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 a University of Massachusetts Boston, Boston, MA 02125, USA b 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 The water masses and flow fields in the Boston, 100 Morrissey Boulevard, Boston, MA 02125, USA. Tel.: +1-617-287-7419; fax: +1-617-287-7474. Bransfield and Gerlache Straits, Antarctica, have E-mail address: [email protected] (M. Zhou). been of interest to both physical and biological

0967-0637/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0967-0637(01)00062-0 268 M. Zhou et al. / Deep-Sea Research I 49(2002) 267–280 oceanographers because of the complexity of flow is bounded to the north from the Drake Passage structure and water sources, and the high produc- by the steep continental margin of the South tivity of all trophic levels. The shallows and bays Shetland Islands (Fig. 1). The southern boundary of the southwestern Bransfield Strait and Gerlache rises much more gradually to the Antarctic Strait are the nursery grounds for a host of biota, Peninsula. At its western end, shallow ridges especially krill (Brinton, 1991; Huntley et al., 1990; (o400 m) traverse between Brabant, Smith and Zhou et al., 1994). Since zooplankton feed in the Snow Islands. The Gerlache Strait forms the upper water column, the near surface circulation deepest western connection to this deep central has great effects on the residence time and basin. However, sills shallower than 100 m at the dispersion of Antarctic krill in this area. This southwest entrance of the Gerlache Strait restrict study was part of the Research on Antarctic the large scale circumpolar flow. Coastal Ecosystems and Rates (RACER; Huntley Corresponding to such topography, Drake et al., 1990). Two groups of Lagrangian mixed- Passage water intrudes into the Bransfield Strait layer drifters were released into the Gerlache Strait from a deep gap between Brabant and Smith in 1988–1989 and 1990–1991. The objective of this Islands (Amos, 1987; Capella et al., 1992; Clowes, drifter program is to demarcate the paths of near 1934; Gordon and Nowlin Jr., 1978; Niiler et al., surface water during the period of high biological 1991). The intruding water remains near the productivity, and to obtain quantitative measure- vicinity of the South Shetland Islands, and does ments of the circulation and its interaction with not offer much guidance on the specific nature of the complex topography and orography of this the circulation in this area. Relatively fresh and area. warm water in the Bransfield Strait originates on The bottom topography of the Bransfield Strait the Weddell Sea shelf that flows westward around consists of a central basin deeper than 1000 m that 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) 267–280 269

