Variational Assimilation of Geosat Data into an Eddy-resolving Model of the Gulf Stream Extension Area

Abstract

A variational inverse technique is applied to assimilate sea surface height (SSH) measurements into a simple eddy-resolving quasigeostrophic ocean model. The data used were measured by Geosat in the spring of 1987 in an area in the Gulf Stream extension. The assimilation technique minimizes the weighted least-squares difference between model and observations, 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 also the wind forcing and internal model parameters describing, for example, diffusion or stratification. The method is first tested systematically 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 nonlinear and baroclinic. It can fit the data to less than 5-cm rms difference, which is within the error estimates of the satellite measurements. Special consideration is given to studying the possibilities and limitations of the retrieval of model parameters. It is found that the assimilation period has to exceed two repeal cycles of the satellite to determine model parameters. For longer assimilation periods, however, the discrepancy between the complex dynamics of the meandering Gulf Stream and the simple dynamics of the model becomes more and more apparent. Verification of model results with an independent dataset shows that modeled currents compare reasonably well with in situ measurements made by drogued buoys.