TITLE: Palmer LTER: Upper Ocean Circulation in the LTER Region Historical Sources AUTHOR: Eileen E Hofmann, Cathy M Lascara, John M. Klinck Submitted July 1992 Antarctic Journal of The United States Palmer LTER Contribution #05 (partly tex formatting ) The LTER sampling region encompasses an area along the Antarctic Peninsula that is roughly 900 km alongshore and 200 km offshore. This region includes portions of Bransfield Strait, Gerlache Strait, and the Bellingshausen Sea and is influenced by adjacent areas such as Drake Passage, the Weddell Sea and Marguerite Bay. The Antarctic Circumpolar Current forms the northern boundary of the study region. Historical descriptions of water mass distributions and circulation patterns are available for selected portions of the LTER region. These previous studies were either concentrated on subareas within the LTER study area or were at the periphery of the LTER region. The purpose of this paper is to synthesize the existing hydrographic and current observations to provide a description of the major circulation features in the LTER study region. This circulation pattern is shown schematically in Figure 1 and is described below. The circulation of the northern section of the LTER region is based on descriptions from Bransfield Strait. A dynamic topography map (relative to 1800 db) constructed from data collected during the R.V. Discovery cruises (Clowes, 1934) shows flow from Drake Passage into Bransfield Strait through the gap (maximum 500 m, Fig. 2) between Smith and Snow Islands. This flow continues to the northeast along the southern side of the South Shetland Islands and at the eastern end of Bransfield Strait turns north to exit between King George Island and Elephant Islands or continues eastward towards the Weddell Sea. A recent analysis of historical temperature data from the Bransfield Strait (Capella et al., 1992a) indicates that Circumpolar Deep Water (CDW) enters the Strait from Drake Passage between Smith and Snow Islands. This water mass, which is characterized by temperatures 0 deg. C, is found throughout Drake Passage and the Bellingshausen Sea between 200 and 700 m (Sievers and Nowlin, 1984). Inside the Strait, CDW is found along the southern side of the Islands and between King George and Elephant Islands. Thus, the climatological temperature distributions support the circulation described by Clowes (1934). Additionally, in 1979 a FGGE buoy drifted from Drake Passage between Smith and Snow Islands into the Strait and continued eastward to about 55\deg W, which in agreement with the circulation proposed by Clowes (1934) and Capella et al. (1992a). However, the buoy then turned and drifted to the west (Klinck, 1991), suggesting the presence of a cyclonic gyre in the surface waters of the Bransfield Strait. To the north of the South Shetland Islands, below 200-300 m, is the westward flowing Polar Slope Current (Nowlin and Zenk, 1988). This current, which is thought to originate in the Weddell Sea, is narrow (10 km) and cold (0 deg. C) and flows counter to the predominant surface flow in that region. The circulation of the central and southern sections of the LTER region is based on descriptions from hydrographic surveys and surface drifter studies in the Gerlache Strait and eastern Bellingshausen Sea made as part of the RACER program (Huntley et al., 1991). The dynamic topography maps (relative to 200 db) and drifter trajectories show that surface flow in the Gerlache is northeast along the axis of the Strait and exits to the northeast into the southern reaches of the Bransfield Strait (Amos et al., 1990; Niiler et al., 1990). Similarly, the surface flow in the near-shore coastal waters off Brabant Island appears to be to the north towards Bransfield Strait (Amos et al., 1990; Niiler et al., 1991). This northward flow turns southeastwards as it enters the southwest portion of Bransfield Strait. However, the circulation in this region is complex and likely has seasonal variations. For example, a westward surface flow has been observed in this region (Stein, 1982). Similarly, seasonal variations are associated with flow from the Weddell Sea into the Bransfield Strait (Capella et al., 1992b). Hydrographic surveys conducted in the eastern Bellingshausen Sea as part of the BIOMASS program show southward flow offshore of Anvers Island (Stein, 1982; Stein, 1988) and between Anvers and Renaud Islands (Kock and Stein, 1978). More recently Stein (1991) presented dynamic topography contours (relative to 200 db) constructed from a larger scale hydrographic survey which suggest that the circulation in the upper water column consists of two cyclonic gyres: one near Anvers and Brabant Islands and one near Adelaide Island. This is similar to the circulation pattern presented in Stein (1988). However, given the spatial resolution of the measurements used by Stein (1991), it is equally possible that there is only a single cyclonic gyre in the Bellingshausen Sea. In fact, the trajectory traced by a FGGE buoy that drifted thorough the Bellingshausen Sea in 1979 (Klinck. 