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African Journal of Marine Science

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An upwelling filament North-West of Cape Town, South Africa

G Nelson, AJ Boyd, JJ Agenbag, CM Duncombe

Abstract


Satellite images frequently show a thin filament of water stretching from the Cape Peninsula upwelling cell to beyond the shelf edge north-west of Cape Town. The filament carries nutrients and weakly motile biological
organisms from the shelf zone towards the coastal transition zone, where eddies are observed. In order to probe the dynamics of the filament and its generating mechanism, a cruise was undertaken from 7 to10 February 1996. At that time, there appeared to be a filament in retreat, following an upwelling episode. There were two apparent eddies beyond the shelf edge observed in the satellite-derived SST. The southern feature did not have a clear hydrographic structure, but the northern one did, and it appears to be a semi-permanent feature. The region is forced by a number of oceanic and meteorological boundary conditions, none of which is entirely predictable. One is the sporadic advection of warm water from the Agulhas Bank onto the southern shelf. The cruise took place following such an event. The anticipated shelf-edge jet was greatly diminished and forced inshore. The possible effect of barotropic shelf waves on the configuration of the upwelling tongue and the formation of filaments is discussed.



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