i86 DISCOVERY REPORTS 



about 20 knots in 60° S. Brennecke (1921) found by measurement a depth of 50 m. in 

 the Weddell Sea. The total transport of water in a pure drift current is directed at 90° 

 cum sole from the direction of the wind ; and if the wind is from the west in the southern 

 hemisphere, the total transport is towards the north. If the depth of the sea is less than 

 1-25 D, the angle between the surface current and the wind decreases; and when the 

 depth is o-i D, the surface current is in approximately the same direction as the wind. 



(2) A SLOPE CURRENT is the direct result of the transport of surface water in the 

 pure drift current which will cause the level of the sea to rise at one place and fall at 

 another. To prevent this slope of the sea surface a deep current flows in a direction at 

 right angles am sole from the direction of the slope, i.e. in the direction of the wind. 

 The velocity of this slope current is constant with depth, if the sea is homogeneous and 

 friction constant, until a depth is reached where the frictional influence of the bottom 

 is felt. 



(3) A BOTTOM CURRENT, which is the slope current modified by the effect of friction 

 with the sea-bottom. Below a height from the bottom which is analogous to the depth 

 of frictional influence at the surface— it may be called the depth of lower frictional 

 influence or the effective height of bottom friction — the slope current is slowed down by 

 friction and is turned coiitra soleni to flow in the direction of the surface slope. When 

 stationary conditions have been reached, the bottom flow in the direction of the slope is 

 all that compensates for the surface flow up the slope. 



4. Convection currents. Ekman also recognizes the presence of currents, if the 

 water is not homogeneous, which are due to instability in the distribution in density. 

 Their speeds and directions depend on the angles and directions of slope of the surfaces 

 of equal density. 



In the Antarctic surface layer the effect of the prevailing west wind north of 66° S is 

 to set up pure drift currents with surface velocities towards the north-east, and with a 

 total transport of water towards the north. As far as can be shown at present the depth 

 of frictional influence is about 60-80 m. The surface stratum of Antarctic water is very 

 often uniform to this depth, especially after a storm, and it will be shown in the section 

 on salinity and temperature of Antarctic surface water, that the changes with depth 

 below 80 m. are always much greater than those above it. In sub-Antarctic water, also, 

 there are changes at about 80 m. which are caused by a component of movement 

 southwards. It therefore seems probable that the effect of pure drift currents is confined 

 to the surface 80 m. of water, and the movement of the colder water at the bottom of the 

 layer will be part of the deep current. According to Ekman it will move as a slope 

 current or a convection current or a combination of the two. The total effect of the 

 prevailing westerly wind on the Antarctic surface layer will be to produce in the cold 

 stratum a current towards the east, and in the surface stratum a current which has at 

 each level the velocity of the pure drift current at that level superimposed on the easterly 

 movement of the cold stratum. 



South of 66° S the prevailing wind is from the east and the direction of movement 

 will be towards the west at the bottom of the layer, and at the surface, south of west. 



