General Theory of Ocean Currents in a Homogeneous Sea 401 



to e~'" = 1/23 of the surface value. This velocity is already so small that by com- 

 parison with the surface value it can usually be neglected. The depth D can therefore 

 be taken as a measure of the depth of penetration into the sea of a v/ind-generated 

 ocean current on the rotating Earth. It can in general also be taken as a measure of 

 how far downwards the effect of a steadily flowing horizontal layer penetrates into the 

 adjacent water masses. It was termed by Ekman the ''frictional depth'"; for drift 

 currents the additional word "upper" is used in order to indicate that here solely 

 conditions in the top-layer of the ocean are dealt with. 



According to equation (XIII.26) D can also be taken as a measure of the internal 

 turbulent friction. It should be noted that the shearing stress T is not involved in the 

 equation relating D and rj ; this could be interpreted to mean that the vertical thickness 

 of the drift current should be independent of the wind intensity producing it and 

 maintaining it against friction. This apparent contradiction is clarified by consider- 

 ing that the frictional coefficient increases with increasing wind strength as does also 

 the frictional depth D. 



Figure 168, according to Ekman, shows the vertical structure of a pure drift current; 

 the arrows projecting from the central column which are also shown in a projection 



Fig. 168. Vertical structure of a pure drift current (according to Ekman). 



2D 



