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Using standard dynamic methods, we show the 7-year mean upper ocean circulation in the Elephant Island area relative to the 200- and 500-decibel (db) levels for two periods: January/February and February/March. Means were calculated using all conductivity-temperature-depth (CTD) stations within each 0.25° latitude x 0.5° longitude bin making up the Antarctic Marine Living Resources (AMLR) grid (see Martin, Hewitt, and Holt, Antarctic Journal, in this issue).
In a larger geographical context, the AMLR grid straddles the southern Drake Passage, the Antarctic Continental Shelf, and the northern Bransfield Strait (figure 1 in Martin et al., Antarctic Journal, in this issue). Thus, one would expect to see the surface circulation include the southern arm of the eastward-setting Antarctic Circumpolar Current (ACC), called the Continental Water Boundary (Nowlin and Clifford 1982), the western boundary of the Weddell-Scotia Confluence (Nast et al. 1988), the counter-flowing East Wind Drift, the Bransfield Current, and perhaps some manifestation of water flowing out of the Weddell Sea and entering the Bransfield Strait (Gordon and Nowlin 1978).
Over the 7 years, we have been impressed with the consistency of the upper-level water structure and geostrophic flow calculated from CTD casts. We have noted month-to-month and year-to-year differences in the thermohaline structure (Amos 1993), but the geostrophy remains similar, both interannually and between the mid- and late-summer seasons of the two AMLR legs.
The general flow is from southwest to northeast and is similar at both the 200-db and the 500-db level ( figure 1A, B), indicating that the motion of the upper few hundred meters is nearly uniform. Three main flow paths are apparent:
It should also be noted that the AMLR grid pattern is not designed for complete coverage of some of the areas of interest to the oceanographer; rather, the coverage must be a compromise to accommodate all of the areas of krill research (acoustics, zooplankton, phytoplankton, krill distribution, penguin and fur seal tracking, etc.) in the available cruise time.
Major reverse flow and East Wind Drift are not evident, but two or possibly three meanderlike features are discernible:
Quasi-permanent* features such as these might affect krill distribution. We do not know if the Drake Passage feature is a meander in the flow or an eddy, but during some years' surveys, the streamlines are closed, indicating an eddy. The concentration of streamlines running southwest to northeast marks the boundary front that separates Drake Passage waters from those of the continental shelf and the Bransfield Strait. The northwest corner of the Drake water where the meander occurs is generally a region of low surface and integrated chlorophyll-a concentrations (Villafañe, Helbling, and Holm-Hansen 1994) and low inorganic nutrients (Silva et al. 1994) but variable krill abundance from acoustic profiles (e.g., Hewitt and Demer 1994) and variable krill and salp abundance in net tows (Loeb and Siegel 1994).
Does the mean flow pattern and location of meanders vary substantially seasonally? We compare the slope of the sea surface relative to 500 db for all stations made during January/February ( figure 2A) and February/March (figure 2B). The time periods display no essential differences. Both meanderlike features are present, as well as the smaller eddy feature between Clarence and Elephant Islands. Also shown in figure 2A are the tracks of three barometer drifters deployed during January 1995. Their tracks follow the mean streamlines closely with the circular motion of the northernmost drifter in the general vicinity of the meander in the northwest corner, although it describes both clockwise and anticlockwise rotations.
This research was supported by National Oceanic and Atmospheric Administration contract 50ABNF600015. We wish to thank the Captain and crew of the R/V Yuzhmorgeologiya, and the invaluable help from the various crews of the NOAA ship Surveyor for the previous six field seasons of the AMLR program.
___________________*Quasi-permanent, because we know only that they exist during the summer AMLR study period.
References
Amos, A.F. 1993. AMLR program: Interannual variability in the Elephant Island surface waters in summer. Antarctic Journal of the U.S., 28(5), 201-204.
Gordon, A.L., and W.D. Nowlin, Jr. 1978. The basin waters of the Bransfield Strait. Journal of Physical Oceanography, 8, 258-264.
Hewitt, R.P., and D.A. Demer. 1994. AMLR program: Distribution and abundance of krill near Elephant Island in the 1993-1994 austral summer. Antarctic Journal of the U.S., 29(5), 183-184.
Loeb, V., and V. Siegel. 1994. AMLR program: Krill stock structure and macrozooplankton abundance near Elephant Island, January to March 1994. Antarctic Journal of the U.S., 29(5), 185-188.
Martin, J.E., R.P Hewitt, and R.S. Holt. 1996. The U.S. Antarctic Marine Living Resources (AMLR) program: 1995-1996 field season activities. Antarctic Journal of the U.S., 31(2).
Nast, F., K.-H. Kock, D. Sahrhage, M. Stein, and J.E. Tiedtke. 1988. Hydrography, krill, and fish and their possible relationships around Elephant Island. In D. Sahrhage (Ed.), Antarctic ocean and resource variability. Berlin: Springer-Verlag.
Nowlin, W.D., Jr., and M. Clifford. 1982. The kinematic and thermohaline zonation of the Antarctic Circumpolar Current at Drake Passage. Journal of Marine Research (Supplement), 481-507.
Silva, N., M. Ramos, E.W. Helbling, and O. Holm-Hansen. 1994. AMLR program: Depletion of inorganic nutrients in the area around Elephant Island, Antarctica. Antarctic Journal of the U.S., 29(5), 195-197.
Villafañe, V.E., E.W. Helbling, and O. Holm-Hansen. 1994. AMLR program: Phytoplankton distribution and species composition around Elephant Island, Antarctica, January to March 1994. Antarctic Journal of the U.S., 29(5), 191-193.