94 DISCOVERY REPORTS 



low temperature in the deep layer (sections 5-7 ; Brennecke, 1921, pis. 4-6 ; and Mosby, 

 1934, figs. 1 1— 18) show that the low temperature and salinity of the water are due partly 

 to the upwelling of cold and poorly saline bottom water, but they also suggest that the 

 deep current which flows northwards along the east coast of Graham Land and east- 

 wards in the northern part of the Weddell Sea, mixed with the upwelling bottom water, 

 continues eastwards across the Atlantic Ocean as far as 30 E. As it flows eastwards, 

 particularly in the eastern half of the Atlantic Ocean, the current diminishes in volume, 

 and its temperature and salinity increase : the vertical sections across it and the charts 

 in Figs. 19-22 indicate that the diminution is caused by its gradual sinking towards 

 the north below the Atlantic water, and especially in the terminal region of the current, 

 by mixing with the Atlantic and Indian deep waters. 



The conclusions drawn so far with regard to the movements of the deep water apply 

 chiefly to the more saline deeper part of the layer ; the water in the warmer less saline 

 upper part of the layer may have somewhat different movements. Sverdrup (1933) has 

 suggested that the water in this part of the layer in the Antarctic Zone is formed just 

 north of the Antarctic convergence by the mixing of the water which sinks from the 

 Antarctic surface current with the highly saline deep water, and the current arrows 

 which he has drawn in his sections (figs. 3-23) show that he regards the Antarctic water 

 as the greater component of the mixture. Sverdrup's conclusions were, however, drawn 

 chiefly from an examination of the temperature sections, and it seems to me that in- 

 sufficient weight given to the evidence of the salinity distribution, which he himself 

 recognized as showing a different picture (1933, pp. 150-1), has made him exaggerate 

 the importance of the Antarctic contribution to the southward movement. 



The vertical distribution of salinity north of the Antarctic convergence (see Fig. 13, 

 p. 47) suggests that most of the Antarctic water which leaves the surface continues 

 towards the north, but Sverdrup regards the bending of the isotherms back towards the 

 south round the end of the cold stratum of the Antarctic layer in the temperature section 

 (Fig. 12) as an indication that the surface water bends back towards the south to feed 

 the warmest stratum of the deep layer. The relatively high salinity of the water, even 

 at this level, seems, however, to prove that it contains more water from a southward 

 movement such as the North Atlantic deep current, than surface water. Such a con- 

 clusion also explains the temperature distribution ; the high temperature of the water at 

 the level of maximum temperature is just as likely to be the result of a southward 

 movement in the deep layer as it is to be caused by the sinking of surface water. The 

 small variation in the properties of the upper stratum of the warm deep layer from 

 summer to winter is an argument in favour of this conclusion. The question of seasonal 

 changes in the deep water has not yet been closely examined, but a comparison of the 

 data obtained in winter at Sts. WS 251-310 (Station List, 1930) with observations made 

 in the same region in summer, shows that the upper part of the layer is not appreciably 

 colder in winter. If a large part of the Antarctic water turned back to the south in the 

 upper part of the warm deep layer, from a point just north of the convergence, a large 

 seasonal change would be expected. 



