An agulhas ring in the South Atlantic ocean and its interaction with the Benguela upwelling frontal system

Abstract

An Agulhas ring was detected using XBT probes on a cruise between Cape Town and Vema Seamount in the south-east Atlantic Ocean in April 1989. CTD and nutrient data, collected on a second cruise in May 1989, GEOSAT altimeter data for February–April 1989 and cloud-free NOAA-11 satellite imagery from June 1989 were used to characterize the ring. The ring was elliptical (330 km E-W and 165 km N-S, relative to the 16°C isotherm at 200 m depth), evident to at least 1200 m, and centred on 30.5°S, 9.2°E in May, about 700 km west of the Orange River. Its drift velocity was 6.4 ± 1 cm s −1 to the NW. Maximum anticyclonic geostrophic surface currents near its edge were 55 cm s −1 relative to 1150 db. The available potential energy was estimated to be 38.8 × 10 15 J and the kinetic energy 2.3 × 10 15 J using a two layer model of the ring. A cool filament extending 450 km offshore from the Benguela upwelling front was identified in the hydrography and the NOAA-11 imagery. Entrainment velocities (maximum of 75 cm s −1) of mature upwelled water from the Benguela frontal region were inferred from feature tracking.