SUB-ANTARCTIC WATER 49 



evident indication of the existence of a southward current and shows that it is colder 

 than the surface water ; the oxygen section suggests the presence of a southward current 

 of water poor in oxygen. 

 I The main volume of sub-Antarctic water, between the subsurface current and the 

 warm deep layer, is clearly indicated in the figures as a large uniform area of low 

 salinity and temperature and high oxygen content. • Its situation in the section and its 

 properties are sufficient evidence that it is formed by the mixing of the Antarctic water 

 which sinks at the Antarctic convergence with water which flows southwards in the 

 subsurface and warm deep currents. 1 Longitudinal sections through each of the three 

 main oceans show that this region of mixing is the birthplace of the Antarctic inter- 

 mediate currents which flow northwards between more saline surface and deep layers 

 as far as and beyond the equator ; the extension of the intermediate currents for such a 

 great distance is conclusive evidence that the main volume of sub-Antarctic water flows 

 northwards. The movement seems to comprehend all the water between the subsurface 

 and warm deep currents, but there are two strata which retain distinctive properties. 



The most prominent stratum is one of minimum salinity ; it is formed just north of 

 the Antarctic convergence by the sinking of the poorly saline water from the surface 

 stratum of the Antarctic water. Below it, the sub-Antarctic mixture contains a greater 

 proportion of water sinking from the more saline cold stratum of the Antarctic layer, 

 and has a greater salinity. Nearer the surface the salinity is also higher owing to the 

 southward movement in the subsurface stratum. The level of minimum salinity is in- 

 dicated in Figs. 12-14 by the line BB. The smallness of the changes of temperature, 

 salinity, and oxygen content from south to north in the stratum suggest that it is the 

 path of a large northward current, which because of the large volume of water it carries 

 is not readily influenced by mixing with the neighbouring water masses. 



Below the stratum of minimum salinity there is one of colder water marked by the 

 lines CC. In 30 W, and throughout the western half of the South Atlantic Ocean, the 

 stratum is one in which the temperature falls to a secondary minimum below warmer 

 less saline sub-Antarctic water and above warmer highly saline deep water. The low 

 temperature suggests that the stratum is a sub-Antarctic mixture which contains a large 

 proportion of the water which sinks from the cold stratum of the Antarctic layer. The 

 path of the current is not so clearly indicated as that of the poorly saline current from 

 the Antarctic surface stratum, and the temperature and salinity sections both suggest 

 that the northward movement of the colder and heavier water is more interrupted by 

 turbulent mixing and eddy movements. 



The observations at St. 668 point to the existence of a southward eddy of sub- 

 Antarctic water at a depth of 800 m., where the temperature rises to a secondary 

 maximum 0-2° C. higher than the temperature at 600 m. Sverdrup (1933, fig. 18) also 

 regards the maximum temperature as evidence of a southward movement, but he further 

 supposes that the movement is much more than an eddy and regards it as the principal 

 origin of the water at the level of maximum temperature at a depth of 400-600 m. in the 

 Antarctic Zone. It seems to me that such a conclusion is not justified, since even the 