Bransfield Strait. Because islands, shallow sills and mately 90 days, with a large variability from 7 to ridges to the north and west of the Bransfield 200 days. Strait act as barriers restricting intermediate and The drifters used in this study were programmed deep water exchanges, the Bransfield Strait is semi- to transmit daily. On average, 8 position fixes of a enclosed. drifter were received during these one-day operat- The relative geostrophic circulation estimates ing periods. The raw ARGOS positions had served to identify the existence of the Bransfield minimum error of 300 m. A sensor mounted at Current and other circulation features (Garcia the center of a drogue constantly sent signals to et al., 1994; Niiler et al., 1991). The stratification in the central processor in the surface float indicating this area is weak, which would favor the develop- the drogue status. Without the drogue, the velocity ment of barotropic circulation; however, this of a drifter would be significantly affected by the cannot be computed from the hydrographic data, surface wind and would deviate from the mean especially in the presence of complex topographic velocity in the surface mixed layer. After we features. Thus, the absolute surface circulation removed those data without drogues, positions remains unknown. The weak stratification at high were then interpolated to 0.2 day intervals at the latitudes leads to a small baroclinic Rossby Radius SVP Data Assembly Center at NOAA/AOML in of 10 km (Huntley and Niiler, 1995). Resolving the Miami using the objective analysis technique baroclinic circulation at such resolution in the developed based on an analytical spectral function Bransfield and Gerlache Straits requires a number that was fitted to the raw spectral estimate of hydrographic stations which have neither been (Hansen and Poulain, 1996; van Meurs, 1996). historically taken nor could be afforded in Velocities were estimated by the central difference RACER. Direct measurements with drifters were between two positions. The original drifter data thus adopted in the second and third years of contains inertial and semi-diurnal tidal motions as RACER as the principal means for determining shown in Fig. 2. We applied a 2-day low-pass filter the surface currents and advective rates of to eliminate these motions. The filtered data were biological fields. decimated to 1-day time series. Most of the drifters were deployed southwest of the Gerlache Strait and in the channel between 2. Drifters and data processing Brabant Island and the Antarctic Peninsula (Fig. 2). All drifters exited the Gerlache Strait to The drifters used in this study consist of a the northeast. Nine drifters deployed on the spherical surface float, a coated wire tether and continental shelf southwest of Anvers Island did drogues that were a diamond shaped triplanar or a not enter the Gerlache Strait, which clearly Holey-sock centered at 15 and 40 m (Niiler et al., indicates that the shallow sills at the southwest 1987, 1990, 1995; Sybrandy and Niiler, 1991). entrance of the Gerlache Strait restrict the large- They were designed to follow a water parcel at the scale circumpolar flow. center of the drogue within 1 cm s1 error under The circulation in the Bransfield and Gerlache wind conditions up to 10 m s–1. There is no Straits is complex and varies seasonally. Because statistical difference between velocities measured we had a very limited number of drifters, ensemble by drifters with drogues at 15 and 40 m in this averages of surface circulation in most locations study. The specific dimension of each drifter and are not statistically significant. Thus, we present entire raw data were recorded and maintained in the data both as forms of daily mean velocity the Marine Environmental Data Service (MEDS), vectors located at the daily mean positions, and as Ottawa, Canada. Table 1 lists ARGOS ID ensemble means in 7 km-square bins. Both the numbers of all drifters, locations of deployments, daily mean velocity field along tracks and the and the starting and end dates of drifters with ensemble mean velocity field in bins revealed the drogues attached. The mean life of drifters similar flow pattern in Bransfield and Gerlache in this environment of patchy sea-ice was approxi- Straits. 270 M. Zhou et al. / Deep-Sea Research I 49(2002) 267–280

Table 1

Id no. Dates No. of days Deployment locations

Start End Longitude Latitude

11510 11/20/89 11/23/89 2 61112.350W6418.030S 11511 11/11/89 9/14/90 306 62145.210W64131.940S 11512 11/4/89 7/25/90 263 61144.200W64123.710S 11513 11/22/89 4/11/90 139 61119.940W64111.410S 11514 11/12/89 4/18/90 156 61150.220W6413.920S 11515 11/10/89 11/12/89 1 61115.060W64115.060S 11516 11/6/89 5/5/90 180 61112.880W63153.960S 11517 11/4/89 12/16/89 41 6210.220W64118.880S 11518 11/17/89 11/18/89 0 62124.720W64134.140S 11520 11/18/89 1/1/90 44 62122.600W64133.540S 11521 11/4/89 4/8/90 154 61159.230W64118.500S 11522 11/12/89 2/27/90 106 61150.160W6414.090S 11523 11/11/89 11/19/89 8 61115.090W64115.320S 11524 11/4/89 1/22/90 78 61144.240W64123.980S 11525 11/20/89 11/30/89 9 61112.640W6418.120S 11526 11/11/89 11/14/89 2 62144.490W64131.850S 11527 11/22/89 12/20/89 27 61119.570W64110.930S 15832 12/20/91 3/4/92 74 61115.960W64118.850S 15833 1/6/92 2/20/92 45 64136.610W64154.870S 15834 1/5/92 1/29/92 23 66125.330W65117.960S 15835 1/6/92 1/21/92 15 6418.810W64150.690S 15836 1/1/92 1/22/92 21 63155.750W6513.400S 15837 1/5/92 3/7/92 60 66127.440W65117.410S 15838 1/5/92 2/11/92 36 66124.990W65117.930S 15839 1/6/92 1/14/92 7 64125.780W64154.980S 15840 1/5/92 2/10/92 35 64158.540W64156.020S 15841 12/11/91 12/20/91 8 62128.320W64134.780S 15842 1/6/92 4/1/92 84 64114.260W64152.360S 15843 12/11/91 4/4/92 114 62129.840W64135.300S 15844 1/5/92 2/17/92 42 64149.900W6413.420S 15845 12/11/91 3/26/92 105 62121.160W64133.080S 15846 12/20/91 1/22/92 33 61119.600W6411.920S 15847 12/20/91 7/ 2/92 194 61112.900W64113.050S 15848 12/12/91 4/ 1/92 109 6214.820W64123.470S 15849 12/12/91 1/24/92 43 61150.100W64116.850S 15850 12/17/91 4/25/92 128 61144.090W6410.770S 15851 12/18/91 12/25/91 7 61156.150W64112.510S 15852 12/9/91 3/21/92 101 6318.090W64148.060S 15853 12/18/91 2/16/92 59 61156.100W64112.010S

