VARIATIONAL ASSIMILATION OF GEOSAT DATA INTO AN EDDY-RESOLVING MODEL OF THE GULF-STREAM EXTENSION AREA

A variational inverse technique is applied to assimilate sea surface h eight (SSH) measurements into a simple eddy-resolving quasigeostrophic ocean model. The data used were measured by Geosat in the spring of 1 987 in an area in the Gulf Stream ''tension. The assimilation techniqu e minimizes the weighted least-squares difference between model and ob servations, while the dynamical model equations are satisfied exactly. Fitting the model to data by applying the adjoint technique allows us not only to solve for the best model trajectory in phase space but al so the wind forcing and internal model parameters describing, for exam ple, diffusion or stratification. The method is first tested systemati cally by performing a number of identical twin experiments with model- produced ''observations.'' A hierarchy of ocean models is then applied to test their performance in assimilating two repeat cycles of Geosat sea surface height (SSH) measurements. The most successful model is n onlinear and baroclinic. It can fit the data to less than 5-cm rms dif ference, which is within the error estimates of the satellite measurem ents. Special consideration is given to studying the possibilities and limitations of the retrieval of model parameters. It is found that th e assimilation period has to exceed two repeat cycles of the satellite to determine model parameters. For longer assimilation periods, howev er, the discrepancy between the complex dynamics of the meandering Gul f Stream and the simple dynamics of the model becomes more and more ap parent. Verification of model results with an independent dataset show s that modeled currents compare reasonably well with in situ measureme nts made by drogued buoys.