2o6 DISCOVERY REPORTS 



described later. The high nitrate content of the Antarctic surface layer is probably 

 also maintained by upwelling. 



SUB-ANTARCTIC WATER 



NATURE AND DEPTH OF THE SUB-ANTARCTIC SURFACE LAYER, AND 

 THE ORIGIN AND MOVEMENTS OF SUB-ANTARCTIC WATER 



The surface layer in the sub-Antarctic Zone is a much thicker layer than that in 

 the Antarctic Zone, and it is composed of much warmer water. Its depth increases from 

 south to north, and it is about five times as thick as the Antarctic surface layer. Its sur- 

 face temperature increases from south to north from about 3 to 11-5^ C. in winter, and 

 from 6 to 14-5° C. in summer. It hes entirely within the region of westerly winds, and 

 the water in it flows towards the east in a continuous movement around the Southern 

 Ocean. 



Plate VIII shows the vertical distribution of temperature in the layer along 30° W, 

 where the sub-Antarctic Zone extends from 49° 30' to 40° 30' S. Between these lati- 

 tudes the temperature generally decreases with depth down to a level to which the 

 Antarctic surface water has sunk. This level is shown in Plate VIII by the bends in the 

 isotherms towards the north, which indicate the presence of a cold stratum of water con- 

 tinuous with the cold stratum of the Antarctic surface layer. An Antarctic layer will not, 

 however, be distinguished below sub-Antarctic water, since the two are so mixed that it 

 is not possible to decide where one ends and the other begins. The whole layer will be 

 called the sub-Antarctic surface layer, but it is useful to remember that the water in the 

 bottom of the layer is probably more Antarctic in origin than the rest. 



Below the cold stratum, which contains the remains of the coldest Antarctic water, 

 the temperature rises owing to the presence of a warm deep layer. This is continuous 

 with the warm deep layer which lies below Antarctic surface water, although all the 

 warm deep water in it is not necessarily of the same origin. In the top of the warm 

 deep layer there is a secondary temperature maximum which is maintained by warm 

 water flowing southwards, and at the bottom of the sub- Antarctic layer a secondary 

 temperature minimum maintained by cold water flowing northwards. Between these 

 two levels lies the boundary between the two layers, and it is sufficiently accurate to 

 assume that it lies half way between them. In 30° W the depth of the sub-Antarctic 

 surface layer is about iioo m. in 45° S and 1450 m. in 40° 30' S. 



Plate VIII also shows the vertical distribution of saHnity in 30° W. In the sub- 

 Antarctic Zone the change of salinity with depth is much less rapid than it is in the 

 Antarctic Zone, but there is still a discontinuity between the sub-Antarctic water and 

 the warm deep water. The discontinuity is not so well defined as that south of the 

 Antarctic convergence, and vertical mixing takes place across it to a greater extent, 

 particularly north of 45' S. Between the surface and 60-80 m. there is a transport to 

 the north as a result of the pure wind drift currents, and it is unusual to find any 

 appreciable change of salinity or temperature with depth. The stratum is uniform 



