Deep-Sea Research I 49 (2002) 267–280
Surface currents in the Bransfield and Gerlache Straits,
Antarctica
Meng Zhoua,*, Pearn P. Niilerb, Jian-Hwa Huc
b
a
University of Massachusetts Boston, Boston, MA 02125, USA
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, USA
c
National Taiwan Ocean University, Keelung, Taiwan
Received 26 March 2001; accepted 27 August 2001
Abstract
We used 39 tracks of mixed layer drifters deployed during the period from November 1988 to January 1990 to study
the surface flow characteristics in the Bransfield and Gerlache Straits, Antarctica. The results revealed both the
Gerlache Strait Current and the Bransfield Strait Current, which flows along the deep channel of the Gerlache Strait,
northeastward to the southern continental margin of the South Shetland Islands following the 750 m isobath. The
observed strongest sustained daily mean current reached approximately 40 cm s1 in the Bransfield Strait and was
confined to the shelf break south of the South Shetland Islands. The computed acceleration of drifters in the Bransfield
Strait Current indicates the southward transversal component limits drifters from approaching isobaths shallower than
750 m. The southern side of the Current is rich in cyclonic eddies. Drifters spun off and circulated in cyclonic eddies over
deep basins. The residence time of a water parcel in the current is approximately 10–20 days. Anticyclonic circulations
were observed around Tower, Hoseason and Liege Islands, and long residence times were found for drifters in shallows
and bays of up to 70 days. Results also indicate the Gerlache Strait water can extend along the shelf of the Antarctic
peninsula to Tower Island, where it meets the southewestward Weddell Sea water. Most of the Gerlache Strait water
exits northward and enters the Bransfield Strait Current. It Spins off and mixes with other waters in the Bransfield
Strait. Several long tracks indicated the existence of a cyclonic large circulation gyre in the Bransfield Strait during the
ice-free condition. The circulation patterns in both Bransfield and Gerlache Straits change seasonally. The analysis of
force balance indicates that currents and eddies are geostrophic though the ageostrophic components are important to
maintain currents and form eddies. This composition of eddies and currents provides ideal physical settings for
zooplankton growth in eddies and bays and zooplankton dispersion in currents. r 2002 Elsevier Science Ltd. All rights
reserved.
Keywords: Gerlache Strait; Bransfield Strait; Surface current; Drifters; Eddies; Shelf; Continental margin
1. Introduction
*Corresponding author. Department of Environmental,
Coastal and Ocean Sciences, University of Massachusetts
Boston, 100 Morrissey Boulevard, Boston, MA 02125, USA.
Tel.: +1-617-287-7419; fax: +1-617-287-7474.
E-mail address: [email protected] (M. Zhou).
The water masses and flow fields in the
Bransfield and Gerlache Straits, Antarctica, have
been of interest to both physical and biological
0967-0637/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 7 - 0 6 3 7 ( 0 1 ) 0 0 0 6 2 - 0
268
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
oceanographers because of the complexity of flow
structure and water sources, and the high productivity of all trophic levels. The shallows and bays
of the southwestern Bransfield Strait and Gerlache
Strait are the nursery grounds for a host of biota,
especially krill (Brinton, 1991; Huntley et al., 1990;
Zhou et al., 1994). Since zooplankton feed in the
upper water column, the near surface circulation
has great effects on the residence time and
dispersion of Antarctic krill in this area. This
study was part of the Research on Antarctic
Coastal Ecosystems and Rates (RACER; Huntley
et al., 1990). Two groups of Lagrangian mixedlayer drifters were released into the Gerlache Strait
in 1988–1989 and 1990–1991. The objective of this
drifter program is to demarcate the paths of near
surface water during the period of high biological
productivity, and to obtain quantitative measurements of the circulation and its interaction with
the complex topography and orography of this
area.
The bottom topography of the Bransfield Strait
consists of a central basin deeper than 1000 m that
is bounded to the north from the Drake Passage
by the steep continental margin of the South
Shetland Islands (Fig. 1). The southern boundary
rises much more gradually to the Antarctic
Peninsula. At its western end, shallow ridges
(o400 m) traverse between Brabant, Smith and
Snow Islands. The Gerlache Strait forms the
deepest western connection to this deep central
basin. However, sills shallower than 100 m at the
southwest entrance of the Gerlache Strait restrict
the large scale circumpolar flow.
