ANTARCTIC SURFACE WATER: MOVEMENTS n 



the combined effect of the wind, the earth's rotation and friction, will give rise to a 

 relatively shallow-reaching current which he has called a pure drift current. In the 

 southern hemisphere the surface water in this current flows in a direction 45 ° to the left 

 of the wind ; below the surface the current turns more and more towards the left and its 

 velocity decreases until at a depth which has been called the "depth of the wind's 

 frictional influence " the velocity is negligibly small and directed opposite to the surface 

 current. Between the surface and this depth, which in the Southern Ocean is probably 

 not more than 60-100 m., the total transport of water is directed 90 to the left of the 

 wind. 



These conclusions are in approximate agreement with the facts as far as they are 

 known at present ; the angle of 45 between the wind and the current, an important part 

 of the theory, has frequently been confirmed. The theory further states, however, that 

 the action of the pure drift current in piling up water to the left of the wind against a 

 coast or another water mass will give rise to a deep current in the direction of the wind, 

 and this current, called by Ekman a slope or gradient current, influences the greater 

 part of the water column in the ocean ; between the relatively thin surface and bottom 

 layers which are influenced by friction with the wind and the sea-floor, it flows with 

 uniform velocity. 



Where such a deep current exists the actual movement at the surface is the resultant 

 of the pure drift current and the deep current, and it will therefore be directed at an 

 angle less than 45 from the wind; the total transport within the depth of the wind's 

 frictional influence will also be made to have a component in the direction of the wind. 

 The more recent work of Ekman suggests, however, that under the actual conditions 

 found in the sea the part played by the deep current is not likely to be as comprehensive 

 as was first supposed. In regions where the wind is not constant, where the depth of the 

 sea varies, or where the deep current flows from one latitude to another, the current 

 will be modified by mixing and eddy movements. Thorade (1933), in a note on Ekman's 

 most recent paper (1932) and the development of the theory, mentions that the frequent 

 confirmation of the angle of 45 ° between the wind and current gives some indication 

 that the drift current generally exists without the deep current. In the Southern Ocean 

 Krummel ( 1 9 1 1 , 11, pp. 680- 1 ) shows that there is poor agreement between the theoretical 

 wind currents and the observed currents, but the angle between the surface current and 

 the wind cannot be regarded as a reliable indication of the deep water movements 

 owing to the existence of a northward movement caused by other factors. 



The density distribution points to the existence of a strong deep current ; the surfaces 

 of equal density and specific volume (isosteres) slope downwards to the north and the 

 isobaric surfaces in the opposite direction, towards the south. According to Ekman the 

 first effect of the wind on a non-homogeneous sea is to set up a pure drift current which 

 transports the light surface water to the left of the wind, and where this movement is 

 restricted by a long straight coast or another water mass which belongs to a different 

 current system it will give rise to such density and pressure distributions, and to a deep 

 current, which he has called a convection current, in the direction of the wind. 



