Improving the utility of ocean circulation models through adjustment of the momentum balance

We propose a new method to improve the utility of three-dimensional ocean c irculation models. The method uses climatological temperature and salinity data to adjust the momentum balance of the model, while leaving the tracer equations fully prognostic and unconstrained. The adjustment is accomplishe d by replacing density in the hydrostatic equation by a linear combination of model-computed and climatological density. The procedure is equivalent t o adding a forcing term to the horizontal momentum equation through a modif ication of the model's horizontal pressure gradient term. The forcing term modifies the model-computed velocity field, which, in turn, affects the mod el-computed temperature and salinity fields through the advection term (the re is no adjustment of the tracer equations carried by the model). Assuming the linear combination coefficient to be invariant in time and space, we s uggest a statistical approach to estimating its optimal value. We apply thi s "semiprognostic" method to the northwest Atlantic. A primitive equation c irculation model is initialized with January climatological temperature and salinity and is forced by monthly mean Comprehensive Ocean-Atmosphere Data Set surface wind stress and heat flux, by restoration of the surface salin ity, to monthly mean climatology, and by flows through the open boundaries. Both the model-computed tracer and velocity fields produced using the semi prognostic method show significant improvement over those produced by a pur ely prognostic calculation; drift of the tracer and velocity fields away fr om climatology is greatly reduced. Further, convective mixing is explicitly represented, thus improving the utility of results over those obtained fro m pure diagnostic calculations. The velocity fields obtained with the new a pproach are somewhat more realistic than those obtained from pure diagnosti c calculations. The method reproduces many well-known circulation features in the region, including the Labrador Current, the Gulf Stream, and the Nor th Atlantic Current. More significantly, the method reproduces reasonably w ell the seasonal evolution of temperature and salinity in the region despit e the fact that the model's tracer fields are not constrained directly by t he new method. This result suggests that the semiprognostic approach will b e useful for examining the evolution of tracers that are not easily determi ned by observations.