190 



DISCOVERY REPORTS 



the island, and the Weddell Sea water east of it. North of the island there is Weddell 

 Sea water with some Bellingshausen Sea water and south of the island a greater propor- 

 tion of Bellingshausen Sea water. 



EXTENT OF THE ANTARCTIC ZONE 



The Antarctic convergence, which is the northern limit of Antarctic surface water at 

 the surface, is usually sharp and well defined. It is distinguished by a sudden change 

 in the surface temperature of the sea when it is crossed. From south to north in winter 

 the temperature increases suddenly from about i to 3-5° C. and in summer from about 

 3 to 5-5° C. The continuous temperature record on the left-hand side of Fig. 10 shows 



Fig. 10. Thermograph records showing the sudden change in temperature at the Antarctic (left) and sub- 

 tropical (right) convergences (vertical scale in ° C). 



a drop of surface temperature from 3-2 to — 0-2' C, registered when the Antarctic 

 convergence was crossed from north to south: it represents a sharp convergence. 

 Occasionally, and especially if the convergence is crossed obliquely, we have found 

 that it is not straight, and the ship passes through cold and warm patches of water 

 alternately. This has been noticed particularly after bad weather, when the speed and 

 direction of the surface pure drift currents will have been varying considerably. It is, 

 however, found that there is always either a sharp convergence or these patches, and 

 never a gradual change from one kind of water to the other. The convergence is usually 

 not ver}^ well defined in the bend to the west in about 50° W, nor in the northerly bend 

 to the north of South Georgia, and in both places there is a tendency for sub-Antarctic 



