The Agulhas Current — Surface Temperature, 

 Eddies, Water Content 

 June 1979 



The false-color CZCS pigment images (93-1 and 93-2) cover the area 

 from the Breede River in the west to Cape Seal in the east, and extend 

 about 200 km offshore. Off Cape Seal the Agulhas Bank shelf width is 

 about 80 km, widening rapidly westward to about 270 km off Cape Infanta 

 (not shown, west of Breede River). 



The relative difference in surface temperature makes the Agulhas 

 Current very easy to identify in thermal infrared images (93-1). The image 

 shows the main flow of the current in the bottom right hand corner. An 

 eddy with a scale of 100 to 200 km is observed inshore of this flow, 

 enclosing a cold core. There is an indication of an increase in total 

 suspended solids (TSS), perhaps due to the upwelling known to be 

 associated with such a cold core. 



The temperature gradients across the frontal areas are considerable, 

 with changes of 3 to 4° C taking place over a few pixels. The structure 

 within the eddy itself is also apparent, while smaller scale features can be 

 seen developing on the northern front. This may be part of the "energy- 

 cascade" to smaller scale processes, and a means whereby the Agulhas 

 Current surface waters can penetrate onto the shallower bank regions. 



The thermal infrared image does not show whether there is a 

 countercurrent associated with the sheared-off tongue of Agulhas Current 

 water. However, if it is assumed that the main source of TSS is in the 

 coastal rivers, then this image tends to indicate total movement toward the 

 southwest. This would occur by a process of entrainment. 



Cooler surface water is shown on the wider reaches of the bank, with the 

 pronged temperature structure apparently correlating with the pattern of 

 TSS. With the limited sea surface temperature gradients in this area, it is 

 not possible to use these for determining circulation patterns. 



On the other hand, it is known that the Breede River, and probably also 

 the Gourits River, were flowing strongly after general rains in their 

 catchments prior to the image being taken. Other minor contributions 

 would also have come from smaller rivers along the coast. The resultant 

 sediment distribution tends to lie in a band less than 20 km wide all along 

 the coast, giving an indication that alongshore currents dominated the 

 flow structures. 



It is probably the wind that is the primary driving force in these inshore 

 regions, with westerlies generally dominant in winter when the image was 

 taken. An eastward coastal jet would result, though the change in coastline 

 near the Gourits River may affect this flow. Offshore there is a tongue of 

 TSS apparently progressing further east. 



In such an analysis, TSS can be used as a tracer, although to be really 

 useful a series of such images separated by periods of a day or so could 

 provide information on the development of current systems. It is also 

 acknowledged that TSS, whether they be organic or inorganic, play a very 

 important part in chemical and biological processes operating in the shelf 

 areas. A knowledge of the distribution of different TSS (confirmed by sea 

 truth data), their origin and eventual fate would therefore be invaluable in 

 the determination of trophic pathways. Such information would be 

 important to the fishing industry. 



The actual distribution of sediment deposited in such a shelf region 

 could also be investigated with the TSS images. In this area it is known 

 that there is a mud and sediment band parallel to the coast and fairly close 

 inshore, which tends to tie in with the conclusions reached earlier. 



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