ANTARCTIC SURFACE WATER: TEMPERATURE AND SALINITY 43 



temperature rises to about i° C. where the convergence is far north, and to o° C. where 

 it is far south. 



In summer the layer has a surface stratum which is warmer and less saline than the 

 deeper water. Near to land or pack-ice the stratum is usually very shallow ; the warm 

 water derived from melting ice and heated by the sun is not well mixed into the main 

 body of the Antarctic water but lies on the surface in warm patches. During a cruise 

 in January 1931, through and along the edge of the pack-ice between 67 and 69 S in 

 the Pacific Ocean, the temperature at a depth of 1-2 m. was found to vary between 

 - 1-9 and + 2-0° C. Farther north, owing to more intensive vertical mixing, the warm 

 summer water is more evenly mixed throughout the Antarctic layer, and there is a deeper 

 though less conspicuous surface stratum. 



Observations made in the neighbourhood of South Georgia show that there is a well- 

 mixed stratum extending to a depth of 55-85 m. ; the changes of temperature and salinity 

 with depth in this stratum are much smaller than those in the deeper water. The homo- 

 geneous nature of the stratum also suggests that it is kept well mixed by the strong west 

 winds ; the mixing may be caused by turbulent drift currents, and the depth of 55-85 m. 

 probably represents the depth of the wind's frictional influence and the depth of these 

 currents. 



The temperature of the surface stratum depends on the strength of the northward 

 current, but in the northern part of the zone it rises in the warmest months to about 

 3-5° C. where the convergence lies in 50 S, and to 2-5° C. where the convergence is 

 in 6o° S. The temperature of the cold stratum in the northern part of the zone is 

 generally 2-3 C. less than that of the surface stratum. The difference may show 

 nothing more than the greater effect of the summer warming on the surface stratum, 

 but there is a further possibility that the low temperature in the cold stratum is partly 

 preserved by a cold northward movement. Near the convergence east of the Falkland 

 Islands and south of Australia the temperature difference between the two strata was 

 large enough to suggest that they belonged to different currents and to contradict the 

 assumption that the surface water had been formed from the deeper water in situ (see pp.8, 

 9, 36, 37). The northward movement in the cold stratum may be caused by the colder 

 and heavier surface water from the southern part of the zone sinking below the warmer 

 and lighter water farther north. This appears to happen at the convergence between the 

 Weddell Sea and Bellingshausen Sea currents, the large difference of temperature be- 

 tween the surface and cold strata just north of the convergence showing that Weddell 

 Sea water sinks into the cold stratum. 



The salinity of the Antarctic water is greatest in winter and least in summer. In 

 winter it is increased by mixing with the highly saline warm deep water and by the salt 

 which is deposited when the surface water freezes, but in summer it is diminished by 

 the large volume of fresh water that drains into the sea from the melting ice and snow. 

 There are probably not enough data to give a reliable picture of the distribution of 

 salinity in the Antarctic Zone in winter, but there are one or two main features that 

 appear to be certain. In the south there is a zone of high salinity from which the salinity 



6-2 



