4° 



DISCOVERY REPORTS 



data, indicate ; and they do not, like the Antarctic convergence, mark lines along which 

 there are large changes in the structure of the sea. 



THE DEPTH OF THE SURFACE LAYER 



The lower boundary of the Antarctic layer is the level at which the northward move- 

 ment of the surface water changes over to the southward movement in the warm deep 

 water. There is a considerable stratum of mixed water between the two layers ; but they 

 have such a different temperature, salinity, and oxygen content that the discontinuity 

 layer in which these properties change rapidly with depth, is clearly distinguished. The 

 exact depth of the surface layer can only be determined when more is known about the 

 speeds of the two currents and of the friction between them, but the level at which the 



300 m 



400 m 



Fig. io. The temperature-depth curves for series of observations made every 4 hours at St. 461, and the 

 depths of the - 1-5, - i-o, -0-5, o-o and 0-5° C. isotherms after the same intervals. 



temperature, salinity, and oxygen content change most rapidly with depth between the 

 two layers gives an approximate measure of it. 



The depth of the Antarctic layer changes not only from place to place but also from 

 time to time. This variation has been illustrated by the up and down movements of the 

 thermocline between the Antarctic and warm deep waters that are brought to light when 

 observations are repeated at intervals in the same place. The curves in Fig. 10 show the 

 results of observations made at intervals of 4 hours during a whole day in a small area 

 between 56 41' S-56 45' S and 02 21' W-02 24' W (St. 461 A-G, Station List, 

 1932). The ship was not anchored, but the observations were made at points so close 

 together that they can be regarded as made at the same point. 



