The absolute velocity field of Agulhas eddies and the Benguela Current

Abstract

Acoustic Doppler current profiler (ADCP) data referenced to Global Positioning System navigation were obtained in May 1993 from the Royal Research Ship Discovery within the Benguela Current as part of the Benguela Source and Transport (BEST) project. These data are used in combination with hydrographic data collected during the cruise to investigate the absolute geostrophic velocities in the Benguela Current and the transient eddies. Four anticyclonic eddies were encountered during the cruise, of which three were determined to be Agulhas Retroflection eddies of various ages and one was determined to be an eddy derived from the Brazil Current. ADCP velocities averaged between conductivity‐temperature‐depth stations have a high linear correlation with geostrophic velocities derived from the hydrographic data (correlation coefficient of 0.93) along the entire cruise track. The magnitudes of the two velocity estimates, however, were notably different within the eddies. It was determined that these discrepancies are probably due to a significant barotropic component of the flow near the eddy center. As much as 50% of the total flow in the eddy is barotropic. The horizontal length scale (radius of maximum velocity) of this eddy determined from both 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 partitioning of total mechanical energy between available potential and kinetic energy. It is also expected to have a significant effect on the climatically important exchange of mass between the Indian and South Atlantic Oceans. Total geostrophic velocities were computed for the Benguela Current usine the averaged ADCP at 250 m as a reference. The ADCP referenced geostrophic transport across 30°S of water warmer than 9°C in the Benguela Current was found to be 17 Sv (1 Sv= 106 m3 s−1) to the north and that of the upper kilometer was 25 Sv to the north. These values are largely consistent with previous estimates, suggesting that the upper layer flow across this section is dominated by the baroclinic field.