Satellite-Derived Shelf-Slope Exchanges in the Southern Benguela Coastal Current

Shelf-slope exchanges are investigated off western South Africa between 32°-35°S. Our specific objectives are to:

1. Quantify the relative roles of shelf-slope exchanges caused by geostrophic and wind-driven transports across the shelf edge; and
2. Determine the extent to which the geostrophic transports are controlled by the passage of offshore eddies.

The geostrophic component of the shelf-slope exchanges are estimated from altimeter surface height fields, using data from the six years of interleaved reference altimeter orbits, during which the spatial resolution is maximum. Gridded sea level anomaly (SLA) and absolute dynamic topography (ADT) fields are interpolated to the shelf edge, across which geostrophic velocities are calculated. During the same periods, an interleaved track (122, figure below) runs approximately along the outer shelf and slope, allowing cross-track geostrophic velocities to be calculated from these more basic alongtrack altimeter heights. These quantify the geostrophic exchanges, for comparison to wind-driven surface Ekman transports across the same boundaries. Wind data come from scatterometers and atmospheric model reanalysis. The presence of offshore eddies is indicated by the same gridded SLA fields. With these data sets we address the two objectives listed above during this six-year period. To establish the generality of the results, similar calculations using the gridded SLA data are made during the entire 22-year period of data. High-resolution numerical ocean circulation models also provide fields of transports for comparison to the satellite-derived results. Preliminary results reveal a greater number of mesoscale circulation features with cross-isobath flow in the altimeter fields than in the model fields, some of which are due to the stationary Mean Dynamic Topography field that is added to the altimeter SLA to produce the ADT. Further analysis and comparison to previous observations will determine the degree of realism in both the altimeter and model fields.