3. Results westward counter current. Drifters deployed in shallow bays show weak currents and eddy-like 3.1. Horizontal circulation in the Gerlache Strait circulations and have long residence times of weeks and months (Fig. 3). The main surface current (Gerlache Strait The flow exiting the Gerlache Strait forms three Current) follows the middle deep channel and paths in the western part of the Bransfield Strait exceeds 30 cm s–1. On the continental margin of the (Fig. 3). The most common path is the stream that Antarctic Peninsula, there exists a weak south- 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 Â 7km2 bins. 272 M. Zhou et al. / Deep-Sea Research I 49(2002) 267–280

Gerlache Strait northward to the southern con- features than the southern edge where drifters tinental margin of the South Shetland Islands. The spun off the jet and recirculated in elongated southern path flows northeastward along the cyclonic circuits along the continental margin of continental margin of the Antarctic Peninsula to the South Shetland Islands. The currents in Tower Island. The third preferred path was cyclones are much weaker and random. These revealed by the ejection of drifters to the west cyclones can be interpreted as either elongated between Hoseason and Liege Islands. eddies spinning off from the Bransfield Strait Current, or circulation cells formed above deep 3.2. Horizontal circulation in the western basin basins by the northeastward Bransfield Strait of the Bransfield Strait Current and the southwestward counter flow at the central axis of the Bransfield Strait. The flow structure, in correspondence to the topographic features, consists of the continuation 3.4. Single drifter trajectories and seasonal of the mainstream from the Gerlache Strait along variability the deep channel (Fig. 3). The mainstream in the deep channel feeds into the Bransfield Strait We take advantage of high-resolution measure- Current as its origin (Niiler et al., 1991). The ments of drifter trajectories for understanding the southern path of the Gerlache Strait Current flows experience of water parcels in both mean circula- northeastward along the Antarctic Peninsula shelf, tion and random motion. Fig. 4 shows the daily and then forms constant anticyclonic eddies mean velocity vectors of drifters at their daily around Trinity and Tower Islands. From the mean locations. Hence, the number of vectors opposite direction, three drifters entered the eddy indicates the number of days for a drifter to around Tower Island from the eastern Bransfield remain in a feature. Two drifters (Argos ID# Strait. Thus, the flows from the Gerlache Strait 11512 and 11524) were deployed at the same and the eastern Antarctic Peninsula shelf must location 6-days apart (Figs. 4a and b). Both of converge in this area. West of Tower Island, the them eventually ended up in the Bransfield Strait flow splits into a re-circuit around the island and a Current, but followed two different pathways. westward flow along the 400 m isobath. This Drifter 11512 took the southern path of the branch follows the 400 m isobath westward and Gerlache Strait Current and flowed northeastward crosses the passage between Hoseason and Liege along the continental margin of the Antarctic Islands, or turns to the north along the 400 m Peninsula to Tower Island. The drifter was isobath joining the Bransfield Strait Current. trapped in the anticyclonic eddy around Tower Island for more than 40 days, before spinning off 3.3. Horizontal circulation on the shelf break of the eddy along the 400 m isobath westward. It was the Bransfield Strait then swept into the Bransfield Strait Current on the continental margin. If the trajectory of Drifter The constant jet on the southern continental 11512 reveals a typical southern path, Drifter margin of the South Shetland Islands exceeds 11524 represents a typical northern path. It flowed 40 cm s1 and is known as the Bransfield Strait around the cyclonic eddy in the deep basin Current (Fig. 3). This jet is the continuation of the between Lower, Deception, and Hoseason Islands mainstream originating from the deep channel in for 18 days, and then followed the path of Drifter the Gerlache Strait and the western Bransfield 11512 into the Bransfield Strait Current. Strait. Drifters remained in a narrow band on the The circulation pattern of the Gerlache Strait shelf break along the isobath deeper than 750 m. Current and the Bransfield Strait Current may No drifter crossed the isobath of 750 m to the vary seasonally. Drifter 11516 was deployed in shallower water. the same period as Drifters 11512 and 11524 Along the sides of the Bransfield Strait Current, (Fig. 4c). It followed the same path of Drifter the northern edge of the Current has fewer eddy- 11524. Instead of exiting the Bransfield Strait by M. Zhou et al. / Deep-Sea Research I 49(2002) 267–280 273 following the Bransfield Strait Current, however, it Strait, and eventually recirculated southwestward spun off the current and was trapped in a cyclonic on the continental margin of the Antarctic eddy in the deep basin south of the Bransfield 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 23.710S, 611 44.200W on 11/04/89; (b) Drifter 11516 deployed at 641 23.980S, 611 44.240W on 11/04/89; (c) Drifter 11524 deployed at 631 53.960S, 611 12.880W on 11/06/89; and (d) Drifter 15847 deployed at 641 13.050S, 611 12.900W 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 11524 was trapped in the anticyclonic eddy around could be associated with the fresh and warm Tower Island, and then spun off the eddy along the water originating on the Weddell Sea shelf (Niiler 400 m isobath westward. The interesting phenom- et al., 1991). Similar to Drifter 11512, Drifter enon is that Drifter 11524 did not end up in the M. Zhou et al. / Deep-Sea Research I 49(2002) 267–280 275

