HYDROLOGY OF THE SOUTH ATLANTIC 



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342 made in 1928-9: in the diagram there is also a curve which shows the surface 

 temperature changes at Grytviken, in South Georgia. At Grytviken the surface tempera- 

 ture is at a maximum in February and at a minimum in July. In the open sea the 

 maximum and minimum are reached a little later than they are in inshore waters, which 

 react more quickly to seasonal changes : the maximum temperature is probably at the 

 end of February or early in March, and the minimum in August. In summer the tem- 

 perature in the open sea decreases with depth from 2-45° C. at the surface to 0-17° C. 

 at a depth of 105 m. With the approach of winter the surface is cooled, and in August the 



MARCH 



JUNE 



SEPTEMBER DECEMBER 



Fig. 14. The seasonal changes in temperature of Antarctic surface water 50 miles 

 north of Prince Olaf Harbour, South Georgia. 



surface was found to be colder than the rest of the layer. This is no doubt a temporary 

 phenomenon due to the presence of water of slightly less salinity in the surface, but it 

 shows that warming of the surface had not started at the end of August. Apart from the 

 surface 5 m. the layer is homogeneous down to a depth of 80 m., but it begins to lose its 

 homogeneous nature after November. The surface is warmed first, and in early summer 

 the difference in temperature between the water at the surface and at 50 m. is greatest. 

 By midsummer the layer is warmed down to a depth of about 80 m. and then there is a 

 rapid decrease in temperature from 1-89° C. at 80 m. to 0-31° C. at 90 m. The surface 

 80 m. is still almost uniform, because of the great amount of vertical mixing which takes 

 place in it during the turbulent movement of the pure drift currents, caused by the wind. 