1991) supports a single cyclonic gyre. This possibility is indicated by the dashed line in Fig. 1. One of the goals of the LTER study is to define the large-scale distribution of water masses and circulation in a region of sufficient size to encompass areas that could potentially contribute larvae of Euphausia superba to the region around Palmer Basin. Previous studies (e.g. Capella et al., 1992b) have shown that E. superba larvae can be transported considerable distances (100s km) from their spawning area. In the LTER region, potential source sites of E. superba larvae are Marguerite Bay, the eastern Bellingshausen Sea, Gerlache Strait and Bransfield Strait. This then defines the north and south extent of the hydrographic sampling grid. The across shelf extent of the grid must be sufficient to include CDW, which is believed to be important to reproducing E. superba (Hofmann et al., 1992). Given these requirements, the LTER hydrographic survey grid (Fig. 2) was designed with an along-shelf spacing of 100 km and an across-shelf station spacing of 20 km. Most across-shelf transects extend offshore beyond the 1000 m isobath and terminate inshore of the 200 m isobath. It should be stressed that this is the basic hydrographic grid. Modifications will be made as needed to accommodate sampling requirements. In particular, stations may be added in certain regions to better define circulation features. We anticipate sampling the entire grid during an LTER cruise scheduled for austral fall (March-April) 1993. We thank T. Whitworth for comments on the schematic circulation pattern. This work was supported by National Science Foundation grant DPP-9011927. REFERENCES \noindent\hangindent=24pt\hangafter=1 Amos, A.~F., S.~S.~Jacobs and J.-H.~Hu.~1990. RACER: Hydrography of the surface waters during the spring bloom in the Gerlache Strait. {Antarctic Journal of the U.S.}, 25(5), 131-134. \noindent\hangindent=24pt\hangafter=1 Clowes, A.I.J.~1934. Hydrography of the Bransfield Strait. {\it Discovery Reports}, 9, 1-64. \noindent\hangindent=24pt\hangafter=1 Capella, J.~E., R.~M.~Ross, L.~B.~Quetin and E.~E.~Hofmann.~1992a. A note on the thermal structure of the upper ocean in the Bransfield Strait-South Shetland Islands region. {\it Deep-Sea Research}, in press. \noindent\hangindent=24pt\hangafter=1 Capella, J.~E., L.~B.~Quetin, E.~E.~Hofmann and R.~M.~Ross.~1992b. Models of the early life history of {\it Euphausia superba}-Part II. Lagrangian calculations. {\it Deep-Sea Research}, in press. \noindent\hangindent=24pt\hangafter=1 Hofmann, E.~E., J.~E.~Capella, R.~M.~Ross and L.~B.~Quetin.~1992. Models of the early life history of {\it Euphausia superba}-Part I. Temperature dependence during the descent-ascent cycle. {\it Deep-Sea Research}, in press. \noindent\hangindent=24pt\hangafter=1 Huntley, M.~E., D.~M.~Karl, P.~P.~Niiler, O.~Holm-Hansen.~1990. Research on Antarctic Coastal Ecosystem Rates (RACER): an interdisciplinary field experiment. {\it Deep-Sea Research}, 38, 911-941. \noindent\hangindent=24pt\hangafter=1 Klinck, J.~M.~1991. Large scale physical structure of the Southern Ocean. In: GLOBEC: Southern Ocean Program, GLOBEC Workshop on Southern Ocean Marine Animal Populations and Climate Change, GLOBEC Report Number 5, 74-83 pp. \noindent\hangindent=24pt\hangafter=1 Kock, K.-H. and M.~Stein.~1978. Krill and hydrographic conditions off the Antarctic Peninsula. {\it Meeresforschung}, 26, 79-95. \noindent\hangindent=24pt\hangafter=1 Niiler, P.~P., A.~Amos and J.-H.~Hu.~1991. Water masses and 200 m relative geostrophic circulation in the western Bransfield Strait region. {\it Deep-Sea Research}, 38, 943-959. \noindent\hangindent=24pt\hangafter=1 Niiler, P., J.~Illeman and J.-H.~Hu.~1990. RACER: Lagrangian drifter observations of surface circulaiton in the Gerlache and Bransfield Straits. {\it Antarctic Journal of the U.S.}, 25(5), 134-137. \noindent\hangindent=24pt\hangafter=1 Nowlin, W.~D., Jr., and W.~Zenk.~1988. Westward bottom currents along the margin of the South Shetland Island Arc. {\it Deep-Sea Research}, 35, 269-301. \noindent\hangindent=24pt\hangafter=1 Sievers, H.~A. and W.~D.~Nowlin, Jr.~1984. The stratification and water masses at Drake Passage. {\it Journal of Geophysical Research}, 89, 10,489-10,514. \noindent\hangindent=24pt\hangafter=1 Stein, M.~1991. Variability of local upwelling off the Antarctic Peninsula, 1986-1990. {\it Archiv.~f\"ur Fischwiss.}, in press. \noindent\hangindent=24pt\hangafter=1 Stein, M.~1988. Variation of geostrophic circulation off the Antarctic Peninsula and in the southwest Scotia Sea, 1975-1985. In: {\it Antarctic Ocean and Resources Variability}, D.~Sahrhage, editor, Springer-Verlag, 81-91. \noindent\hangindent=24pt\hangafter=1 Stein M.~1982. Fischereiozeanographische untersuchungen w\"ahrend FIBEX 1981. {\it Archiv.~f\"ur Fischwiss.}, 33, 35-51. LIST OF FIGURES Fig.~1.\quad Schematic of the circulation in the upper 200-300 m in the LTER study region. Circulation patterns were derived from the sources discussed in the text. The Polar Slope Current is indicated by the dotted arrow. The vectors with lighter shading in the eastern Bellingshausen Sea represent the possibility of a single cyclonic gyre in this region. Islands are identified as: 1-Adelaide, 2-Renaud, 3-Anvers, 4-Brabant, 5-Smith, 6-Snow, 7-Livingston, 8-King George, 9-Elephant. Gerlache Strait is between Anvers and Brabant Islands and the Antarctic Peninsula. Fig.2. The LTER hydrographic grid. Hydrographic stations are indicated by triangles. Depth in meters is indicated by the shading. The 200 and 1000 m isobaths are indicated by the dashed and dotted lines, respectively. The bathymetric contours are from the ETOPO5 bathymetry data set, which is available through the National Center for Atmospheric Research in Boulder, Colorado.