24 DISCOVERY REPORTS 



determined by factors which govern the flow of the bottom water, it follows that the 

 same factors fix the position of the Antarctic convergence. 1 



It will be shown in the following section of the report that although the position of the 

 convergence is fixed primarily by the movements of the deep and bottom waters, it may 

 be influenced within narrow limits by the surface currents themselves. In certain 

 regions the water in the lower stratum of the Antarctic layer flows towards the north, 

 and sinks at what would be the usual position of the Antarctic convergence, while the 

 water at the surface flows in the opposite direction, towards the south. Owing to this 

 movement sub- Antarctic water is carried southwards over the Antarctic water, and the 

 convergence is found 100-150 miles to the south of its normal position. 



Since the northward current of Antarctic water sinks as soon as it has passed the 

 latitude where the deep water makes its sudden upward movement, it seems impossible 

 that an increase in the strength of the current can affect the conditions at the surface to 

 the north of this latitude and drive the convergence northwards. 



According to the observations which have been made so far in the Falkland Sector, 

 the position of the convergence does not vary much, probably not more than 60 miles 

 between its extreme positions, and it shows no regular seasonal change. Such facts 

 are in keeping with the conclusion that its position is fixed primarily by the move- 

 ments of the deep and bottom waters, which are themselves only subject to minor 

 variations. 



Further information with regard to the movements of the Antarctic water in the 

 neighbourhood of the convergence will be found in the following section of the report 

 and in the section on sub-Antarctic water. 



THE WEST WIND DRIFT 



In the Drake Passage the Antarctic water flows mainly towards the east. Along the 

 line of section 1, between Elephant Island and the Falkland Islands, the boundary 

 between the Antarctic and sub-Antarctic waters is very well defined. Between 55 20' S 

 and 55 10' S the surface temperature increases from 1-5 to 5-5° C, and the isotherms 

 and isohalines run almost vertically from the surface to a depth of 600 m. Such a sharp 



1 Before arriving at this conclusion I searched for other possible explanations of the close agreement 

 between the surface and deep observations. The agreement might lead conversely to the conclusion that the 

 movements of the bottom water were determined in some way by those of the surface water, or it might 

 suggest that both were regulated by some other factor which governed the movements of the deep water. 

 Neither of these suggestions seems, however, to provide a possible explanation, and a closer examination 

 of them helps to confirm the impression that the bottom water, being the heaviest water in the sea, has an 

 overruling influence. The way in which the surface and deep currents accommodate themselves to the 

 movements of the bottom water when these are determined by the bottom configuration supports this 

 conclusion. 



The sudden slope of the layers of surface, deep, and bottom waters in the neighbourhood of the con- 

 vergence might also be due to the existence of a strong wind towards the east in this locality, with lesser 

 winds to the north and south. There are, however, no such sharp changes in the strength of the wind, and 

 as far as can be decided at present, no agreement between wind and slope. 



