149 ø120øW
9
119 ø90øW
10
89 ø60øW
11
59 ø_
30øW
12
29øW 0
39.8
41.9
30.6
36.1
45.3
51.2
34.1
70.4
30.8
33.8
48.6
51.4
18.1
19.0
27.2
40.0
19.4
21.2
34.1
38.7
17.1
22.8
20.9
73.5
10.1
9.0
17.0
33.8
9.0
9.0
19.5
34.8
9.3
10.6
21.1
21.9
-27.3
44.5
35.1
45.2
30.5
34.4
21.7
25.7
13.4
33.1
PFZ
21.3
34.9
39.5
39.1
30.5
38.0
31.5
30,7
29.3
37.5
31.3
33.4
PF
AAZ
40.9
22.7
52.0
35.7
50.9
33.2
43.1
33.1
34.5
23.3
43.1
32.6
27.8
28.9
34.5
31.9
30.9
19.1
25.4
30.4
40.1
27.9
57.5
20.0
The number
of observations
available
front
and
nonfront values are more pronouncedin some sectors.Sector
6, which encompassesthe region south of Australia, is one
suchexample.A possiblereason for this is suggestedwhen the
buoy tracks and historical front locations are compared with
the bathymetry in the individual sectors.From 90øE to 135øE
(Figure 6a) the STF locations and buoy tracks exhibit considerable variability in latitude. However, eastward of the
Tasman Plateau (near 145øE) the latitudinal variability in the
STF locations and buoy tracks decreases.As the buoys and
STF flow acrossthe Tasman Plateau and toward the Campbell Plateau (near 170øE),they remain near a latitude of approximately 48øS.Eastward of the Campbell Plateau (Figure
6b) the STF and buoys drift to the north, although the drifter
trajectories are not coincidental with the STF locations. There
Sectors
STZ
STF
SAZ
SAF
Pacific
for each zone in the individual
sectors is the same as in Table 2.
Valuesfor sector12 are basedon one observationand do not representmeans.
7094
HOFMANN' LARGE-SCALEHORIZONTALSTRUCTUREOF THE ANTARCTICCIRCUMPOLARCURRENT
Fig. 6a. Buoy tracksin the region90øEto 180ø. Line segmentsare the locationsof the SubtropicalFront (heavylines),
SubantarcticFront (light lines),and Polar Front (mediumlines)determinedby Clifford [1982]. Light shadingindicates
depthsshallowerthan 3500 m.
Fig. 6b. Sameas 6a exceptfor the regionextendingfrom 180ø to 90øW.
HOFMANN.' LARGE-SCALE HORIZONTAL STRUCTURE OF THE ANTARCTIC CIRCUMPOLAR CURRENT
Fig. 6c. Sameas 6a exceptfor the regionextendingfrom 90øWto the GreenwichMeridian.
Fig. 6d. Sameas6a exceptfor theregionextendingfromtheGreenwichMeridianto 90øE.
7095
7096
HOFMANN: LARGE-SCALEHORIZONTAL STRUCTUREOF THE ANTARCTIC CIRCUMPOLARCURRENT
is no obvious agreement between buoy tracks and STF locations across the central
Pacific.
There is relatively good correspondencebetween the buoy
tracks and SAF locations over the region south of Australia
(Figure 6a). Note that in this area the SAF flows along and
over the Indian-Antarctic Rise, over the Macquarie Ridge
(near 158øE), and around the tip of the Campbell Plateau.
Similarly, the PF locations and buoy tracks show the best
agreement where the PF flows over and along the IndianAntarctic Rise. Heath [-1981] suggestedthat the locations of
these three fronts were controlled by the bathymetry in this
region.
Betweenthe Indian-Antarctic Rise and Macquarie Ridge, at
52øS to 56øS, the buoy tacks trace a large-amplitude wave.
This feature appears in all the tracks, which suggeststhat
there may be a permanent standing wave between these two
bathymetricfeatures.A similar though lesspronouncedwave
pattern is seenat 58øS-60øS.
In the regionof the East PacificRise (Figure 6b) the buoy
tracks and SAF locations tend to align along latitudes of 57øS
to 58øS.However, as reflectedby the mean buoy densitiesand
near-surface speeds,the distinction here between front and
nonfront regionsis not as pronouncedas that south of Australia. To the east of South America (Figure 6c) along the
northern edge of the Falkland Plateau there is again good
agreementbetweenthe buoy tracks and SAF locations.
From
30øW to the Greenwich
Meridian
the front locations
are not defined; thereforeit is not possibleto comment on the
effect of the Mid-Atlantic Ridge. However, eastward of the
Mid-Atlantic Ridge, at 1øE to 2øE, there is a correspondence
betweenthe buoy tracks and PF locationswhich persistsover
much of the region south of Africa.
The buoy tracks in the region of the Crozet Plateau (near
45øE; Figure 6d) indicate that the buoys are either confinedto
the region of the STF near 40øS or confined to the SAF along
the southern edge of the Plateau. To the east of the Crozet
Plateau the buoy tracks indicate considerable meandering.
This region is known to have a high incidence of mesoscale
(100-300 km) disturbances [Lutjeharms and Baker, 1979;
Colton and Chase, 1983; Patterson, 1985].
The above comparisonssuggestthat the location of a front
may not exhibit much spatial variability in regions where it
flows over large bathymetric features. Hence, in these areas,
partitioning of the buoys on the basis of historical front locationswill give a fairly accuraterepresentationof the number
of buoys in front and nonfront regions.In other regions,such
as the central Pacific, the fronts may exhibit considerablespatial variability. Therefore,the historicalfront location may not
reflect that observedby the buoys.
4.
SUMMARY
The fact that the FGGE buoys appeared to have an affinity
for frontal regions brings up an interestingpoint concerning
the calculationof mean quantities,suchas velocity,from drifter data. Becauseof the nonuniform distribution of the buoys
such quantitieswould be biased by the contribution of the
high-speedcurrentsassociatedwith the frontal regionsand,
therefore, may not representa true area average.
The mean near-surfacespeedspresentedin Table 3 suggest
that the SAF and PF are circumpolar in nature. Nowlin and
Clifford [1982] show that at Drake Passagethesetwo fronts
account for approximately 60% of the total transport of the
ACC. If this observationis representativeof the generalcirculation of the ACC, then over half of the transport of the ACC
would occur in two narrow high-speedcurrents.
Acknowledgments.I gratefully acknowledgeadvice and assistance
from Rudolf Freund of the Institute of Statistics, Texas A and M
University. Discussionswith Steve Worley and Julie Ambler and
commentsfrom two anonymousreviewerswere most helpful. Steve
Patterson provided the FGGE buoy data. This study was supported
jointly by the Global AtmosphericResearchProgram of the National
ScienceFoundation and the SpecialProgramsOffice of the National
Oceanic and Atmospheric Administration under grant number
ATM8316640.
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(ReceivedOctober 22, 1984;
acceptedDecember 17, 1984.)