Modelled time-dependent flow in the Agulhas retroflection region as deduced from altimeter data assimilation

Abstract

The past decade has seen the rapid development of sophisticated, eddy-resolving models of ocean circulation. What has been missing thus far in creating realistic descriptions of ocean flows is the availability of sufficient synoptic data that can be incorporated into such models to initialize the models and keep their forward integration on track. Observation of the ocean from space provides such a possibility for assimilating the dynamic signal needed to steer the models towards reality and to allow oceanographers to create a synthesis and description of time-varying currents in a region of interest. The very large eddy signal present in the region south of South Africa makes that area ideal for studies of the assimilation of data derived from the Geosat (the U.S. Navy's geodetic satellite) altimeter. Maps of the eddy variability have been constructed over a few hundred days of the repeat orbit mission (December 1986 – February 1988) for data-assimilation purposes. The method of successive corrections was used to carry out the optimal interpolation from the track data to a regular ¼-degree latitude/ longitude grid. These data are then assimilated into a regional ocean model of the Agulhas Retroflection region. The model is a five-layer, ¼-degree horizontal resolution, quasigeostrophic model that covers the region from 20 to 54°S and from 2,5°W to 65°E, including the southern part of the African continent. Open lateral boundary conditions allow for inflows and outflows appropriate to this region. Numerical experiments, with and without assimilation of the altimeter data set, are carried out and a variety of assimilation techniques examined. The best of the results provides a four-dimensional description of the ocean circulation in the region for the time period covered.