286 DEACON [('HAT. 12 



must eventually be the source of most of the Antarctic bottom water. North- 

 east of the Ross Sea the divergence occurs in about 65° to 70°S, and the cyclonic 

 movement is on a much smaller scale. More observations close to the continent 

 may show similar eddying deep movements during periods of strong east winds. 



B. Antarctic Bottom. Water 



It is evident from meridional sections of temperature and salinity distribu- 

 tion that the cold water found at the bottom of much of the Atlantic, Indian 

 and Pacific Oceans comes from the Antarctic, but it is apparently not formed 

 all round the continent. It must be formed by the cooling of the warm deep 

 water, which is the only general southward movement towards the continent, 

 but the cooling is of a special kind. The bottom water is 1° or 2°C colder than 

 the deep water, but only 0-05 parts per thousand less saline. The deep water is, 

 therefore, cooled strongly, but diluted very little. As far as we know it is never 

 exposed at the surface, and there are only very few places where the cold, less 

 saline water always found above it becomes saline enough to cool it sufficiently 

 by mixing without diluting it to a salinity less than that of the bottom water. 



Using observations made some 30 years ago in the Weddell Sea by the Deutsch- 

 land, Norwegia and Discovery II, Fofonoff (1956) has shown that the tempera- 

 ture and salinity of the much weakened deep current can be taken as 0.6°C 

 and 34.70% o . Taking account of the changes in volume and density on mixing, 

 he shows that water cooled to freezing point must have a salinity greater than 

 34.51 % before it can form mixtures with the deep water that are heavier than 

 the deep water itself. As soon as freezing water round the coast of Antarctica 

 reaches such a high salinity, it will, therefore, be likely to form mixtures with 

 the deep water that will be heavy enough to sink down the continental slope 

 and feed the bottom layer. Fofonoff further showed that if the freezing water 

 reaches a salinity as high as 34.63% , it will already be heavier than any mixture 

 it could form with the deep water and will sink directly down the continental 

 slope if not confined in a depression on the shelf. Water with a salinity less than 

 34.51 % , even though it is cooled to its freezing point, cannot form mixtures 

 heavier than the deep water and must float above it in the Antarctic surface 

 layer. 



Most of the Antarctic bottom water appears to be formed in the Weddell 

 Sea. The surface water in the Antarctic coastal current is warmed and diluted 

 in summer till its temperature is well above freezing point and its salinity less 

 than 34% . In winter it is cooled, and its salinity rises as ice separates out, 

 drainage from the land ceases, snow lies unmelted above the sea-ice, the cold 

 air over relatively warm water leads to increased evaporation, and vertical 

 mixing is easier because the density gradients are weaker. Within a month 

 after midsummer the salinity of the coastal water flowing westwards in about 

 70°S across the Greenwich meridian is already greater than 34% and rising 

 rapidly. On the broad continental shelf farther south and west in the Weddell 

 Sea it feels the full effect of winter and reaches salinities of 34.75% or more. 

 There are no observations in the southernmost part of the sea in winter, but 



