io DISCOVERY REPORTS 



depth of 80-150 m. with a temperature of - 1 -8 to - 1-9° C. and a salinity of 34-4 to 

 34'5 °/oo- O ur observations made in January show that the cold stratum still had a 

 temperature of — 1-52 to — i-86° C. and a salinity of 34-34 to 34-50 °/ 00 . 



At the surface, however, the conditions in summer are very different from those of 

 winter, though the changes are confined for the most part to a very shallow stratum, 

 which has a very low salinity and a temperature 0-3 to o-6° C. higher than that of the 

 cold stratum. South of 6o° S in section 5 the salinity of the surface water varied from 

 32-82 to 33-82 °/ 00 , and the temperature from - 0-76 to - 1-55° C. The observations at 

 St. 816 show how shallow a stratum is affected. The salinity of the water down to a 

 depth of 10 m. was only 33-25 °/ 00 , whilst that of the water at 20 m. was as much as 

 34-33 °/ o0 . An examination of all the data south of 6o° S in the section shows that the 

 water at a depth of 40 m. had in the summer an average salinity only o-i °/ 00 below that 

 of the water in the cold stratum, but between 10 and 40 m. the average difference was 

 about o-8 °/oo- 



A similar sharply defined surface stratum is nearly always found in summer near the 

 Antarctic Continent and Antarctic islands situated in high latitudes. In such regions 

 there is usually an abundant supply of fresh water, which because of its low density 

 floats above the colder highly saline water in a shallow stratum. The winds in the high 

 latitudes are also generally less strong than those farther north, and the surface water is 

 often protected to some extent from even their relatively small disturbing influence by 

 a covering of drift-ice. Farther north, where the winds are stronger and the seas less 

 sheltered, the poorly saline water is more evenly distributed through a deeper stratum. 

 Owing to their stability the poorly saline strata do not readily pass on to the deeper water 

 the heat which they receive as radiation from the sun ; on the contrary, their rising tem- 

 perature increases their stability and prevents vertical mixing. The continuous records 

 of the temperature at a depth of 2-3 m., obtained while cruising in high latitudes in 

 summer, show that the water close to drifting ice may be as much as 4 C. warmer than 

 the water some distance away from it. Such large differences have, however, only been 

 found on very calm sunny days, and their absence on the succeeding stormy days shows 

 that they are only short-lived. 



THE MOVEMENTS OF ANTARCTIC SURFACE WATER 



The principal movements of the Antarctic surface water are towards the east in 

 latitudes north of 65 S, towards the west farther south, and a general northward move- 

 ment, stronger in some meridians than in others, throughout the whole of the zone. 

 The movements have two chief causes ; the eastward and westward currents appear to be 

 caused primarily by the effect of the prevailing winds, but the northward movement, 

 though partly the effect of the wind, seems to be due mainly to the influence of the 

 cold Antarctic climate on the density distribution. 



The action of the winds on the sea cannot be examined very closely, but their in- 

 fluence on the surface water is fairly well known and there are some indications as to 

 what happens in the deep water. Ekman (1928) has shown that in a homogeneous sea 



