Variability and sources of the southeastern Atlantic circulation

The 1992-1993 Benguela Sources and Transport (BEST) time series provide a quantitative view of the Benguela Current transport and the eddy held crossing 30S, as well as an estimate of the relation between its barotropic and baroclinic components. This is done by a simultaneous analysis of the BEST data derived from inverted echo sounders, pressure sensors, current meter moorings, CTD, and ADCP stations. The analysis of the time Series indicates that the annual mean baroclinic transport of the BengueIa Current is 13 Sv with a total transport of 16 Sv. Through the combination of instruments the total baroclinic plus barotropic transport of the upper 2600 m was obtained without making any assumption about the level of no motion. Results from this calculation corroborated the assumption that 1000 m as a level of no motion could be used as a fairly good approximation. The stationary flow of the Benguela Current is mostly confined near the African Continent while a transient flow, composed by large eddies shed from the Agulhas retroflection, dominates the western portion of the Benguela Current. In the stationary part of the Benguela Current, both barotropic and baroclinic components are equally important while in the transient part, the barotropic is more substantial. Several rings were observed during the experiment that migrated toward the west. An initial speed of 12 km/day diminished to 6 to 7 km/day at the Walvis Ridge. The water mass source of the Benguela Current includes Indian and South Atlantic subtropical thermocline water; relatively saline, low oxygen tropical Atlantic water; and the cooler, fresher subantarctic water. Changes in thermocline salinity correlate with transport: in general when the northward transport is increasing the thermocline salinity also increases, without a decrease in oxygen. This indicates that the Benguela Current increases in strength by bringing in more subtropical thermocline water. As the Agulhas input is most effective in boosting the salinity of the upper thermocline (the South Atlantic Current water being deficit in salinity relative to the Indian Ocean source) we suggest that the spatial variations in transport are tied to Agulhas water influx, presumably associated with the eddy field.