Corresponding to such topography, Drake
Passage water intrudes into the Bransfield Strait
from a deep gap between Brabant and Smith
Islands (Amos, 1987; Capella et al., 1992; Clowes,
1934; Gordon and Nowlin Jr., 1978; Niiler et al.,
1991). The intruding water remains near the
vicinity of the South Shetland Islands, and does
not offer much guidance on the specific nature of
the circulation in this area. Relatively fresh and
warm water in the Bransfield Strait originates on
the Weddell Sea shelf that flows westward around
the tip of the Antarctic Peninsula into the
Fig. 1. The bathymetry (m) of the Bransfield and Gerlache Straits. The black box indicates our study area.
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
Bransfield Strait. Because islands, shallow sills and
ridges to the north and west of the Bransfield
Strait act as barriers restricting intermediate and
deep water exchanges, the Bransfield Strait is semienclosed.
The relative geostrophic circulation estimates
served to identify the existence of the Bransfield
Current and other circulation features (Garcia
et al., 1994; Niiler et al., 1991). The stratification in
this area is weak, which would favor the development of barotropic circulation; however, this
cannot be computed from the hydrographic data,
especially in the presence of complex topographic
features. Thus, the absolute surface circulation
remains unknown. The weak stratification at high
latitudes leads to a small baroclinic Rossby Radius
of 10 km (Huntley and Niiler, 1995). Resolving the
baroclinic circulation at such resolution in the
Bransfield and Gerlache Straits requires a number
of hydrographic stations which have neither been
historically taken nor could be afforded in
RACER. Direct measurements with drifters were
thus adopted in the second and third years of
RACER as the principal means for determining
the surface currents and advective rates of
biological fields.
2. Drifters and data processing
The drifters used in this study consist of a
spherical surface float, a coated wire tether and
drogues that were a diamond shaped triplanar or a
Holey-sock centered at 15 and 40 m (Niiler et al.,
1987, 1990, 1995; Sybrandy and Niiler, 1991).
They were designed to follow a water parcel at the
center of the drogue within 1 cm s1 error under
wind conditions up to 10 m s–1. There is no
statistical difference between velocities measured
by drifters with drogues at 15 and 40 m in this
study. The specific dimension of each drifter and
entire raw data were recorded and maintained in
the Marine Environmental Data Service (MEDS),
Ottawa, Canada. Table 1 lists ARGOS ID
numbers of all drifters, locations of deployments,
and the starting and end dates of drifters with
drogues attached. The mean life of drifters
in this environment of patchy sea-ice was approxi-
269
mately 90 days, with a large variability from 7 to
200 days.
The drifters used in this study were programmed
to transmit daily. On average, 8 position fixes of a
drifter were received during these one-day operating periods. The raw ARGOS positions had
minimum error of 300 m. A sensor mounted at
the center of a drogue constantly sent signals to
the central processor in the surface float indicating
the drogue status. Without the drogue, the velocity
of a drifter would be significantly affected by the
surface wind and would deviate from the mean
velocity in the surface mixed layer. After we
removed those data without drogues, positions
were then interpolated to 0.2 day intervals at the
SVP Data Assembly Center at NOAA/AOML in
Miami using the objective analysis technique
developed based on an analytical spectral function
that was fitted to the raw spectral estimate
(Hansen and Poulain, 1996; van Meurs, 1996).
Velocities were estimated by the central difference
between two positions. The original drifter data
contains inertial and semi-diurnal tidal motions as
shown in Fig. 2. We applied a 2-day low-pass filter
to eliminate these motions. The filtered data were
decimated to 1-day time series.
Most of the drifters were deployed southwest of
the Gerlache Strait and in the channel between
Brabant Island and the Antarctic Peninsula
(Fig. 2). All drifters exited the Gerlache Strait to
the northeast. Nine drifters deployed on the
continental shelf southwest of Anvers Island did
not enter the Gerlache Strait, which clearly
indicates that the shallow sills at the southwest
entrance of the Gerlache Strait restrict the largescale circumpolar flow.
