Sea-ice cover dramatically effects ocean–atmosphere exchanges of heat and water. The rejection of brine during freezing and the transport of fresh water as ice are key parts of the fresh water budget of the polar regions and have an influence on the formation of deep oceanic waters.

The sea-ice model has two distinct components, a thermodynamic model and a dynamic model. The thermodynamic model calculates melting and freezing rates to achieve a local balance of heat and water fluxes. It treats the ice and the snow above it as a single layer of variable thickness with partial areal coverage. Some recent developments include changes to the sea-ice/snow albedo parametrization and the calculation of the basal heat flux from the ocean. The dynamic model calculates ice velocity by balancing forcing from windstress, ocean drag and sea-surface tilt with internal ice stresses. Work is under-way to implement an elastic-viscous-plastic ice dynamics scheme in the full coupled climate model.

The figure below to the right shows a comparison of the annual average Arctic sea-ice cover (in terms of ice concentration - the fraction of area covered by sea-ice) between HadCM3 (a 100-year section of the control run) and observational estimates derived from SSM/I satellite data. There is good agreement in general, though the model appears to have too little ice off the Siberian coast, and in the model there is ice cover north of Norway in the Barents Sea where none is seen in the observations.

The corresponding figure for the Antarctic (left) also shows reasonable agreement between the model and observations. However the model shows too much ice near the coast round most of the continent and not enough perennial ice cover in the Weddell Sea.