Response of the Weddell Sea pack ice to wind forcing

Sea ice drift in the Weddell Sea was studied on the basis of data from seven buoys deployed on ice flees in January-February 1996. Six of the buoys formed an array with initial mutual distances of 50 km, which by October 1996 were stretched to similar to 400 km in an east-west direction. The differential kinematical parameters i.e., divergence, shearing rate, and vorticity, were estimated from the buoy array positions by applying a time-dependent method. The large-scale drift of the array was divergent until August 1996, but after the array came under the influence of the Antarctic Circumpolar Current the deformation was mainly shearing. Meteorological data from European Centre for-Medium-Range Weather Forecasts analyses and Special Sensor Microwave Imager derived ice concentrations were interpolated at the buoy sites. The drift velocity was highly wind-dependent; it was also wind-dependent in winter when the momentum balance was mainly between the internal ice stress and the air drag. The drift divergence and shearing rate, were related to the wind forcing, while the array vorticity correlated better with the air pressure itself. We estimated the air-ice drag coefficient to be 1.8 x 10(-3) at a height of 10 m over the ice and the geostrophic drag coefficient to be 6.1 x 10(-4). The stability effects on the coefficients were mostly felt in conditions of light winds. The average ice-water drag coefficient based on 5 day periods was 2.1 x 10(-3). The ice transport through a transect crossing the Weddell Sea was computed on the basis of the geostrophic winds, observed wind-drift dependence, SSM/I-derived ice concentrations, and the literature on ice draft statistics. The resulting annual mean net export in 1996 was 1600 km(3) yr(-1).