Simulating the Baltic Sea ice season with a coupled ice-ocean model

The Baltic Sea ice season climatology is studied with a coupled ice-ocean model. The evolution of sea temperature, ice thickness and ice drift has been simulated for three particular ice seasons, normal (1983/84), severe (1986/87) and mild (1991/92), forced with the prescribed daily atmospheric data. The ice model is a plastic Hibler model with a three-level ice thickness distribution (open water, undeformed ice and ridged ice) and thermodynamics. The ocean model consists of a barotropic circulation model with 4-layer vertical thermodynamic evolution. The models are coupled via the momentum and heat fluxes; the grid size is 10’ in latitude and 20’ in longitude. With the same initial conditions and observed meteorological forcing, a fully prognostic thirteen-months integration was performed for each ice season from 1 May to 31 May in the following year. The model results were compared with routine ice charts. The annual cycles of sea surface temperature, ice thickness and coverage, and the interannual variability of the ice seasons were realistically simulated. Ocean surface temperatures were produced well in the shallow sea areas, but over the deeper central parts of the basins they were too warm in the winter stage. On the average and severe winters the predicted maximum ice extent was about 20% less than that observed, while in the mild winter the agreement was very good. Basin scale features were rather well realized in the ice thickness field. The freezing date and ice growth were in agreement with observations, but the beginning of the ice melting was delayed which led to about 1—2 weeks delay in the ice break-up date.