The circulation in the Bransfield and Gerlache
Straits is complex and varies seasonally. Because
we had a very limited number of drifters, ensemble
averages of surface circulation in most locations
are not statistically significant. Thus, we present
the data both as forms of daily mean velocity
vectors located at the daily mean positions, and as
ensemble means in 7 km-square bins. Both the
daily mean velocity field along tracks and the
ensemble mean velocity field in bins revealed the
similar flow pattern in Bransfield and Gerlache
Straits.
270
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
Table 1
Id no.
Dates
Start
11510
11511
11512
11513
11514
11515
11516
11517
11518
11520
11521
11522
11523
11524
11525
11526
11527
15832
15833
15834
15835
15836
15837
15838
15839
15840
15841
15842
15843
15844
15845
15846
15847
15848
15849
15850
15851
15852
15853
11/20/89
11/11/89
11/4/89
11/22/89
11/12/89
11/10/89
11/6/89
11/4/89
11/17/89
11/18/89
11/4/89
11/12/89
11/11/89
11/4/89
11/20/89
11/11/89
11/22/89
12/20/91
1/6/92
1/5/92
1/6/92
1/1/92
1/5/92
1/5/92
1/6/92
1/5/92
12/11/91
1/6/92
12/11/91
1/5/92
12/11/91
12/20/91
12/20/91
12/12/91
12/12/91
12/17/91
12/18/91
12/9/91
12/18/91
No. of days
End
11/23/89
9/14/90
7/25/90
4/11/90
4/18/90
11/12/89
5/5/90
12/16/89
11/18/89
1/1/90
4/8/90
2/27/90
11/19/89
1/22/90
11/30/89
11/14/89
12/20/89
3/4/92
2/20/92
1/29/92
1/21/92
1/22/92
3/7/92
2/11/92
1/14/92
2/10/92
12/20/91
4/1/92
4/4/92
2/17/92
3/26/92
1/22/92
7/ 2/92
4/ 1/92
1/24/92
4/25/92
12/25/91
3/21/92
2/16/92
3. Results
3.1. Horizontal circulation in the Gerlache Strait
The main surface current (Gerlache Strait
Current) follows the middle deep channel and
exceeds 30 cm s–1. On the continental margin of the
Antarctic Peninsula, there exists a weak south-
Deployment locations
Longitude
2
306
263
139
156
1
180
41
0
44
154
106
8
78
9
2
27
74
45
23
15
21
60
36
7
35
8
84
114
42
105
33
194
109
43
128
7
101
59
0
61112.35 W
62145.210 W
61144.200 W
61119.940 W
61150.220 W
61115.060 W
61112.880 W
6210.220 W
62124.720 W
62122.600 W
61159.230 W
61150.160 W
61115.090 W
61144.240 W
61112.640 W
62144.490 W
61119.570 W
61115.960 W
64136.610 W
66125.330 W
6418.810 W
63155.750 W
66127.440 W
66124.990 W
64125.780 W
64158.540 W
62128.320 W
64114.260 W
62129.840 W
64149.900 W
62121.160 W
61119.600 W
61112.900 W
6214.820 W
61150.100 W
61144.090 W
61156.150 W
6318.090 W
61156.100 W
Latitude
6418.030 S
64131.940 S
64123.710 S
64111.410 S
6413.920 S
64115.060 S
63153.960 S
64118.880 S
64134.140 S
64133.540 S
64118.500 S
6414.090 S
64115.320 S
64123.980 S
6418.120 S
64131.850 S
64110.930 S
64118.850 S
64154.870 S
65117.960 S
64150.690 S
6513.400 S
65117.410 S
65117.930 S
64154.980 S
64156.020 S
64134.780 S
64152.360 S
64135.300 S
6413.420 S
64133.080 S
6411.920 S
64113.050 S
64123.470 S
64116.850 S
6410.770 S
64112.510 S
64148.060 S
64112.010 S
westward counter current. Drifters deployed in
shallow bays show weak currents and eddy-like
circulations and have long residence times of
weeks and months (Fig. 3).
