FITTING DYNAMIC-MODELS TO THE GEOSAT SEA-LEVEL OBSERVATIONS IN THE TROPICAL PACIFIC-OCEAN .2. A LINEAR, WIND-DRIVEN MODEL

The Geosat altimeter sea level observations in the tropical Pacific Oc ean are used to evaluate the performance of a linear wind-driven equat orial wave model. The question posed is the extent to which such a mod el can describe the observed sea level variations. The Kalman filter a nd optimal smoother are used to obtain a solution that is an optimal f it to the observation in a weighted least-squares sense. The total mea n variance of the Geosat sea level observation is 98.1 cm2, of which 3 6.6 cm2 is due to measurement errors, leaving 61.5 cm2 for the oceanog raphic signal to be explained. The model is found to account for about 68% of this signal variance, and the remainder is ascribed to the eff ects of physical mechanisms missing from the model. This result sugges ts that the Geosat data contains sufficient information for testing ye t more sophisticated models. Utility of an approximate filter and smoo ther based on the asymptotic time limit of the estimation error covari ance is also examined and compared with the estimates of the full time -evolving filter. The results are found to be statistically indistingu ishable from each other, but the computational requirements are more t han an order of magnitude less for the approximate filter/smoother. Co rrections to the wind field that drives the model are also obtained by the smoother, but they are found only to be marginally improved when compared with in situ wind measurements. The substantial errors in the Geosat data and the simplicity of the present model prevents a reliab le wind estimate from being made.