SUB-ANTARCTIC WATER: SUBSURFACE CURRENT 63 



A salinity section constructed by Sverdrup(i93i,p. 101) based on the Carnegie results 

 points to the existence of a sharp convergence in about 32 S, 109 W, but the data given 

 are too few to give an accurate idea of the water movements to the north and south of it. 



THE SUBSURFACE CURRENT 



The temperature, salinity, and oxygen content of the southward current in the sub- 

 surface stratum of the sub-Antarctic Zone suggest that it is replenished from two main 

 sources, from a subsurface current in the subtropical zone itself, and from sub- 

 Antarctic water which sinks at the convergence ; its properties are intermediate to those 

 of these two types of water. In the northern part of the zone the stratum is generally 

 sharply distinguished from the less saline surface and deep waters, but farther south the 

 contrast is reduced by vertical mixing, and within a zone which extends 100-200 miles 

 north of the Antarctic convergence there is often no sign of a southward current. 



According to Ekman's theory the prevailing west wind will give rise to a slope or 

 convection current towards the east, and the resultant movement in the subsurface 

 stratum should therefore be towards the south-east. 



In the western part of the Atlantic Ocean, in the neighbourhood of the Brazil current, 

 the surface water in the northern part of the sub-Antarctic Zone is frequently mixed 

 with subtropical water and is therefore more saline than the subsurface stratum. This 

 stratum can, however, still be recognized at St. 72 (Station List, 1932) and Deutschland 

 St. 99 (Brennecke, 1921, p. 97), where the highly saline surface water and the subsurface 

 current are still separated by a less saline stratum of sub-Antarctic water. 



East of the Falkland Islands where there are a large number of observations in the 

 southern part of the sub-Antarctic Zone, the subsurface current can generally be 

 traced to within about 100 miles of the Antarctic convergence. At St. WS 69 (Station 

 List, 1929), for example, at this distance from the convergence, the subsurface stratum 

 had a salinity of as much as 34*16 °/ 00 , whilst the surface and intermediate water had 

 only 34-07 and 34-11 °/ 00 . At some of the stations — WS 251, 252 (Station List, 1930), 

 431, 432, 518 and 520 (Station List, 1932) — the subsurface stratum is more saline than 

 the surface water but of the same salinity or even slightly less saline than the deeper 

 water. The relatively high temperature of the stratum distinguishes it, however, from 

 the deep water, and in conjunction with an occasional difference of salinity shows 

 that there is still a subsurface current towards the south. 



In the central and eastern part of the Atlantic Ocean the existence of the subsurface 

 current is plainly indicated by the observations at Sts. 668 and 671 (Fig. 13), Sts. 7-10 

 (Station List, 1929) and WS 437-9 (Station List, 1932). The level of maximum salinity 

 rises towards the south from about 150 to 100 m., suggesting that the current climbs 

 gradually as it approaches the region of lower water temperatures. At St. WS 439, near 

 the subtropical convergence, in 38°27'S, 5 45' E, the subsurface water was as much as 

 °'5 7oo more saline than the surface or intermediate waters. The average difference was, 

 however, much less than this, and in the central part of the zone it was only about 