The flow exiting the Gerlache Strait forms three
paths in the western part of the Bransfield Strait
(Fig. 3). The most common path is the stream that
follows the middle of the deep channel of the
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
271
Fig. 2. Deployment locations (black dots) and trajectories (solid lines) of drifters in the Bransfield and Gerlache Straits deployed
during RACER II and III.
Fig. 3. Ensemble mean surface velocity vectors in the Bransfield and Gerlache Straits in 7 7 km2 bins.
272
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
Gerlache Strait northward to the southern continental margin of the South Shetland Islands. The
southern path flows northeastward along the
continental margin of the Antarctic Peninsula to
Tower Island. The third preferred path was
revealed by the ejection of drifters to the west
between Hoseason and Liege Islands.
3.2. Horizontal circulation in the western basin
of the Bransfield Strait
The flow structure, in correspondence to the
topographic features, consists of the continuation
of the mainstream from the Gerlache Strait along
the deep channel (Fig. 3). The mainstream in the
deep channel feeds into the Bransfield Strait
Current as its origin (Niiler et al., 1991). The
southern path of the Gerlache Strait Current flows
northeastward along the Antarctic Peninsula shelf,
and then forms constant anticyclonic eddies
around Trinity and Tower Islands. From the
opposite direction, three drifters entered the eddy
around Tower Island from the eastern Bransfield
Strait. Thus, the flows from the Gerlache Strait
and the eastern Antarctic Peninsula shelf must
converge in this area. West of Tower Island, the
flow splits into a re-circuit around the island and a
westward flow along the 400 m isobath. This
branch follows the 400 m isobath westward and
crosses the passage between Hoseason and Liege
Islands, or turns to the north along the 400 m
isobath joining the Bransfield Strait Current.
3.3. Horizontal circulation on the shelf break of
the Bransfield Strait
The constant jet on the southern continental
margin of the South Shetland Islands exceeds
40 cm s1 and is known as the Bransfield Strait
Current (Fig. 3). This jet is the continuation of the
mainstream originating from the deep channel in
the Gerlache Strait and the western Bransfield
Strait. Drifters remained in a narrow band on the
shelf break along the isobath deeper than 750 m.
No drifter crossed the isobath of 750 m to the
shallower water.
Along the sides of the Bransfield Strait Current,
the northern edge of the Current has fewer eddy-
features than the southern edge where drifters
spun off the jet and recirculated in elongated
cyclonic circuits along the continental margin of
the South Shetland Islands. The currents in
cyclones are much weaker and random. These
cyclones can be interpreted as either elongated
eddies spinning off from the Bransfield Strait
Current, or circulation cells formed above deep
basins by the northeastward Bransfield Strait
Current and the southwestward counter flow at
the central axis of the Bransfield Strait.
3.4. Single drifter trajectories and seasonal
variability
We take advantage of high-resolution measurements of drifter trajectories for understanding the
experience of water parcels in both mean circulation and random motion. Fig. 4 shows the daily
mean velocity vectors of drifters at their daily
mean locations. Hence, the number of vectors
indicates the number of days for a drifter to
remain in a feature. Two drifters (Argos ID#
11512 and 11524) were deployed at the same
location 6-days apart (Figs. 4a and b). Both of
them eventually ended up in the Bransfield Strait
Current, but followed two different pathways.
Drifter 11512 took the southern path of the
Gerlache Strait Current and flowed northeastward
along the continental margin of the Antarctic
Peninsula to Tower Island. The drifter was
trapped in the anticyclonic eddy around Tower
Island for more than 40 days, before spinning off
the eddy along the 400 m isobath westward. It was
then swept into the Bransfield Strait Current on
the continental margin. If the trajectory of Drifter
11512 reveals a typical southern path, Drifter
11524 represents a typical northern path. It flowed
around the cyclonic eddy in the deep basin
between Lower, Deception, and Hoseason Islands
for 18 days, and then followed the path of Drifter
11512 into the Bransfield Strait Current.
