The objectives of the present study are to formulate and explore a coupled sea ice–ocean model for semi-enclosed basins. Both thermodynamic and dynamic processes are included. Variations in the on–off shore directions are considered, and the model is applied to the Bothnian Bay, where ridging due to southerly winds is a common feature. The importance of the different ice processes and their simulation are discussed. It is concluded that a one-dimensional model approach to the winter sea ice season can be used, giving realistic information about ice formation, growth and decay. Horizontal gradients in the sea and in the ice, however, imply that a two- or three-dimensional model is in general needed, particularly for the initial advance of the ice edge and for the ice deformation calculation. DOI: 10.1034/j.1600-0870.1990.t01-3-00007.x