A version of the Bryan–Cox–Semtner numerical ocean general circulation model, adapted to include a free surface, is described. The model is designed for the following uses: tidal studies (a tidal option is explicitly included); assimilation of altimetric data (since the surface elevation is now a prognostic variable); and in situations where accurate relaxation to obtain the streamfunction in the original model is too time consuming. Comparison is made between a 300-year run of the original model and the free-surface version, using a very coarse North Atlantic calculation as the basis. The results are very similar, differing only in the streamfunction over topography; this is to be expected, since the treatment of topographic torques on the barotropic flow differs because of the nature of the modifications.