HYDROLOGY OF THE SOUTH ATLANTIC i8i 



facts is explained by a greater mixture of the surface water with the warmer, more 

 saline, and poorly oxygenated deep water. In section II there is shown a great up- 

 welling of deep water at the edge of the continental shelf. The effect of the upwelling 

 on the Antarctic surface layer is shown in the temperature and salinity sections, Figs. 

 3, 4, and also very plainly in Fig. 19 (p. 204), which shows the percentage of oxygen 

 saturation of the water in section II. 



Fig. 17 (p. 203) shows the percentage of oxygen saturation of a surface stratum of 

 water 100 m. thick, in the Falkland Sector. It is a fairly accurate guide to the places 

 where deep water is upwelling. There are objections to its use, but it has been found that 

 where low percentages in the diagram show that poorly oxygenated deep water is up- 

 welling, the upwelling is confirmed by the temperature and salinity data. From the 

 figure it will be seen that the surface layer in the Falkland Sector receives most water 

 from the warm deep layer along the west coast of Graham Land and the South 

 Shetland Islands, and along that part of the ridge known as the Scotia Arc (Herdman, 

 1932, p. 214), which joins Joinville Island to the South Orkney Islands, and the South 

 Sandwich Islands. 



There is also considerable upwelling in other places outside the scope of the diagram, 

 particularly in the centre of the cyclonic current system in the Weddell Sea. In 

 winter the effect of addition of water from the warm deep layer to the Antarctic surface 

 layer is partly the cause of the increasing salinity of the surface layer. In summer its 

 effect is exceeded by that of additions of fresh water, and the salinity of the surface 

 layer decreases. The relative importance of the two sources and their efl'ect on the 

 nutritive conditions in the surface layer has yet to be worked out. 



The heavy precipitation in the Antarctic Zone is a third source of Antarctic surface 

 water. G. Schott (1926) shows that the annual precipitation in the West Atlantic Ocean 

 between 50 and 60° S is greater than 1000 mm. per annum, and this amount is about 

 700 mm. per annum greater than the amount of evaporation. A fourth influence, which 

 increases the salinity of the layer in winter, is the deposition of salt when sea ice is 

 formed. Neither of these last two factors is negligible and their importance is being 

 investigated. 



The sources of Antarctic surface water can be summarized as follows : 



(i) Fresh water from melting ice and snow. 



(2) Warm water of high salinity from the warm deep layer. 



(3) Fresh water from the excess of the precipitation over evaporation, partly allowed 

 for in (i). 



(4) Salt which is deposited when sea ice is formed. 



There are also several influences directly due to the Antarctic climate which are 

 necessary to maintain the layer. These are (i) conduction and radiation of heat into 

 the layer in summer, and out of it in winter, and (ii) strong vertical mixing due to 

 turbulent movement under the influence of strong winds. It is important to remember 

 that north of 66^ S the water in the layer is part of a continuous movement towards the 

 east, and that to the south of this latitude its movement has a westerly component. 



