HYDROLOGY OF THE SOUTH ATLANTIC 207 



because of the vigorous mixing which takes place in the turbulent movements of the 

 drift currents. The total movement in the stratum will be the resultant of the pure 

 wind drift current and the deep current; and at the surface, it is shown by all the 

 available current charts to be a little north of east. 



The winds of the Atlantic Ocean south of 40' S are described by Lieut. R. A.B. Ardley, 

 R.N.R., in the Appendix to this report. The resultant strength of the westerly winds— 

 their average strength multiplied by their frequency — in different latitudes is shown 

 in Table XII, column 4 (p. 236). This resultant is greatest between 45 and 50° S, and 

 the percentage of easterly winds is also least between these latitudes. The surface 

 transport towards the north due to wind influence will therefore be greatest between 

 45 and 50° S, and it will decrease towards the north. Because of this there will be a 

 tendency for the surface water to sink towards the north, and also, in the south of the 

 zone, for water to well up towards the surface. Below the surface stratum the move- 

 ment in the layer is towards the east in a deep current which is the result of the westerly 

 wind. But at the same time there are north and south components of movement, and 

 these are responsible for the maintenance of the temperature and salinity distributions 

 which are typical of the layer. 



On the western side of the South Atlantic, and particularly outside a region of intense 

 mixing, which reaches about 100 miles north of the Antarctic convergence, there is an 

 increase in salinity with depth below about 80 m. Sometimes the temperature also 

 increases, or there is a decrease in the temperature-depth gradient. A stratum of water 

 of greater salinity is found between 80 and 100 m. at St. 668, in 46° 43' S ; and between 

 100 and 150 m. at St. 671 in 43° 08' S. Such a stratum can have a continued existence 

 between waters of lesser salinity only if the water in it has a component of movement 

 towards the south, at least relative to the waters above and below it; but it is not quite 

 certain how such a component can be explained. It may be a return current to com- 

 pensate for the flow of surface water towards the north in the drift current. This ex- 

 planation appears the more likely to be valid because in the South Atlantic Ocean 

 north of 45"' S the amount of northerly transport decreases towards the north, so that 

 the surface water must sink towards the north. Within 100 miles of the Antarctic con- 

 vergence there is not so much evidence of this southward movement below 80 m. ; this 

 may be due to the greater vertical mixing, or to a falling oft' towards the south of the 

 northerly transport which might cause the water of the return current to well up towards 

 the surface. On the western side of the South Atlantic, however, the stratum between 

 80 and 200 m. is more remarkable for its high salinity than its high temperature, and 

 it also has a low oxygen content. If therefore it is composed of water which has sunk 

 below the surface, it must be mixed with water from deeper sources. Just south of 

 the sub-tropical convergence it contains sub-tropical water, and the last traces of this 

 may reach to within 100-150 miles of the Antarctic convergence. 



The presence of a southerly component can also be explained as the result of a small 

 thermohaline circulation, confined to the surface 200 m. of sub-Antarctic water and 

 due to the difference in temperature between the climate in the south and north of the 



