Drifter measurements of near-surface flow over the West Antarctic

Peninsula shelf during austral summer-fall 2001

R. Limeburner, R. Beardsley and B. Owens

Department of Physical Oceanography

Woods Hole Oceanographic Institution

Woods Hole, MA 02543

rlimeburner@whoi.edu

As part of the U.S. Southern Ocean GLOBEC program, we deployed 14 satellite-tracked drifters near Marguerite Bay on the West Antarctic Peninsula shelf during March - May, 2001 to investigate the regional near-surface circulation. The drifters were WOCE SVP instruments with drogues centered at 15 m and equipped with cold weather batteries and ice strengthened buoy hulls since this area becomes ice covered in austral winter (June to December). The drifter tracks show 1) a moderate (10-20 cm/s) cyclonic circulation around Marguerite Bay with broad inflow in the northern side near Adelaide Island and a narrower outflow and greater variability in the southern side near Alexander Island, 2) weak (< 10 cm/s) flow at mid-shelf, and 3) strong (>20 cm/s) alongshelf flow toward the northeast over the outer shelf and shelf break. The Marguerite Bay circulation was not closed; most drifters entering the bay left the bay and a few apparently become stuck in the ice during August. Closed eddies were surprisingly absent in Marguerite Bay except for one instance of weak near-inertial oscillations that decayed within two days and and small eddies (diameter ~ 10-20 km, rotation period ~ 3-5 days) near Rymill Bay. The weak mid-shelf surface drifter velocities were surprising due to the strong winds observed during the deployment cruises. The slow drifter speeds during large wind stress events may be due to the deep surface mixed layer (~ 50 m), resulting in quite weak Ekman currents. Lagrangian time and space scales of 1.8 - 3 days and 18 - 27 km were calculated from the autocorrelation functions for the drifter velocity components.