The circulation pattern of the Gerlache Strait
Current and the Bransfield Strait Current may
vary seasonally. Drifter 11516 was deployed in
the same period as Drifters 11512 and 11524
(Fig. 4c). It followed the same path of Drifter
11524. Instead of exiting the Bransfield Strait by
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
following the Bransfield Strait Current, however, it
spun off the current and was trapped in a cyclonic
eddy in the deep basin south of the Bransfield
273
Strait, and eventually recirculated southwestward
on the continental margin of the Antarctic
Peninsula. This southwestward current on the
11512
(a)
11516
(b)
Fig. 4. Drifter trajectories. The black dots indicate the deployment locations; and velocity vectors indicate the daily mean velocities at
the daily mean locations. (a) Drifter 11512 deployed at 641 23.710 S, 611 44.200 W on 11/04/89; (b) Drifter 11516 deployed at 641 23.980 S,
611 44.240 W on 11/04/89; (c) Drifter 11524 deployed at 631 53.960 S, 611 12.880 W on 11/06/89; and (d) Drifter 15847 deployed at 641
13.050 S, 611 12.900 W on 12/20/91.
274
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
Fig. 4 (continued).
continental margin of the Antarctic Peninsula
could be associated with the fresh and warm
water originating on the Weddell Sea shelf (Niiler
et al., 1991). Similar to Drifter 11512, Drifter
11524 was trapped in the anticyclonic eddy around
Tower Island, and then spun off the eddy along the
400 m isobath westward. The interesting phenomenon is that Drifter 11524 did not end up in the
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
Bransfield Strait Current. It was ejected westward
between Hoseason and Liege Islands. Drifters
11512 and 11516 revealed the two paths of the
Gerlache Strait Current from November to
January, while Drifter 11524 exposed the third
path that is the ejection of Gerlache Strait water to
the west between Hoseason and Liege Islands in
late February and April. Drifter 11847, deployed
in a different year, followed the ejection path of
Drifter 11524 in March.
The residence times of drifters varied. Those
deployed in the Gerlache Strait Current were
quickly advected out of the Gerlache Strait in less
than 7 days, but those deployed in shallow bays
were trapped for more than 70 days. Similarly, in
the western Bransfield Strait, drifters trapped in
eddies around islands and basins had residence
times of around 40 days, and those entering the
Bransfield Strait Current exited the Strait in 13
days. Hence, there are generally two time scales of
surface water exchange in this area: a short time
scale of 10 days in the Gerlache Strait Current and
the Bransfield Strait Current, and a long time scale
of 70 days in the bays and eddies.
4. Discussion
The linearity of the flow field can be simply
evaluated by the Rossby number (Ro), or the ratio
of acceleration of a water parcel (a drifter) to the
Coriolis force, both of which can be calculated
from drifter data, i.e.,
Ro ¼ d~
u=dt=f j~
uj;
ð1Þ
where ~
u is the drifter velocity, and f is the Coriolis
parameter. Because we are only interested in the
mean flow, the acceleration of a drifter is
calculated from the 2-day low-pass-filtered data.
Our estimates indicate that the acceleration is
one order of magnitude smaller than the Coriolis
force in most areas: about 50% of Ro are less than
0.2, and 85% are less than 0.5 (Fig. 5). Though the
Coriolis force plays the dominant role in the force
balance, Ro¼ 0:2 or greater indicates that nonlinear effects in the vorticity balance will be
important (Pedlosky, 1987). Those Ro larger than
0.5 occurred around islands and basins in the
275
Gerlache Strait and the western Bransfield Strait,
where drifters spun off eddies, exited the Gerlache
Strait Current, or entered the Bransfield Strait
Current.
The tendency of the drifter movement can be
examined by the force balance. We set an
orthogonal coordinate system within which the
longitudinal direction is in the current direction,
and the transverse direction is perpendicular to the
current following the right-hand rule (Fig. 6).
