178 



DISCOVERY REPORTS 



upwelling deep water. In summer, however, as shown in Fig. 4, the surface salinity is 

 much less than it is in section I. At the surface the minimum salinity is found at St. 587, 

 which is directly in the path of the surface current from pack-ice which was lying to the 

 south-west. The surface stratum has been diluted with fresh water from mehing ice, and 

 by drainage from Antarctic land. The salinity of the water in the cold stratum is still 

 greater than it is at the same depth in section I : it has not been lessened by dilution 

 to anything Hke the same extent. 



The changes which take place in the layer with the approach of summer are also shown 

 by diagrams illustrating the way in which the temperature, salinity, and density of the 

 water change with depth in different 

 seasons of the year. The four curves in 

 Fig. 5 show the change of temperature 

 with depth at a point 50 miles north of 

 Prince Olaf Harbour, South Georgia, in 

 August, November, December, and 

 January. They showthe changes which 

 take place in the Antarctic surface layer 

 not very far from its northern boundary . 

 In August the layer is cold and homo- 

 geneous, a condition which is typical 

 of winter. In November the surface 

 has been warmed, and so to a much 

 lesser extent has the cold stratum. 

 Warming goes on throughout the 

 summer, until in February the surface 

 is S'Q^C. warmer than it is in winter 

 and there has been a corresponding 

 increase of i-3°C. in the cold stratum. 



Fig. 6 shows the changes in salinity 

 with depth at the same stations in 

 August, December, and February. 

 In August the salinity does not alter 



with depth down to 100 m., and the layer has it greatest salinity. In December the 

 salinity is less, but it is still uniform down to 80 m. In February the layer has its least 

 salinity and there is a greater increase in salinity with depth. The changes in density 

 which take place in the layer are important because of their effect on vertical mixing 

 both in the layer itself and between the layer and the warm deep layer below it. Fig. 7 

 shows the vertical distribution of a, at the same stations ^ 



In August the surface layer has its greatest density. In December it is warmer, less 

 saline and lighter, and in February, when the effects of warming and dilution are 



1 o-( = (Sf - i) 1000, where St is the specific gravity of the sea water at t° C. referred to distilled water 

 at 4° C. 



Fig. 6. The change in salinity with depth in 

 the Antarctic Zone. 



