WIND FORCING AND ICE MOTION IN THE WEDDELL SEA REGION

The wind forcing on the ocean surface and temporal and spatial characteristics of ice motion are analyzed from data of automatic buoy stations, drifting for 14 months on ice floes in the Weddell Sea. Generally fluctuations with periods from 2 to 5 days, which are caused by synoptic scale atmospheric wind fields, primarily contribute to the variance of ice drift. Peaks in the spectra of ice motion are also found at 12 hours due to inertial motion and weak tidal effects, which are nearly coincident in this geographical region. Inertial motion is only missing in winter, when the ice concentration is high in the western branch of the Weddell Gyre. Semidiurnal peaks are stronger over the continental shelf than over the deep ocean. Diurnal peaks are weaker than semidiurnal peaks. They are found over the continental shelf and are missing over the deep ocean at the same time. The tidal motion reflects non-wind-related coupling between water and ice motion for periods of 1/2 to 1 day. The integral length scales of ice motion are between 550 and 680 km (longitudinal correlation) and between 360 and 540 km (lateral correlation), respectively, when the ice concentration exceeds eight tenths. In the marginal ice zone, the lateral length scale of ice motion is reduced to 270 km. The integral length scales of air pressure fields exceed the length scales of ice motion by a factor of at least 1.5. Longitudinal and lateral length scales of ice motion in the Weddell Sea are slightly smaller than those published for the Beaufort Sea, where the ice moves within a basin of similar diameter of approximately 1500 km. The smaller scales in the Weddell Sea presumably are due to differences of the forcing fields. About 70% to 95% of the variance of 12-hourly averaged drift velocities can be linearly related to the wind velocity, except when the mean wind speed drops below 3.5 m/s. The correlation with geostrophic winds is close to that with locally measured winds. The twelve-hourly averaged ice drift amounts to about 3.5% of the actual surface wind velocity at a height of 3 m or to about 1.6% of the geostrophic wind velocity, derived from surface pressure analyses. The drift/geostrophic wind ratio is 30% smaller in central winter with ice concentrations above nine tenths in the central Weddell Sea than during summer periods, when the buoys are closer to the coast, in the periphery of the gyre and in the marginal ice zone. The drift/local wind ratio scatters more than the drift/geostrophic wind ratio. The ice drift in winter for ice concentrations above eight tenths, on an average, is deflected by 20 degrees to the left of the surface wind direction and is parallel to the isobars. In summer under reduced ice concentrations, it deviates by 40 degrees to the left of the surface wind and by 10 to 15 degrees to the left of the geostrophic wind. For periods of 1 month, the ice drift is also linearly well correlated to geostrophic wind velocity. Winds in the Weddell Sea usually do not average out over longer time scales. Ocean currents are too weak to dominate mean ice motion.