VERTICAL CIRCULATION IN THE OCEAN 165 



In winter the pure drift current may therefore reach to depths greater than 100 m. In 

 summer the temperature of the surface layers rises and the sahnity decreases because 

 of melting of ice and excess precipitation. The density therefore decreases and at a 

 depth of 40 to 80 m. a sudden increase in the density is developed. In summer the 

 lower limit of the pure drift current is found at this depth. During the summer the 

 temperature and the salinity of the water below 80 m. increase because of admixture 

 of the ascending water of the return current and the deep water. The observations 

 around South Georgia show that such an increase takes place as the season advances 

 and the feature can be explained only by admixture from below since the surface 

 salinity decreases. The ascending motion is, however, so slow that the layer of low 

 temperature is never completely removed in summer, but many stations are found at 

 which only traces of the cold water are present. The layer of cold water may, therefore, 

 be interpreted not as indicating a flow towards the north but as representing evidence 

 of the cooling in the preceding winter. 



We can now summarize our conceptions as to the structure of the circumpolar 

 Antarctic current. Within the Antarctic water we find a vertical circulation which, when 

 looking towards the east, rotates counter-clockwise. The surface layers are carried to- 

 wards the north by the pure drift current. At the Antarctic Convergence part of the 

 water continues towards the north as one constituent of the Antarctic intermediate 

 current, but part is mixed with deep water and returns to the south as the warm 

 intermediate current within the Antarctic Zone. The surface layer of the Antarctic water 

 is very rich in plant and animal life. Within the Antarctic Zone the dead organisms sink 

 ? and are decomposed and, therefore, the water of the intermediate return current shows 

 a high phosphate content. This phosphate is carried back to the south and can begin a 

 new cycle. The low oxygen values within or directly below the intermediate return 

 current indicate that oxidation of organic matter takes place. 



Since only part of the water which flows north returns to the south it follows that the 

 motion of the deep water must also have a component to the south. The active cooling 

 of the water along the Antarctic Continent leads, on the other hand, to a sinking of the 

 water in high southerly latitudes, and the cold bottom water which is formed must flow 

 away from the Continent towards the north. Below the upper circulation we, therefore, 

 get a lower vertical circulation which rotates clockwise when looking towards the east. 

 These vertical circulations are superimposed on the general easterly current, and the 

 resulting motion has the character of complicated spirals. Within these spirals the 

 physical properties of the water are changed in the upper layers because of heating, 

 cooling, evaporation, precipitation or melting of ice, while in the lower layers they are 

 changed by admixture of water from other regions. Few water particles will describe 

 complete spirals. Some of the water, which at the surface moves north, will continue 

 towards the north within the Antarctic intermediate current, and only part will be 

 brought back again to the south by the intermediate return current. The water, which 

 in high southerly latitudes must be drawn to the surface in order to replace what is 

 carried north, consists partly of water from the intermediate return current and partly 



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