Then the momentum equations for the longitudinal and transversal velocity components, uL
and uT ; in the surface mixed layer can be written as
duL
ð2Þ
¼ FL ;
dt
duT
dt
¼ f huL i þ FT ;
ð3Þ
where / S is the ensemble average, and FL and
FT are the longitudinal and transversal components of the sum of surface gradients and wind
stresses. We do not have enough data to separate
the surface gradients from the wind stresses in this
study. Hydrographic data indicate that the currents appear to be geostrophic in the Bransfield
Strait (Niiler et al., 1991). The spatial scale of the
mean wind field is usually at least one order of
magnitude larger than the mesoscale of the current
field. Thus, the mesoscale current field should
primarily represent the surface gradients. For
convenience, we call the sum of surface gradients
and wind stresses the apparent surface gradients.
We present the Coriolis force vectors and longitudinal and transversal components of acceleration in 7 7 km2 bins (Figs. 7 and 8).
We start from the Gerlache Strait. The transversal acceleration of the Gerlache Strait Current
is northward in the left-hand direction relative to
the current (Fig. 8), which pushes the Gerlache
Strait Current close to Bradant Island. Exiting
from the Gerlache Strait, the mainstream of the
Gerlache Strait Current turns to the north
corresponding to the northward transversal acceleration. The Ro of this mainstream is estimated
greater than 0.2, which indicates the influence of
nonlinear acceleration or the departure from the
geostrophic balance. Fig. 8 shows the nonlinear
276
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
Fig. 5. (A) The histogram of Ro from daily mean velocities. (B) Contours of Ro.
Fig. 6. The orthogonal coordinate system following the trajectory of a drifter.
acceleration which consists of both longitudinal
deceleration and counter-clockwise rotation. Such
nonlinear acceleration can be produced by a rapid
reduction of the surface slope, which causes the
geostrophic unbalance between the Coriolis force
and reduced surface slope.
Following the northern branch of the Gerlache
Strait Current to the vicinity of Deception Island,
the transversal acceleration switches to the righthand direction relative to the current, which means
the transversal apparent surface gradient is greater
than the Coriolis force, and forces the current to
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
277
Fig. 7. Coriolis force vectors.
turn to the right. The longitudinal velocity
accelerates. The longitudinal and transverse accelerations can be explained from the barotropic
geostrophic adjustment induced by a sudden
increase in the downhill slope. In the area south
of Deception Island, the longitudinal velocity
accelerates abruptly, marking the origin of the
Bransfield Strait Current. If the magnitude of
mean wind is weak (less than 2 m s1 in austral
summer months) and the spatial scale of wind is 1–
2 orders of magnitude greater than the spatial scale
of the current, such acceleration must be produced
by the downhill longitudinal surface gradient.
The transverse acceleration of the Bransfield
Strait Current is southward. This southward
acceleration forbids drifters from turning to the
north. This is remarkably consistent with the
drifter trajectories: no drifters crossed the 750 m
isobath to the shallower water. There is no eddy
feature north of the Current. Oppositely, drifters
frequently spun southward off from the Bransfield
Strait Current and were trapped in eddies. The
southern edge of the Current is much more
unstable than the northern edge. The spinningoff of eddies can be explained starting from the
deceleration of currents at the southern edge, the
decrease in Coriolis force in the left-hand direction, to the positive right-hand transverse acceleration, which produces a clockwise circulation.
The trajectories of drifters marked the paths and
boundaries of surface water masses. The origin of
the Gerlache Strait Current and the Bransfield
Strait Current can be clearly followed to the
southwest of the Gerlache Strait (Fig. 3). It is
unlikely that the flow originates by the intrusion of
circumpolar flow over the shallow sills at the
southwest entrance of the Gerlache Strait, because
none of the 9 drifters deployed southwest of the
area entered the Strait (Fig. 2). The Gerlache Strait
water also flows along the Antarctic Peninsula
shelf, and meets the water from the eastern
Antarctic Peninsula shelf in the vicinity of Tower
Island, which marked the eastern boundary of the
Gerlache Strait water. The water east of Tower
Island originates from the Weddell Sea shelf, as
indicated from hydrographic study (Niiler et al.,
1991). Drifters released in the Gerlache Strait
marked the path of Gerlache Strait water, how it
exits the Gerlache Strait and enters the Bransfield
Strait Current. These drifters are restricted to the
278
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
Fig. 8. (a) Longitudinal components of drifter acceleration. (b) Transversal components of drifter acceleration.
continental margin of the South Shetland Islands
deeper than 750 m. Thus, water around the South
Shetland Islands must originate elsewhere. The
trajectories of drifters marked the interface be-
tween these two waters north of the Bransfield
Strait Current.
