HYDROLOGY OF THE SOUTH ATLANTIC 193 



is part of the drift towards the east in the region of westerly winds. In the Falkland 

 Sector this subdivision separates regions affected by Weddell Sea water and Bellings- 

 hausen Sea water. Over the whole zone, however, the layer of Antarctic surface water 

 is continuous: it is deepest in the north and shallowest in the south. 



There is already considerable evidence of the importance of the Antarctic convergence 

 as a boundary in the distribution of marine plankton. There are some species which are 

 confined to waters to the south of it and others which are found only north of it. At least 

 one species has been shown to have different broods north and south of the convergence, 

 with only such mixing between the two as can be explained by some of the Antarctic 

 brood being carried northwards in the sinking Antarctic water, and some of the sub- 

 Antarctic brood being carried southwards in deeper water. The Antarctic convergence 

 is probably the extreme northerly limit of pack-ice ; but pack is rarely found so far north. 

 Ice will stop at the convergence because there is a smaller movement northwards in 

 sub-Antarctic water than there is in Antarctic water, and because of the sudden increase 

 of temperature. 



Judged from a hydrological standpoint the Falkland Island Dependencies are all 

 Antarctic, although the Falkland Islands themselves are sub-Antarctic. Farther east 

 Bouvet Island, Heard Island, and MacDonald Island are Antarctic. Kerguelen lies just 

 on the convergence and there is a mixture of Antarctic surface water and sub-Antarctic 

 water near it. Marion and Prince Edward Islands, the Crozets and Possession Island, 

 are just north of the convergence, but so close to it that the Antarctic surface water has 

 not had time to sink far below the surface. When water upwells, as it will do particularly 

 on the north side of the islands, their hydrological conditions and marine life will be 

 influenced by Antarctic water, as well as by sub-Antarctic water. 



DEPTH OF THE ANTARCTIC SURFACE LAYER 



It has already been mentioned that between the Antarctic surface layer and the warm 

 deep water there is a well-defined discontinuity in the change of temperature and salinity 

 with depth. This indicates the presence of well-defined laj'ering which hinders vertical 

 mixing, but there must be a small amount of mixing taking place, especially in winter. 

 The level at which the temperature, salinity and density change most rapidly with depth 

 can be considered the boundary between the two layers. It has been found that the 

 changes are greatest about 40 m. below the depth of the minimum temperature of the 

 cold stratum of the Antarctic surface layer, and it is also at about this depth that the 

 temperature is the mean of the minimum of the cold stratum and the maximum of 

 the warm deep water. It is in this way that the depth of the layer has been measured. 



The depth of the layer varies with the speed and direction of the surface currents . The 

 influence of the earth's rotation on the layers of equal density in a current is to make them 

 slope downwards to the left of the current in the southern hemisphere. The depth of the 

 discontinuity is therefore greater on the left-hand side of a current, and less on the right 

 and for this reason the depth of the layer changes within the zone. The effect is greatest in 

 the neighbourhood of land masses because the layers slope more steeply when the inflow 



