HYDROLOGY OF THE SOUTH ATLANTIC zii 



South of the Brazil current and the Agulhas current there is a southward movement 

 at the surface which is probably stronger than the transport of water northwards due to 

 wind currents, and south of these two currents the convergence lies farthest south. In 

 30° W the salinity and temperature distribution show that there is a southward movement 

 below 80 m. : if such a movement also occurs at the surface it is reduced by the effect 

 of the wind. In mid-ocean the convergence is farther north; it is not so far removed 

 from the region of the south-east trade winds, which give rise to a water transport in the 

 surface towards the south-west. 



When the sub-tropical convergence is crossed, and especially when it is crossed 

 obliquely, it has been noticed that there are large areas of sub-tropical water cut off by 

 sub-Antarctic water, and the ship passes through patches of sub-tropical and sub- 

 Antarctic water alternately. When a station is made in such a patch of sub-tropical 

 water, it is found to be lying above sub-Antarctic water in a layer about 200 m. deep, 

 but when the patches of sub-Antarctic water are examined they show the changes of 

 temperature and salinity with depth which are typical of the sub-Antarctic Zone. It is 

 therefore the sub-tropical water which pushes southwards over the convergence to cause 

 these isolated patches of water. These patches are formed so extensively south of the 

 Brazil current and the Agulhas current that it is impossible to fix the convergence 

 definitely; but an attempt has been made in Fig. 11 to show the convergence as the 

 northern extent of sub-Antarctic water. South of the Cape of Good Hope it is most 

 likely that sub-Antarctic water will not be encountered north of the position shown, but 

 south of the Brazil current there is not enough information to be certain even of this. 

 The latitude of the convergence between 50° W and 30° E, based on our own observa- 

 tions, is given in the following table. 



Table IV 



Longitude 5°° W 45° W 40° W^ 35° W 30° W^ 25° W^ 



Latitude of sub-tropical convergence 44° 30' S 43° 30' S 41° 30' S 40° S 40° 30' S 39° 30' S 



Longitude 20° W 15° W 10° W 5° W 0° 5° E 



Latitude of sub-tropical convergence 38° 30' S 37° 30' S 37° S 37° S 37° S 37° 30' S 



Longitude 10° E 15° E 16° E 20° E 25° E ^ 30° E 



Latitude of sub-tropical convergence 37° 30' S 37° 30' S 37° 30' S 43° S 43° 30' S 44° S 



41° S 



42° 30' S 



When a station is made near the convergence, the salinity and temperature of the 

 water must be used to decide whether the water is sub-tropical or sub-Antarctic, or how 

 much the two waters are mixed. 



The sub-Antarctic Zone includes Cape Horn, and the Falkland Islands, and as much 

 of the Patagonian coast as is influenced by the Falkland current which flows as far north 

 as the River Plate. Gough Island is sub-Antarctic, and Tristan da Cunha is just sub- 

 Antarctic. The Cape of Good Hope is north of the sub-tropical convergence, but the 

 cold water off the south-west coast of Africa is water which has upwelled from the Ant- 

 arctic intermediate layer into which sub-Antarctic water sinks. The conditions off the 



