212 DISCOVERY REPORTS 



south-west coast are therefore partly sub-Antarctic as well as sub-tropical. Off the south 

 and east coasts the water is sub-tropical, and in summer it is almost tropical owing to 

 the influence of the Agulhas current. 



TEMPERATURE AND SALINITY OF THE SUB-ANTARCTIC WATER 



Both the temperature and salinity of the sub-Antarctic water increase towards the 

 north. The increase is greatest between 80 and 200 m., as would be expected if the 

 water in this stratum was flowing southwards as well as eastwards and losing its heat and 

 salt content by mixing. It is next greatest in the surface stratum, in which the water has 

 a component of movement to the north. The changes are least at the level of minimum 

 salinity, where a large volume of water is sinking towards the north ; and they are least 

 because the properties of a large volume do not change readily. At the level of minimum 

 temperature, and at the bottom of the layer, there is a considerable increase in salinity 

 towards the north because of the mixing which takes place with the warm deep layer. 



The surface temperature of sub-Antarctic water just north of the Antarctic converg- 

 ence in the West Atlantic Ocean is about 3 to 3-5° C. in winter and 6° C. in summer. It 

 increases from south to north, slowly in the southern half of the zone and more rapidly 

 in the northern half, where vertical mixing is not so complete. Near the sub-tropical 

 convergence the surface temperature is about 1 1 -5° C. in winter and 14-5° C. in summer. 

 The salinity in the south of the zone varies between about 33-95 °/oo ^t the end of summer 

 and about 34* 10 °/oo ^t the end of winter. The salinity also increases towards the north. 

 Just south of the sub-tropical convergence it has a value of about 34-4700' t)ut it may be 

 much increased south of the Brazil current and south of the Agulhas current, by mixing 

 with sub-tropical water which has crossed the convergence at the surface. In the east 

 of the Atlantic Ocean the surface stratum has a lesser salinity than in the west, probably 

 because of the additions it has received from the Antarctic surface water as it crosses the 

 ocean. 



Between 80 and 200 m. the changes are slightly greater, starting from about the same 

 values just north of the Antarctic convergence. 



The very small changes in the temperature and salinity at the level of minimum 

 salinity are shown in Table XI (p. 223). Between 47° S and the sub-tropical convergence 

 there is only an increase of about 0-04 7oo in salinity and about 0-5° C. in temperature. 



At the level of minimum temperature near the bottom of the layer there is a large 

 increase of salinity towards the north as the water mixes with warm deep water, and 

 there is also a small increase in temperature. Between 47 and 38° S there is an increase 

 in salinity from 34-23 to 34-53 7oo ^^d an increase in temperature from 2-36 to 2-76° C. 

 The increase in temperature is rapid as the Antarctic water sinks below the surface at the 

 convergence ; it then becomes slower and is approximately regular between 47 and 38° S. 



The way in which the temperature and salinity change with depth has already been 

 described. In the south of the zone the difference in temperature between surface water 

 and that at the bottom of the layer is about 3° C. and near the sub-tropical convergence 

 about 12° C. 



