ATMOSPHERIC AND OCEANIC FORCING OF WEDDELL SEA-ICE MOTION

The data from sea ice buoys, which were deployed during the Winter Wed dell Sea Project 1986, the Winter Weddell Gyre Studies 1989 and 1992, the Ice Station Weddell in 1992, the Antarctic Zone Flux Experiment in 1994, and several ship cruises in Austral summers, are uniformly rean alyzed by the same objective methods. Geostrophic winds are derived af ter matching of the buoy pressure data with the surface pressure field s of the European Centre for Medium Range Weather Forecasts. The ratio between ice drift and geostrophic wind speeds is reduced when winds a nd currents oppose each other, when the atmospheric surface layer is s tably stratified, and when the ice is under pressure near coasts. Over the continental shelves, the spatial inhomogeneity of tidal and inert ial motion effectively controls the variability of divergence for peri ods below 36 hours. Far from coasts, speed ratios, which presumably re flect internal stress variations in the ice cover, are independent of drift divergence on the spatial scale of 100 km. To study basin-scale ice dynamics, all ice drift data are related to the geostrophic winds based a the complex linear model [Thorndike and Colony, 1982] for dail y averaged data; The composite patterns of mean ice motion, geostrophi c winds, and geostrophic surface currents document cyclonic basin-wide circulations. Geostrophic ocean currents are generally small in the W eddell Sea. Significant features are the coastal current near the sout heastern coasts and the bands of larger velocities of approximate to 6 cm s(-1) following the northward and eastward orientation of the cont inental shelf breaks in the western and northwestern Weddell Sea. In t he southwestern Weddell Sea the mean ice drift speed is reduced to les s than 0.5% of the geostrophic wind speed and increases rather continu ously to 1.5% in the northern, central, and eastern Weddell Sea. The L inear model accounts for less than 50% of the total variance of drift speeds in the southwestern Weddell Sea and up to 80% in the northern a nd eastern Weddell Sea.