194 DISCOVERY REPORTS 



of water laterally into the current is prevented. In the centre of a cyclonic movement, 

 clockwise in the southern hemisphere, the layer is shallowest and the deep water nearest 

 to the surface. In the centre of an anticyclonic movement the layer will be deepest. 



Besides the variation in depth caused by the difference in the surface currents there 

 is a general increase from south to north from about lOO to 250 m., and this is probably 

 explained by the relative densities of the surface and deep layer and their movements. 

 Although the Antarctic surface water sinks suddenly at the convergence there is a 

 gradual sinking from south to north in the layer and also a rise in the deep water below 

 it in the opposite direction. There is also some variation in the depth of the layer as a 

 result of internal waves in the sea which have been shown to give a small vertical 

 oscillation to layers of equal density. 



TEMPERATURE AND SALINITY OF THE ANTARCTIC SURFACE WATER 



An examination of salinity sections, such as Fig. 4, which have been made near the 

 edge of the ice or near to the land in summer, shows how great is the influence of 

 additions of fresh water to the layer. At St. 587 (Fig. 4) there is a low salinity of 32-72700 

 at the surface, and still lower values are occasionally obtained closer to ice or land. At 

 most stations made far south in summer there is a surface stratum 20 or 30 m. thick 

 which contains such water with a very low salinity. A sharp discontinuity in the change 

 of salinity with depth separates this stratum from the rest of the Antarctic surface layer 

 below it, in which the conditions are not much different from those in winter. Later in 

 the season, or farther north, the sharp discontinuity becomes destroyed by vertical 

 mixing and the change in salinity from the surface stratum to the colder stratum is more 

 gradual. 



In the south, at least in summer, the winds are not so strong as they are farther north 

 and the low salinity stratum is stable. Because of its stability and the absence of vertical 

 mixing between it and the water below, it does not pass on to the deeper water the heat 

 which it receives by radiation and conduction at the sea surface. The temperature of the 

 stratum therefore rises rapidly and a discontinuity also appears in the change of tem- 

 perature with depth — a factor which still further increases the stability of the stratum. 

 As water is added to the stratum it will flow away towards the north rather than sink : 

 a stream will flow away from the ice, or in the direction of the surface current which is 

 maintained by other additional influences. 



On a very fine day, and especially in sheltered waters, a discontinuity may appear in the 

 surface layer due to the effect of temperature alone. Such a discontinuity is illustrated 

 by observations made at St. 606. At this station the temperature decreased rapidly 

 from 3*30° C. at the surface to i"84° C. at a depth of 5 m., and to i-oo° C. at 10 m. 

 The salinity at 5 m., 33*89 °/„„, was 0-02° j^^ greater than that at 10 m., but o-qi °j^^ less 

 than the surface salinity, which had probably been increased by evaporation. Owing 

 to the almost complete absorption of the sun's radiation near the surface there was a 

 difference of 2-30° C. in temperature, and 0-19 in a, , between the water at the surface and 

 that at a depth of 10 m., whilst the salinity was almost the same. Shallow discon- 



