GEOSAT DATA ASSIMILATION WITH APPLICATION TO THE EASTERN NORTH-ATLANTIC

An attempt is made to determine the three-dimensional ocean circulatio n from satellite altimeter measurements by assimilating Geosat sea sur face height data into an eddy-resolving quasigeostrophic (QG) model of the eastern North Atlantic Ocean. Results are tested against independ ent information from hydrographic field observations and moored curren t meter data collected during the Geosat ERM. The comparison supports the concept of inferring aspects of the three-dimensional flow field f rom sea surface height observations by combining altimetric measuremen ts with the dynamics of ocean circulation models. A Holland-type QG mo del with open boundaries is set up on a 2000 km x 2000 km domain of th e eastern North Atlantic between 25 degrees and 45 degrees N, 32 degre es and 8 degrees W. By using a simple nudging technique, about two yea rs of Geosat altimeter data are assimilated into the model every five days as space-time objective analyses on the model grid. The error inf ormation resulting from the analysis is used during the assimilation p rocedure to account for data uncertainties. Results show an intense ed dy field, which in the surface layer interacts with a meandering Azore s Front. Compared to Geosat, the model leads to smoothed fields that f ollow the observations. Model simulations are significantly correlated with hydrographic data from March 1988 and June 1989, both close to t he surface and in the subsurface. Good agreement is also found between the model velocity fields and moored current meter data in the top tw o model layers. The agreement is visually weak in the bottom layer, al though a coherence analysis reveals an agreement between the model sim ulation and current meter data over the full water column at periods e xceeding 80 days.