HYDROLOGY OF THE SOUTH ATLANTIC 



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Om 



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greatest, its density is least. Since the density of the layer is greatest in winter and least 

 in summer, the discontinuity between it and the warm deep water (whose density 

 remains fairly constant) is in winter 

 least effective as a hindrance to vertical 

 mixing between the two layers, and in 

 summer most effective. 



The conclusions drawn from the 

 changes in temperature and salinity 

 at these stations near the northern 

 boundary of Antarctic surface water 

 are in general the same as those drawn 

 from sections I and II farther south. 

 The surface of the layer is warmed 

 and diluted with fresh water in sum- 

 mer, and there is still a cold stratum 

 which the conditions are least 



250k 



500m 



in 



750- 



lOOOi 



Fig. 7. The change in density (ctj) with depth 

 in the Antarctic Zone. 



changed from those of winter. The 



results show, however, that there is 



an increase in vertical mixing from 



south to north in the layer. Far south 



there are sudden changes in salinity 



with depth near the surface. Farther 



north these changes are more gradual, 



and although the surface stratum no 



longer has such a low salinity there has been a greater reduction in the salinity of the 



cold stratum. 



ORIGIN OF THE ANTARCTIC SURFACE WATER 



The Antarctic surface layer is bounded in the south by the Antarctic Continent, and 

 in the north by sub-Antarctic water along a convergence which has recently been shown 

 to be continuous throughout the Southern Ocean. North of 66° S the direction of move- 

 ment of the water in the surface of the layer is in general towards the north-east ; south 

 of 66° S it moves towards the west or south-west. The movement north of 66° S can be 

 considered as having two components, one to the east and the other to the north. The 

 easterly component will maintain a continuous current towards the east across the 

 Southern Ocean but the northerly component will move water northwards towards sub- 

 Antarctic water. 



It will be shown later that along the Antarctic convergence, where the two waters 

 meet, Antarctic surface water sinks below sub-Antarctic water, and after a good deal of 

 mixing with it continues to flow northwards as a deep current. This water is lost from 

 the Antarctic surface layer, and must be replaced. 



It has been found that the surface currents in the Antarctic Zone are much more 



