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Surface Currents in the Bransfield and Gerlache Straits, Antarctica

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Surface Currents in the Bransfield and Gerlache Straits, Antarctica 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 sÀ1 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 sÀ1 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
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