ANTARCTIC SURFACE WATER: MOVEMENTS 13 



bottom current similar to the Antarctic current, but another combination of circum- 

 stances, mainly perhaps the strong anticyclonic wind and current systems and the high 

 evaporation in the subtropical part of the North Atlantic Ocean, lead to the sinking of 

 the highly saline North Atlantic deep current which enters the South Atlantic Ocean 

 above the Antarctic bottom current. The thermodynamical compensation for the 

 bottom current is probably effected by this southward movement of North Atlantic deep 

 water into the Antarctic regions instead of by a southward movement at the surface. 



The Antarctic surface water is a mixture of fresh water from melting ice and snow 

 with highly saline water from a warm deep current such as the North Atlantic deep 

 current, and its properties are modified while it is exposed at the surface by freezing 

 in winter and thawing in summer. It has a low temperature, which makes it heavier 

 than the surface waters farther north, but owing to its low salinity it is lighter than 

 the highly saline warm deep water. Vertical sections through the region of westerly 

 winds show that it lies above the warm deep water in a layer whose thickness increases 

 gradually from about 80 m. in the south to 250 m. in the north ; the heaviest water 

 with the lowest temperature and highest salinity lies at the southern end of the sections, 

 and the isosteric surfaces slope gradually downwards to the north. Such a slope of 

 these surfaces may, as described above, be partly due to the influence of the west wind, 

 but it cannot be overlooked that the greater amount of radiation falling on the northern 

 part of the region compared with the southern part will lead to a density distribution 

 with the same features. 



Although the density gradient arising from the differences of climate will tend to 

 make the Antarctic water sink and be replaced by a southward movement at the 

 surface, the Antarctic surface water cannot sink vertically like the Antarctic bottom 

 water because it is not heavy enough to force its way through the highly saline warm 

 deep layer. It can, however, find its way gradually to a deeper level by spreading 

 northwards in the lower part of the surface layer, and farther north, where the warm 

 deep layer rises abruptly towards the south, it is free to sink more steeply. 



In the absence of a wind from the west the sinking of the Antarctic water to the lower 

 part of the layer in the southern part of the zone would tend to set up a southward 

 current at the surface ; there are some indications that such a movement may exist in 

 certain regions (see pp. 8, 36, 37), but generally the current appears to be directed 

 towards the north, the influence of the wind seeming usually to predominate. The 

 combined effects of the wind and thermohaline differences may therefore be supposed to 

 lead to a northward movement in both the upper and lower strata of the surface layer. 

 In summer the northward movement will be assisted by the addition of thaw-water to 

 the layer from melting ice and snow, water which was originally deposited on the sea and 

 neighbouring land in the form of precipitation, or removed from the sea during the 



winter as sea ice. 



South of 65 S the prevailing east wind will set up a drift current which carries the 

 surface water to the west and south. In winter it will be assisted by the sinking of cold 

 heavy surface water in the neighbourhood of the continent. In the Weddell Sea part of 



