DYNAMICS OF THE SOUTHERN OCEAN 137 



more northerly trend than the isobaths: it seems impossible that the water should 

 circumnavigate the Southern Ocean along the same path as the isobaths and it is more 

 reasonable to suppose that it gradually drifts away to the north. The isobaths seem, 

 however, to serve as a measure of the northward movement, bending northwards where 

 it is strong and southwards where it is weak. 



The 600 decibar surface (Fig. 2) has a topography similar to that of the o surface and 

 the dynamic isobaths of the two are practically parallel. As at the o surface the topo- 

 graphy suggests that the current flows mainly towards the east, but when allowance has 

 been made for various deviations towards the north and south, the resultant trend in the 

 greater part of the ocean is slightly south of east. 



An examination of the temperature and salinity distribution suggests, however, that 

 the topographical chart minimizes the strength of the northward and southward move- 

 ments. In the Antarctic Zone the 600 decibar surface lies within the vertical limits of 

 the warm deep layer between the colder and less saline surface and bottom layers, and 

 such a layer cannot retain its relatively high temperature and salinity unless it is con- 

 stantly reinforced by a movement of warm and highly saline water from the north ; its 

 presence at any point in the Southern Ocean is therefore a certain indication of the 

 existence of a southward movement. 



If the isobaths are regarded as the stream lines of the current they show that the deep 

 water travels round the whole circle of the Southern Ocean without being appreciably 

 diluted by mixing with the colder and less saline surface and bottom waters or reinforced 

 by warm highly saline water from the north, an occurrence which seems most im- 

 probable. Vertical mixing must take place as the current flows onwards between the 

 colder and less saline waters and the temperature and salinity will both decrease in the 

 direction of movement. The isotherms and isohalines will therefore lie at an angle to the 

 direction of movement, partly across the current. The temperature of the warmest 

 stratum of the current, an approximate measure of the heat content, is shown in 

 Fig. 3. The current must have some movement towards the south across the isotherms, 

 and since these are approximately parallel to the isobaths (Fig. 2) it follows that the 

 current must also flow southwards across the isobaths. In the Falkland Sector an un- 

 qualified acceptance of the isobaths as stream lines of the current leads to the con- 

 clusion that the deep water found north of South Georgia, and in 30 W as far north 

 as 46 S is derived, as a result of a current towards the north-east, from the Pacific 

 Ocean (Clowes, 1933), but a survey of the temperature and salinity data shows that a 

 large proportion of it must belong to a southward movement in the Atlantic Ocean 

 (Deacon, 1937, p. 88). 



In the southern part of the sub-Antarctic Zone, north of the 07-0-8 dynamic metre 

 isobaths in Fig. 2, the water at the level of the 600 decibar surface is a mixture of 

 Antarctic and sub-Antarctic waters sinking towards the north to give rise to the Ant- 

 arctic intermediate current ; but the isobaths have no corresponding northward trend. 

 Before condemning the isobaths altogether as inaccurate measures of the current 

 direction it must be remembered that those described have shown the topographies of 



