On the interaction between the subtropical gyre and the Subtropical Cell on the shelf of the SE Atlantic
The interaction of the subtropical gyre and the Subtropical Cell was studied with emphasis on the Benguela upwelling system by observations of sea surface height, surface wind vectors, and precipitation. Additionally, current profiles, drifter data, and hydrographic data completed the data set. The subtropical gyre of the South Atlantic is governed by a Sverdrup balance between the curl of the wind stress and the planetary vorticity. The coastal and shelf edge upwelling of cold nutrient enriched water in the Benguela are driven by the south east trade and the negative curl of the wind stress concentrated at the shelf edge of the Benguela, respectively. The coastal upwelling is maintained by wind fluctuations in the coastal wave guide with time scales of less than 10 days since coastal upwelling is stopped and the associated jets are arrested by coastal Kelvin waves. The shelf edge upwelling is maintained by wind stress curl fluctuations with time scales less than a few months since it is controlled by propagating long baroclinic Rossby waves. The compensation current of the Ekman offshore current ventilates the bottom water on the shelf of the Benguela more efficient in austral winter than in summer with well oxygenated Eastern South Atlantic Central Water which is advected by the Benguela current along the shelf edge. Wind stress curl localised just north of the Angola-Benguela front forces the Angola Dome which disperses into the Angola Gyre by long baroclinic Rossby waves. The eastern part of the Angola Gyre, as part of the southern Subtropical Cell in the South Atlantic, is governed by Sverdup balance while its western part is modified by heat accumulated by upwelled water along its path within the South Equatorial Current. The corresponding sea level elevation terminates the gyre at about 20°W and forms a ridge at whose southern edge the South Equatorial Counter Current develops. The upwelling in the Angola Gyre lifts its thermocline water into the euphotic zone causing high biologic productivity. The degradation of sinking plankton implies a suboxic thermocline water mass in the centre of the gyre with high nutrient concentration. Interaction between the subtropical gyre and the Subtropical Cell in the South Atlantic occurs on the West African shelf by poleward outbreaks of the Angola Current through the Angola-Benguela front carrying suboxic South Atlantic Central Water with high nutrient concentration into the northern Benguela. These outbreaks maintain the nutrient balance of the northern Benguela and are controlled by variations of the meridional wind stress in the Cape Frio cell and by elevated coastal sea level north of the Angola-Benguela front. Coastal sea level is elevated in September to November by an equatorial Kelvin wave which is triggered by relaxation of zonal wind stress in the equatorial wave belt of the western Atlantic and in February to April by freshwater surplus due to precipitation in the eastern tropical Atlantic and river discharge at the African coast. The coincidence of outbreaks of the Angola current onto the shelf of the northern Benguela with weakened ventilation of the bottom water in austral summer support the formation of anoxic bottom water due to diffusion of hydrogen sulphide from the mud belt into the water column during austral summer.