1 84 



DISCOVERY REPORTS 



It has been shown by Jeffreys (1925) that if a difference in temperature is maintained 

 over any level surface within, or in contact with a fluid, the fluid will move, and continue 

 to move, until such difference of temperature ceases to exist. From the application of 

 this principle to the South Atlantic Ocean it is evident that there must be a movement 

 of Antarctic water to the north, and of warmer water to the south, in a continuous 

 circulation. 



The circulation is not so simple as that described by Lenz (1847), who imagined a 

 symmetrical circulation in both hemispheres with water sinking near the poles and rising 

 towards the surface near the Equator. It is, on the contrary, the result of heavy Antarctic 

 surface and bottom waters sinking to different levels in the south, and of water of sub- 

 tropical origin sinking into the intermediate level from the North Atlantic. The circula- 

 tion will be discussed in detail in the section of the report dealing with the deep waters. 



LATITUDE 35S 



STATION 675 ' 



40 5 



45°S 



673 



671 



668 



50"S 

 66G 



663 



55"S 



G£l 



lOCM 



20 CM 



Fig. 9. The slope of the o, 300, 600, and looo decibar isobaric surfaces in 30° W. 



For the present it can be seen from Plate VIII that the shape of the isotherms and iso- 

 halines in vertical sections which show the distribution of temperature and saHnity in 

 the South Atlantic Ocean indicates that both Antarctic surface water and sub-Antarctic 

 water sink towards the north. This is also seen in Plates IX and X which give the 

 vertical distribution of phosphate, nitrate, and oxygen content, and it is also indicated 

 by the presence of water farther north in the deep levels which can only have had its 

 origin south of the convergences. Such a movement of Antarctic surface water is ex- 

 plained by Jeffreys' work. Although unable to sink vertically because of the still heavier 

 water which is found below it, it can sink towards the north and still remain above the 

 heavier water. The south to north movements of Antarctic surface water can be explained 

 therefore by the effect of the thermohaline influence. 



The greatest movement in the layer, except in some parts of the Falkland Sector, is, 

 however, probably to the east, north of 66° S, and to the west, south of 66° S, and 

 these movements can only be explained as due to the influence of the prevailing winds. 



