SUCCESSIVE CORRECTION OF THE MEAN SEA-SURFACE HEIGHT BY THE SIMULTANEOUS ASSIMILATION OF DRIFTING BUOY AND ALTIMETRIC DATA

A simultaneous assimilation model of drifting buoy and altimetric data is proposed to determine the mean sea surface height (SSH) as well as the temporal evolution of the surface circulation on synoptic scales. To demonstrate the efficiency of our assimilation model, several iden tical twin experiments for the double-gyre circulation system are perf ormed using a 1 1/2-layer primitive equation model. An optimal interpo lation for the multivariate is used for the assimilation scheme that a ssumes the geostrophic relationship between the error fields of the ve locity and the interface depth. To identify the nature of the assimila tion of the buoy-derived velocities into the dynamical ocean model, th e authors first conduct the assimilation experiment using the drifting buoy data alone. The result shows that realistic buoy deployment (32 in a 40 degrees square) can effectively constrain the model variables; that is, both the absolute (mean plus time varying) velocity and SSH (interface depth) fields are significantly improved by this buoy data assimilation. Moreover, in the case of denser buoy deployment in the e nergetic western boundary current regions, where the mean SSH is compa rable to the time-varying part and the geoid error is relatively large , the assimilation provides a better determination of the absolute vel ocity and SSH. This is because significant changes in the mean SSH lea d to an improvement along the extensive buoy trajectories associated w ith the strong current. It is worth noting that the assimilation of dr ifting buoy data is more effective than that of moored velocity data, thanks to the Lagrangian information content of the drifting buoys. Su ccessive correction of the mean SSH is made with simultaneous assimila tion of drifting buoy and altimetric data. Consequently, a better corr ection of the mean SSH is obtained: The initial error of the mean SSH is reduced by approximately 40% after the I-year experiment. In contra st, the assimilation experiment of altimetric data alone corrects only the time-varying part, but yields little error reduction for the mean SSH in our model. These results clearly show that the simultaneous as similation of drifting buoy and altimetric data into the dynamical mod el is a very useful tool for improving the model's realism.