Observed and modelled sea-ice drift response to wind forcing in the northern Baltic Sea

Abstract

The wind dependence of sea-ice motion was studied on the basis of ice velocity and wind observations, and weather model output. The study area was a transition zone between open water and the ice-covered ocean in the northern Baltic Sea. In the centre of the basin the sea-ice motion was highly wind-dependent and the linear relationship between the wind and the drift velocities explained 80% of the drift's variance. On the contrary, the wind-drift dependence was low near the coast. The wind-drift coherence was significant over a broader frequency range in the central part of the basin than for the coastal drift. The ice motion was simulated by a numerical model forced with five types of wind stress and with two types of current data, and the outcome was compared with the observed buoy drift. The wind and the wind-induced surface current were the main factors driving the ice in the basin's centre, while internal ice stresses were of importance in the shear zone near the fast ice edge. The best wind forcing was achieved by applying a method dependent on atmospheric stability and ice conditions. The average air-ice drag coefficient was 1.4 x 10-3 with the standard deviation of 0.2 x 10-3. The improvement brought about by using an accurate wind stress was comparable with that achieved by raising the model grid resolution from 18 km to 5 km.