A prognostic one-dimensional mixed layer--pycnocline model describing the vertical structure of the upper ocean is coupled to a thermodynamic sea ice model. The coupled prognostic model is compared with the more usual sea ice model overlying a fixed mixed layer with constant oceanic heat flux and is then applied to investigate the effect of temporal and regional variations of the entrained oceanic heat flux on the sea ice cover in both polar regions. In the southern ocean a standard simulation is compared with two perturbation experiments which both describe the occurrence of a polynya. In the Arctic Ocean the effect of possible Soviet river diversions is investigated. Finally, the response of the coupled model to paleoclimatic forcing and boundary conditions is presented. ¿ American Geophysical Union 1987