HYDROLOGY OF THE SOUTH ATLANTIC 



201 



is either because the layer is deeper in this particular region in winter, when the surface 

 currents are slower, or because there is a smaller tendency for deep water to upwell. 



Fig. 1 6 shows the seasonal change in density in the surface layer. The Antarctic sur- 

 face water has its least density at about the end of February, and its greatest density in 

 August and September. In summer the density increases with depth, slowly in the 

 surface 50 m., and rapidly below 80 m.; in winter it is almost uniform down to 100 m. 



MARCH 



JUNE 



SEPTEMBER DECEMBER 



26' 80 



2690 



27 00 



27' 10 



27-20 



2730 



27-40 



Fig. 1 6. The seasonal changes in density (a,) of Antarctic surface water 50 miles 

 north of Prince Olaf Harbour, South Georgia. 



It can easily be seen how such uniformity in winter is brought about. In the two 

 previous diagrams it is shown that the surface 50 m. of water is practically uniform, even 

 in summer. Below 50 m. the changes are slightly greater; but the curves in Fig. 16 

 show how closely the water at 80 m. is related to the surface water in its properties, 

 and how great is the difference between the 0-80 m. stratum and the rest of the layer. 

 These facts have led to the assumption that the surface 80 m. is affected by pure drift 



