412 



General Theory of Ocean Currents in a Homogeneous Sea 



-* « • -t ai 



= /^i8. 



Thereby jS was presumed to be the direction of the wind stress. At the same time it 

 should be noted that under the influence of the turbulence of the wind the frictional 

 coefficient is largest at the sea surface and decreases with depth. In order to satisfy 



02 0-4 0-6 0-8 



0-2 0-4 



02 04 Q6 08 



Fig. 176. Turbulent bottom current according to Prandtl (full lines: n = 5, /?i = 174 m; 

 dotted lines: bottom current according to Ekman (see Fig. 172). 



these conditions the vertical current structure in the drift current will diff'er from that 

 in the bottom current where the frictional coefficient converges to zero at the bottom ; 

 it will have a similar form as compared with that shown in Fig. 172. 



A theory of drift and gradient currents based on similar principles was put forward 

 by RossBY (1932) and later extended by Rossby and Montgomery (1935). This was 

 based on the principles of the newer turbulent flow theories and introduces in place 

 of the earlier used frictional coefficient the Prandtl mixing length. In drift currents this 

 is largest in the surface layers where the intensity of movement is greatest and decreases 

 with depth to vanish at the frictional depth. The theoretical treatment of this assump- 

 tion is very complicated and the results can only be shown by means of tables. Also 

 here the deflection angle of the wind drift comes out to be dependent on both the wind 

 speed and latitude, while according to the Ekman theory it should have a constant 

 value of 45°. The ratio of the velocity of the surface current to the wind speed (wind 

 factor, p. 418) results as equally dependent in a rather complicated way on the same 