Bransfield Strait Current. It was ejected westward Gerlache Strait and the western Bransfield Strait, between Hoseason and Liege Islands. Drifters where drifters spun off eddies, exited the Gerlache 11512 and 11516 revealed the two paths of the Strait Current, or entered the Bransfield Strait Gerlache Strait Current from November to Current. January, while Drifter 11524 exposed the third The tendency of the drifter movement can be path that is the ejection of Gerlache Strait water to examined by the force balance. We set an the west between Hoseason and Liege Islands in orthogonal coordinate system within which the late February and April. Drifter 11847, deployed longitudinal direction is in the current direction, in a different year, followed the ejection path of and the transverse direction is perpendicular to the Drifter 11524 in March. current following the right-hand rule (Fig. 6). The residence times of drifters varied. Those Then the momentum equations for the long- deployed in the Gerlache Strait Current were itudinal and transversal velocity components, uL quickly advected out of the Gerlache Strait in less and u ; in the surface mixed layer can be written as T than 7 days, but those deployed in shallow bays duL were trapped for more than 70 days. Similarly, in ¼ FL; ð2Þ dt the western Bransfield Strait, drifters trapped in  eddies around islands and basins had residence duT times of around 40 days, and those entering the ¼fuhiL þ FT; ð3Þ dt Bransfield Strait Current exited the Strait in 13 days. Hence, there are generally two time scales of where / S is the ensemble average, and FL and surface water exchange in this area: a short time FT are the longitudinal and transversal compo- scale of 10 days in the Gerlache Strait Current and nents of the sum of surface gradients and wind the Bransfield Strait Current, and a long time scale stresses. We do not have enough data to separate of 70 days in the bays and eddies. the surface gradients from the wind stresses in this study. Hydrographic data indicate that the cur- rents appear to be geostrophic in the Bransfield 4. Discussion Strait (Niiler et al., 1991). The spatial scale of the mean wind field is usually at least one order of The linearity of the flow field can be simply magnitude larger than the mesoscale of the current evaluated by the Rossby number (Ro), or the ratio field. Thus, the mesoscale current field should of acceleration of a water parcel (a drifter) to the primarily represent the surface gradients. For Coriolis force, both of which can be calculated convenience, we call the sum of surface gradients from drifter data, i.e., and wind stresses the apparent surface gradients.

