THE ABSOLUTE VELOCITY-FIELD OF AGULHAS EDDIES AND THE BENGUELA CURRENT

Acoustic Doppler current profiler (ADCP) data referenced to Global Pos itioning System navigation were obtained in May 1993 from the Royal Re search ShipDiscovery within the Benguela Current as part of the Bengue la Source and Transport (BEST) project. These data are used in combina tion with hydrographic data collected during the cruise to investigate the absolute geostrophic velocities in the Benguela Current and the t ransient eddies. Four anticyclonic eddies were encountered during the cruise, of which three were determined to be Agulhas Retroflection edd ies of various ages and one was determined to be an eddy derived from the Brazil Current. ADCP velocities averaged between conductivity-temp erature-depth stations have a high linear correlation with geostrophic velocities derived from the hydrographic data (correlation coefficien t of 0.93) along the entire cruise track. The magnitudes of the two ve locity estimates, however, were notably different within the eddies. I t was determined that these discrepancies are probably due to a signif icant barotropic component of the flow near the eddy center. As much a s 50% of the total flow in the eddy is barotropic. The horizontal leng th scale (radius of maximum velocity) of this eddy determined from bot h the ADCP data and the thermal field was found to be approximately 60 km, considerably smaller than previous estimates, which are about 120 km. The barotropic component in Agulhas eddies leads to an equal part itioning of total mechanical energy between available potential and ki netic energy. It is also expected to have a significant effect on the climatically important exchange of mass between the Indian and South A tlantic Oceans. Total geostrophic velocities were computed for the Ben guela Current using the averaged ADCP at 250 m as a reference. The ADC P referenced geostrophic transport across 30 degrees S of water warmer than 9 degrees C in the Benguela Current was found to be 17 Sv (1 Sv= 10(6) m(3) s(-1)) to the north and that of the upper kilometer was 25 Sv to the north. These values are largely consistent with previous es timates, suggesting that the upper layer flow across this section is d ominated by the baroclinic field.