HYDROLOGY OF THE SOUTH ATLANTIC 209 



with it. So complete is the mixing in the region extending 100-150 miles north of the 

 convergence, that it is not often that the minimum saHnity at a depth of 300-400 m. 

 can be detected. In summer, or at the end of summer, vertical mixing is not so com- 

 plete, and a surface stratum is usually formed which is similar to that in the Antarctic 

 Zone but not nearly so well defined. 



In the Drake Passage, where the components of movement perpendicular to the con- 

 vergence are restricted by the land masses to the north and south, neither the minimum 

 salinity nor the minimum temperature can be detected. Near Cape Horn the surface 

 is considerably diluted by coastal water. 



At stations east of the Falkland Islands both the minimum salinity caused by the 

 Antarctic surface water, and also the minimum temperature caused by the cold stratum 

 of Antarctic surface water, are found. The depth of the minimum temperature is shown 

 in Fig. 20. South of the Antarctic convergence it shows the depth of the cold stratum of 

 Antarctic surface water, and north of the convergence the depth to which the water 

 from the cold stratum sinks. The depth of the minimum salinity north of the convergence 

 is also shown ; at this depth there is more water from the surface stratum mixed with 

 sub-Antarctic water. 



In the bend of the Antarctic convergence to the east of the Falkland Islands, and 

 also in the bend north of South Georgia (Fig. 1 1) there appears to be the strongest flow 

 of Antarctic water below sub-Antarctic water and also of sub-Antarctic water eastwards 

 over Antarctic water. In both regions the convergence is never very well defined, and 

 it is the more difficult to fix with accuracy because the rise in surface temperature usually 

 occurs south or east of the position where the Antarctic surface water starts to sink 

 rapidly. A stratum of sub-Antarctic water is often found above a stratum of Antarctic 

 water which is still at its usual depth and conditions in these regions can change 

 suddenly with depth. 



In the eastern half of the South Atlantic, particularly south of the Cape of Good 

 Hope, the movement at right angles to the convergence seems to be restricted, as a result 

 of a general southerly component of movement in sub-Antarctic water. Such a com- 

 ponent is probably caused by the sub-Antarctic water being driven southwards to round 

 the Cape and the southerly extension of sub-tropical water caused by the Agulhas 

 current. As a result the minimum salinity is not found except at a greater distance north 

 of the convergence, and the minimum temperature is not well defined. 



The mixture of water which results from the addition of Antarctic surface water to the 

 sub-Antarctic water moves towards the east, and at the same time sinks towards the 

 north . Its path is shown by the depth of the levels of minimum temperature and minimum 

 salinity which can be followed a long way north in the Atlantic — the minimum salinity 

 as far north as 25° N. This movement will be described in the section on the Antarctic 

 intermediate water. 



In the Northern Half of the Zone there is a surface stratum 60-80 m. thick in 

 which the saHnity and temperature are almost uniform with depth. Below this surface 

 stratum, the salinity increases until it reaches a secondary maximum value between 80 



