The Benguela Current — Coastal Upwelling 

 February 1979 



The false-color CZCS pigment image on 19 February (89-1) shows the 

 distribution of chlorophyll off the west coast of South Africa. The 

 chlorophyll concentrations range from less than 0. 1 mg m-^ to over 

 10.0 mg m-3. Clearly shown is the extent of coastal waters with a cool 

 surface layer and chlorophyll concentrations exceeding 0.1 mg m"' 

 separated from the warmer, chlorophyll-poor oceanic water by a well- 

 defined front. The corresponding thermal infrared image (89-2) shows the 

 surface temperature distribution and confirms that the thermal and color 

 fronts coincide. The thermal infrared image temperature range is ±12° C 

 (dark blue) to ±20° C (red). 



There are three areas of high chlorophyll concentrations within the 

 coastal waters (89-1). The first area of high chlorophyll, off the Cape 

 Peninsula, is separated from the land by a strip of very cold, freshly 

 upwelled water (89-2) containing comparatively low concentrations of 

 chlorophyll. This cold surface water is forced offshore fairly rapidly by the 

 continued upwelling; and the plankton, needing time to grow, achieves its 

 maximum concentration some distance offshore. Growth is also 

 accelerated by the warming of the surface water as it moves offshore. 



The second area of high concentration, off Saldanha Bay, is most likely 

 the result of the same wind event, but because of the less rugged 

 topography along the coast, the upwelling is not as intense and hence 

 chlorophyll has had time to develop alongside the coast without being 

 inhibited by a strong flow of very cold upwelled water, as is the case off the 

 Cape Peninsula. 



The third high-chlorophyll area in the north is the southern extremity of 

 the quasi-stationary "straight-coast" upwelling which extends northward 

 along the entire west coast of southern Africa. 



Other features of note are the meanders in the front, with a large 

 cyclonic eddy west of Saldanha Bay, probably the result of a breakdown in 

 the front following intensification of the front by a previous upwelling 

 event. Such intensification is evident west of the peninsula where there are 

 signs that the meander in that area could develop into an eddy at some 

 later stage. 



North of Cape Columbine, the complex flow regime in the area is clearly 

 illustrated by the chlorophyll concentration patterns which confirm the 

 extreme patchiness of the chlorophyll distribution. 



The patch of high chlorophyll in St. Helena Bay could be evidence of a 

 cyclonic eddy entraining high chlorophyll concentrations from the 

 northern extension of the Saldanha Bay upwelling cell, or it could be a 

 resuh of the input of nutrients from the Berg River which runs into the St. 

 Helena Bay at about the point where the plankton "plume" touches the 

 land. 



Both the thermal infrared and the pigment images show very clearly the 

 expected north to northwesterly flow of surface water away from the coast 

 under the influence of the prevailing southeasterly wind that maintains the 

 intense offshore oceanic front during the summer months. Winter imagery 

 available does not show evidence of a continuous, intense oceanic front. 



Although the CZCS does not provide data on the vertical distribution of 

 chlorophyll in the water columns, it does provide a major step forward in 

 the study of upwelling regions such as that of the southern Benguela 

 region, and could play a role in fisheries management if imagery was 

 routinely available which could be processed to provide a quantitative 

 assessment of primary production. 