We present the Coriolis force vectors and long- R ¼ d~u=dt =f jj~u ; ð1Þ o itudinal and transversal components of accelera- where ~u is the drifter velocity, and f is the Coriolis tion in 7 Â 7km2 bins (Figs. 7 and 8). parameter. Because we are only interested in the We start from the Gerlache Strait. The trans- mean flow, the acceleration of a drifter is versal acceleration of the Gerlache Strait Current calculated from the 2-day low-pass-filtered data. is northward in the left-hand direction relative to Our estimates indicate that the acceleration is the current (Fig. 8), which pushes the Gerlache one order of magnitude smaller than the Coriolis Strait Current close to Bradant Island. Exiting force in most areas: about 50% of Ro are less than from the Gerlache Strait, the mainstream of the 0.2, and 85% are less than 0.5 (Fig. 5). Though the Gerlache Strait Current turns to the north Coriolis force plays the dominant role in the force corresponding to the northward transversal accel- balance, Ro¼ 0:2 or greater indicates that non- eration. The Ro of this mainstream is estimated linear effects in the vorticity balance will be greater than 0.2, which indicates the influence of important (Pedlosky, 1987). Those Ro larger than nonlinear acceleration or the departure from the 0.5 occurred around islands and basins in 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.

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 right- hand direction relative to the current, which means Fig. 6. The orthogonal coordinate system following the trajec- the transversal apparent surface gradient is greater tory of a drifter. 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 deceleration of currents at the southern edge, the accelerates. The longitudinal and transverse accel- decrease in Coriolis force in the left-hand direc- erations can be explained from the barotropic tion, to the positive right-hand transverse accel- geostrophic adjustment induced by a sudden eration, which produces a clockwise circulation. increase in the downhill slope. In the area south The trajectories of drifters marked the paths and of Deception Island, the longitudinal velocity boundaries of surface water masses. The origin of accelerates abruptly, marking the origin of the the Gerlache Strait Current and the Bransfield Bransfield Strait Current. If the magnitude of Strait Current can be clearly followed to the mean wind is weak (less than 2 m s1 in austral southwest of the Gerlache Strait (Fig. 3). It is summer months) and the spatial scale of wind is 1– unlikely that the flow originates by the intrusion of 2 orders of magnitude greater than the spatial scale circumpolar flow over the shallow sills at the of the current, such acceleration must be produced southwest entrance of the Gerlache Strait, because by the downhill longitudinal surface gradient. none of the 9 drifters deployed southwest of the The transverse acceleration of the Bransfield area entered the Strait (Fig. 2). The Gerlache Strait Strait Current is southward. This southward water also flows along the Antarctic Peninsula acceleration forbids drifters from turning to the shelf, and meets the water from the eastern north. This is remarkably consistent with the Antarctic Peninsula shelf in the vicinity of Tower drifter trajectories: no drifters crossed the 750 m Island, which marked the eastern boundary of the isobath to the shallower water. There is no eddy Gerlache Strait water. The water east of Tower feature north of the Current. Oppositely, drifters Island originates from the Weddell Sea shelf, as frequently spun southward off from the Bransfield indicated from hydrographic study (Niiler et al., Strait Current and were trapped in eddies. The 1991). Drifters released in the Gerlache Strait southern edge of the Current is much more marked the path of Gerlache Strait water, how it unstable than the northern edge. The spinning- exits the Gerlache Strait and enters the Bransfield off of eddies can be explained starting from the 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 tween these two waters north of the Bransfield deeper than 750 m. Thus, water around the South Strait Current. Shetland Islands must originate elsewhere. The