TROPICAL ATLANTIC WATER WITHIN THE BENGUELA UPWELLING SYSTEM AT 27-DEGREES-S

A CTD-O-2 and ADCP section across the African Atlantic continental mar gin near 27 degrees S, obtained during R.R.S. Discovery cruise 165B in May 1987, reveals the water mass structure and associated velocity fi eld of the shelf and upper slope of the Benguela upwelling system. Con tinental shelf water upwelling within the Benguela Current is drawn fr om the 12 degrees C (about 200 m) level. The upwelling water is drawn from oxygen depleted, tropical South Atlantic thermocline water that i s advected along the shelf floor by a southward flowing subsurface cur rent. Lower thermocline and intermediate water from the tropical South Atlantic are also observed flowing southward over the continental slo pe. Tropical Atlantic water generally resides north of the Angola-Beng uela Front at 16 degrees S. A narrow band of upwelled water is observe d well seaward of the shelf, along the western edge of a large Agulhas eddy, indicating that Agulhas eddies play a role in stirring eastern boundary upwelled water into the ocean interior. These eddies also dra w into the interior tropical Atlantic water found over the upper conti nental slope. The net transport between the 120 and 350 isobaths as me asured by the ship-mounted ADCP, referenced to the sea floor, is 0.9 x 10(6) m(3) s(-1) to the south, with 1.6 x 10(6) m(3) s(-1) of southwa rd Rowing tropical Atlantic water and 0.7 x 10(6) m(3) s(-1) of northw ard Bowing upwelled surface water. The tropical thermocline water mass advected to the south is not observed offshore within the northward f lowing Benguela Current, in an unaltered state, thus the 0.9 x 10(6) m (3) s(-1) must feed shelf upwelling south of 27 degrees S, implying a net offshore flux of upwelled water between Luderitz (26 degrees) and Cape Columbine (33 degrees S).