A general cyclonic basin-scale circulation is
found in the Bransfield Strait, with complex
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
temporal and spatial features. We can say that the
Bransfield Strait Current bounds the water of the
Gerlache and Bransfield Straits in the north. But
we cannot define a clear boundary in the south.
The interfaces between waters in the Gerlache
Strait, Bransfield Strait and Antarctic Peninsula
shelf have eddy-rich features. The hydrographic
data indicate that Antarctic Peninsula shelf water
can reach the western Bransfield Strait, Hoseason
Island and Brabant Island (Niiler et al., 1991). A
detailed interpretation of flow and water masses is
difficult from our limited drifter data.
Both western Bransfield and Gerlache Straits
are extraordinarily productive bodies of water in
the Antarctic Peninsula area. The most abundant
species, such as Calanoides acutus, Euphausia
crystallorophias and Euphausia superba, spawn
in the Gerlache Strait from November–March.
High
concentrations
of
phytoplankton
(>10 mg chl m3) may be present in the upper
50 m from late October–January (Holm-Hansen
and Mitchell, 1991; Holm-Hansen and Vernet,
1992), providing a food-rich environment. It is
critical for these larval populations to remain in
such a food-rich environment. Drifters entrained
in the Gerlache Strait Current and the Bransfield
Strait Current will exit the Bransfield Strait in 10–
20 days, but those entrained in eddies and bays can
have a time scale from 50–100 days. Hence,
individuals in an eddy or bay will have enough
residence time in such food-rich environments for
their development. For example, C. acutus could
develop from a late naupliar stage to copepodite
CIV, E. crystallorophias could develop from eggs
through early furcilia stages, and E. superba, could
reach late furcilia stages (Brinton and Townsend,
1991; Huntley and Brinton, 1991; Huntley and
Escritor, 1991). The currents and jets are also
important for zooplankton recruitment and dispersion.
Is it true that individuals of zooplankton
entrained in the Bransfield Strait Current might
not meet the common fate? Our drifter measurements show that the northern boundary of the
Bransfield Strait Current behaves like a barrier
where the southward acceleration limits any
crossover transport to the shallower water. Conversely, the southern boundary of the Current is
279
rich in eddies. A drifter could remain in the
Bransfield Strait Current less than 10 days before
its exit. But drifters were frequently ripped off
from the Bransfield Strait Current and trapped in
cyclonic eddies in the deep basins, which provides
a mechanism for zooplankton individuals to
recirculate back to the western Bransfield Strait.
Such flow configuration leads to a long residence
time and high abundance of zooplankton in the
western Bransfield and Gerlache Straits, bounded
by the Bransfield Strait Current in the north
(Brinton and Townsend, 1991; Huntley and
Escritor, 1991).
The seasonal third path (from late February to
April) of the Gerlache Strait Current that describes the ejection of Gerlache Strait water
westward between Hoseason and Liege Islands
may determine the seasonal variation of zooplankton distribution. Lower zooplankton were found
in the western Bransfield Strait in general, but
more zooplankton were found west and north of
the South Shetland Islands during that particular
season (Brinton and Townsend, 1991; Huntley and
Escritor, 1991).
Acknowledgements
This work was supported under NSF Grant
numbers DPP85-19908 and OPP95-23748. We
thank Judy Illeman for processing the original
drifter data.
References
Amos, A.F., 1987. Hydrography of the Bransfield Strait during
the RACER field season: December 1986–April 1987. Eos
68, 1685–1685.
Brinton, E., 1991. Distribution and population structures of
immature and adult Euphausia superba in the western
Bransfield Strait region during the 1986–87 summer.
Deep-Sea Research II 38, 1169–1194.
Brinton, E., Townsend, A., 1991. Development rates and
habitat shifts in the Antarctic neritic euphausiid Euphausia
crystallorophias, 1986–87. Deep-Sea Research II 38,
1195–1211.