A general cyclonic basin-scale circulation is trajectories of drifters marked the interface be- found in the Bransfield Strait, with complex M. Zhou et al. / Deep-Sea Research I 49(2002) 267–280 279 temporal and spatial features. We can say that the rich in eddies. A drifter could remain in the Bransfield Strait Current bounds the water of the Bransfield Strait Current less than 10 days before Gerlache and Bransfield Straits in the north. But its exit. But drifters were frequently ripped off we cannot define a clear boundary in the south. from the Bransfield Strait Current and trapped in The interfaces between waters in the Gerlache cyclonic eddies in the deep basins, which provides Strait, Bransfield Strait and Antarctic Peninsula a mechanism for zooplankton individuals to shelf have eddy-rich features. The hydrographic recirculate back to the western Bransfield Strait. data indicate that Antarctic Peninsula shelf water Such flow configuration leads to a long residence can reach the western Bransfield Strait, Hoseason time and high abundance of zooplankton in the Island and Brabant Island (Niiler et al., 1991). A western Bransfield and Gerlache Straits, bounded detailed interpretation of flow and water masses is by the Bransfield Strait Current in the north difficult from our limited drifter data. (Brinton and Townsend, 1991; Huntley and Both western Bransfield and Gerlache Straits Escritor, 1991). are extraordinarily productive bodies of water in The seasonal third path (from late February to the Antarctic Peninsula area. The most abundant April) of the Gerlache Strait Current that de- species, such as Calanoides acutus, Euphausia scribes the ejection of Gerlache Strait water crystallorophias and Euphausia superba, spawn westward between Hoseason and Liege Islands in the Gerlache Strait from November–March. may determine the seasonal variation of zooplank- High concentrations of phytoplankton ton distribution. Lower zooplankton were found (>10 mg chl m3) may be present in the upper in the western Bransfield Strait in general, but 50 m from late October–January (Holm-Hansen more zooplankton were found west and north of and Mitchell, 1991; Holm-Hansen and Vernet, the South Shetland Islands during that particular 1992), providing a food-rich environment. It is season (Brinton and Townsend, 1991; Huntley and critical for these larval populations to remain in Escritor, 1991). such a food-rich environment. Drifters entrained in the Gerlache Strait Current and the Bransfield Strait Current will exit the Bransfield Strait in 10– Acknowledgements 20 days, but those entrained in eddies and bays can have a time scale from 50–100 days. Hence, This work was supported under NSF Grant individuals in an eddy or bay will have enough numbers DPP85-19908 and OPP95-23748. We residence time in such food-rich environments for thank Judy Illeman for processing the original their development. For example, C. acutus could drifter data. 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, References 1991; Huntley and Brinton, 1991; Huntley and Escritor, 1991). The currents and jets are also Amos, A.F., 1987. Hydrography of the Bransfield Strait during important for zooplankton recruitment and dis- the RACER field season: December 1986–April 1987. Eos persion. 68, 1685–1685. Brinton, E., 1991. Distribution and population structures of Is it true that individuals of zooplankton immature and adult Euphausia superba in the western entrained in the Bransfield Strait Current might Bransfield Strait region during the 1986–87 summer. not meet the common fate? Our drifter measure- Deep-Sea Research II 38, 1169–1194. ments show that the northern boundary of the Brinton, E., Townsend, A., 1991. Development rates and Bransfield Strait Current behaves like a barrier habitat shifts in the Antarctic neritic euphausiid Euphausia crystallorophias, 1986–87. Deep-Sea Research II 38, where the southward acceleration limits any 1195–1211. crossover transport to the shallower water. Con- Capella, J.E., Ross, R.M., Quetin, L.B., Hofmann, E.E., 1992. versely, the southern boundary of the Current is 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

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