HYDROLOGY OF THE SOUTH ATLANTIC 217 



north until, in about 28° S (when the temperature at the surface is 23° C), the increase 

 becomes rapid, and another surface layer of water can be distinguished. 



This second layer lies above sub-tropical water, and is separated from it by another 

 discontinuity of temperature and salinity, forming so sharp a density gradient that vertical 

 mixing between the two layers is almost entirely prevented. The surface layer has been 

 called tropical water. It is almost depleted of its dissolved phosphate and nitrate ; and 

 they cannot be renewed, except perhaps very slowly, because the sharpness of the dis- 

 continuity makes vertical mixing across it almost impossible. The mean temperature 

 in the discontinuity between the tropical water and the sub-tropical water is 23° C, 

 and the depth of this isotherm has been assumed to be the bottom of the tropical 

 water. 



The sub-tropical water which lies below tropical water has been called sub-tropical 

 under-water, and the layer the sub-tropical under-layer, to distinguish it from sub- 

 tropical surface water and the sub-tropical surface layer. The name sub-tropical under- 

 water has already been used by Wiist (1928, p. 514) for water with approximately the 

 same limits of temperature and salinity. The mean temperature in the lower discon- 

 tinuity which separates sub-tropical surface and under-waters from Antarctic inter- 

 mediate water is about 10-5" C, and the mean sahnity 34-85 to 35-0 °/o„. The depth of 

 this temperature and salinity has been assumed to be the depth of the sub-tropical 

 waters. Its depth along 30° W is shown in the following table. 



Table V 



Latitude 



Depth of sub-tropical waters in metres 



The slope of the discontinuity in a north and south direction, follows approximately 

 the slope of the isobaric surfaces and from it the direction of movement of the water in 

 the sub-tropical layer can be obtained. 



Between 38 and 30° S sub-tropical water moves eastwards under the influence of the 

 westerly winds, and the discontinuity slopes downwards towards the north. The easterly 

 movement carries water across the Atlantic Ocean from the Brazil current towards 

 Africa. Some of this water flows south of the Cape of Good Hope, joins water which is 

 turned back from the Agulhas current and flows eastwards across the Indian Ocean. 

 The remainder turns northwards, joins the Benguela current and flows north and east 

 along the west coast of Africa. 



Between 30 and 10° S sub-tropical water flows westwards, and the discontinuity 

 slopes upwards towards the north. This westerly movement is the result of the south- 

 east trade winds; it carries water from the Benguela current back to the Brazil current. 



The two movements of sub-tropical water complete an anticyclonic water movement 

 which extends over the whole width of the ocean. In the centre of this movement, there 

 is a tendency for the surface water to accumulate and to sink downwards owing to the 

 effect of the earth's rotation on the currents. The sub-tropical water is therefore deepest 

 in the centre of the movement, and in 30° S and 30° W its depth is about 500 m. 



