Tropical atlantic water within the Benguela upwelling system at 27°S

Abstract

A CTD-O2 and ADCP section across the African Atlantic continental margin near 27°S, obtained during R.R.S. Discovery cruise 165B in May 1987, reveals the water mass structure and associated velocity field of the shelf and upper slope of the Benguela upwelling system. Continental shelf water upwelling within the Benguela Current is drawn from the 12°C (about 200 m) level. The upwelling water is drawn from oxygen depleted, tropical South Atlantic thermocline water that is advected along the shelf floor by a southward flowing subsurface current. Lower thermocline and intermediate water from the tropical South Atlantic are also observed flowing southward over the continental slope. Tropical Atlantic water generally resides north of the Angola-Benguela Front at 16°S. A narrow band of upwelled water is observed 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 draw into the interior tropical Atlantic water found over the upper continental slope. The net transport between the 120 and 350 isobaths as measured by the ship-mounted ADCP, referenced to the sea floor, is 0.9 × 106 m3 s−1 1 to the south, with 1.6 × 106 m3 s−1 of southward flowing tropical Atlantic water and 0.7 × 106 m3 s−1 of northward flowing upwelled surface water. The tropical thermocline water mass advected to the south is not observed offshore within the northward flowing Benguela Current, in an unaltered state, thus the 0.9 × 106 m3 s−1 must feed shelf upwelling south of 27°S, implying a net offshore flux of upwelled water between Ld¨eritz (26°) and Cape Columbine (33°S).