ON VERTICAL CIRCULATION IN THE 

 OCEAN DUE TO THE ACTION OF THE 

 WIND WITH APPLICATION TO CON- 

 DITIONS WITHIN THE ANTARCTIC 

 CIRCUMPOLAR CURRENT 



T 



By H. U. Sverdrup 



(Text-figs. 1-23) 



H E theoretical studies of the ocean currents have principally dealt with the horizontal 

 currents which arise because of the effect of the wind and because of the distribution 

 of density. The question as to the relative importance of the wind and the distribution 

 of density has often been discussed and seems now to be answered in favour of the wind. 

 Defant, in his excellent survey of the present status of dynamic oceanography, says 

 (1929, p. 136) that the greater influence on the development of the horizontal currents 

 must be ascribed to the wind, while the differences in density are of special importance 

 to the vertical circulation. In this paper it is intended to show that the wind also main- 

 tains systems of vertical circulation and that the factors which influence the density of 

 the sea water, such as heating, cooUng, evaporation and precipitation, are of equal im- 

 portance to the horizontal currents as is the wind. Before doing so it is necessary to give 

 a brief review of our present theoretical knowledge of the ocean currents. Compre- 

 hensive investigations have especially been undertaken by Ekman (1928), who has 

 examined the currents which arise in homogeneous and non-homogeneous water both 

 under the action of the wind and as a resuh of the distribution of density. We shall 

 follow his classification and terminology. 



In homogeneous water Ekman discriminates between three different current systems, 

 which all arise under the action of the wind : 



(i) The pure drift current, which is limited to the uppermost layer in which the 

 current at the surface is directed at 45° cum sole from the direction of the wind. With in- 

 creasing depth this current turns cum sole and decreases in velocity until it becomes 

 negligible at the " depth of frictional resistance." Within this current the total transport 

 of the water is directed at 90° cum sole from the direction of the wind. 



(2) The slope current, which arises because the oceans are limited and, therefore, the 

 water is piled up along the coast towards which the transport by the pure drift current 

 is directed. Because of this piling up, the surface of the sea becomes inclined and a 

 current is set up which runs with uniform direction and velocity from the surface to the 

 bottom, except in the immediate vicinity of the bottom where the influence of the 

 friction along the bottom has to be considered. Above the "lower depth of frictional 

 resistance" the slope current is directed at right angles cum sole from the direction of 

 the slope. Below the " lower depth of frictional resistance " we find the third constituent 

 of the current system. 