Capella, J.E., Ross, R.M., Quetin, L.B., Hofmann, E.E., 1992.
A note on the thermal structure of the upper ocean in the
280
M. Zhou et al. / Deep-Sea Research I 49 (2002) 267–280
Bransfield Strait-South Shetland Islands region. Deep-Sea
Research I 39, 1221–1229.
Clowes, A.J., 1934. Hydrology of the Southern Ocean.
Discovery Reports 9, 1–64.
Garcia, M.A., Lopez, O., Sospedra, J., Espino, M., Gracia, V.,
Morrison, G., Rojas, P., Figa, J., Puigdefabregas, J.,
Arcilla, A.S., 1994. Mesoscale variability in the Bransfield
Strait region (Antarctica) during austral summer. Annales
Geophysicae 12, 856–867.
Gordon, A.L., Nowlin Jr., W.D., 1978. The basin waters of the
Bransfield Strait. Journal of Physical Oceanography 8,
258–264.
Hansen, D.V., Poulain, P.M., 1996. Quality control and
interpolations of WOCE-TOGA drifter data. Journal of
Atmospheric and Oceanic Technology 13, 900–909.
Holm-Hansen, O., Mitchell, B.G., 1991. Spatial and temporal
distribution of phytoplankton and primary production in
the western Bransfield Strait region. Deep-Sea Research II
38, 961–980.
Holm-Hansen, O., Vernet, M., 1992. RACER: Distribution,
abundance, and productivity of phytoplankton in the
Gerlache Strait during austral summer. Antarctic Journal
of the United States 27, 154–156.
Huntley, M.E., Brinton, E., 1991. Mesoscale variation in
growth and early development of Euphausia superba Dana
in the western Bransfield Strait region. Deep-Sea Research
II 38, 1213–1240.
Huntley, M.E., Escritor, F., 1991. Dynamics of Calanoides
acutus (Copepoda: Calanoida) in antarctic coastal waters.
Deep-Sea Research II 38, 1145–1167.
Huntley, M.E., Niiler, P.P., 1995. Physical control of population dynamics in the Southern Ocean. ICES Journal of
Marine Science 52, 457–468.
Huntley, M.E., Brinton, E., Lopez, M.D.G., Townsend, A.,
Nordhausen, W., 1990. RACER: Fine-scale and mesoscale
zooplankton studies during the spring bloom, 1989.
Antarctic Journal of the United States 25, 157–159.
Niiler, P.P., Davis, R.E., White, H.J., 1987. Water-following
characteristics of a mixed layer drifter. Deep-Sea Research I
34, 1867–1881.
Niiler, P.P., Illeman, J., Hu, J.H., 1990. RACER: Lagrangian
drifter observations of surface circulation in the Gerlache
and Bransfield Straits. Antarctic Journal of the United
States 25, 135–137.
Niiler, P.P., Amos, A.F., Hu, J.H., 1991. Water masses and
200 m relative geostrophic circulation in the western
Bransfield Strait region. Deep-Sea Research II 38, 943–959.
Niiler, P.P., Sybrandy, A.S., Bi, K., Poulain, P.M., Bitterman,
D.M., 1995. Measurements of the water-following capability of Holey–sock and TRISTAR drifters. Deep-Sea
Research I 42, 1951–1964.
Pedlosky, J., 1987. Geophysical Fluid Dynamics. Springer, New
York.
Sybrandy, A.L., Niiler, P.P., 1991. WOCE/TOGA Lagrangian
drifter construction manual. SIO Reference 91/6, Scripps
Institution of Oceanography. University of California, San
Diego.
van Meurs, P., 1996. The importance of spatial variabilities on
the decay of near-inetial mixed layer current: theory,
observations and modeling. Ph.D. thesis, Scripps Institution
of Oceanography, University of California, San Diego, pp.
256.
Zhou, M., Nordhausen, W., Huntley, M.E., 1994. ADCP
measurements of the distribution and abundance of
euphausiids near the Antarctic Peninsula in winter. DeepSea Research I 41, 1425–1445.