HYDROLOGY OF THE SOUTH ATLANTIC 231 



From north to south the temperature at the level of maximum saHnity decreases from 

 3-93° C. in 14° 27' N to 0-67° C. in 57° 36' S, and the saHnity decreases from 34-97 to 

 34-68 °/q^ over the same distance. 



The stream of North Atlantic deep water is most saturated with oxygen north of the 

 Rio Grande ridge, which crosses the West Atlantic basin in about 34° S. It contains 

 most oxygen at a depth of 2000-3000 m., where the water is about 75 per cent saturated 

 and contains from 5 to 5-5 cc. Oo/litre. There is a second stratum of high saturation in 

 the layer between 3000 and 4000 m., and the average saturation of the whole layer is 

 about 70 per cent. 



The oxygen content decreases only slowly towards the south until the water has 

 crossed the Rio Grande ridge. Then the decrease is more rapid, and in the upper 

 stratum the oxygen content falls to 3-8 cc./litre and the saturation to 50 per cent. The 

 rapid decrease south of the ridge is probably due to the greater consumption of oxygen 

 by the greater amount of animal life and oxidizable matter in the water. It may also be 

 due to a decrease in the southerly component of the deep-water movement. 



The deep water in the Antarctic Zone is thus poorly oxygenated compared with the 

 deep water farther north ; it contains most oxygen near the bottom of the layer, where it 

 is enriched by mixing with Antarctic bottom water. The oxygen content of the layer in 

 the Antarctic Zone is shown in Figs. 18 and 19 (p. 204) expressed as a percentage 

 saturation, and in Plate X as cc. OJ litre. 



The seasonal changes in the temperature and salinity of the warm deep water have 

 not yet been worked out, but it has been shown that there are considerable changes 

 whose period is not yet known. There are patches of different temperature and salinity 

 in the North Atlantic deep current as it flows southwards, and the changes in the pro- 

 perties of the deep water at a particular station may be the result of the arrival of these 

 patches at different times. 



ANTARCTIC BOTTOM WATER 



THE ANTARCTIC BOTTOM LAYER, AND THE ORIGIN AND MOVE- 

 MENT OF ANTARCTIC BOTTOM WATER 



Below the warm deep layer there is a colder layer of water, which has its origin in the 

 Antarctic regions. In the Antarctic Zone this layer is not separated from the warm deep 

 layer by a discontinuity in the changes of temperature and salinity with depth, and it is 

 difficult to decide upon the level at which the movement of warm deep water southwards 

 changes to one of Antarctic bottom water northwards. 



The formation of Antarctic bottom water has been explained in various ways. It is 

 clear from its properties, and the level at which it is found, that it must consist of warm 

 deep water which has been cooled without being appreciably diluted. It could be formed 

 if the warm deep water were exposed for some period at the surface, especially in winter ; 

 but such exposure is not known to take place, for the layer is always found to be covered 



by colder and less saline Antarctic surface water. 



8-3 



