General Theory of Ocean Currents in a Homogeneous Sea 



413 



quantities. Table 119 gives some values for these relationships. A comparison of these 

 results with those of the Ekman theory and with observational data is given later on 

 (see p. 418). The introduction of a mixing length decreasing with depth and vanishing 

 at the frictional depth should give a correct representation of actual conditions only if 

 the turbulence arises solely from the wind drift and not from other currents which may 

 be present (for instance, tidal currents, gradient currents). If such influences exist, it is 

 necessary to introduce in the theory of wind-drift currents the vertical distribution of 

 the turbulent coefficients which corresponds to the total current. This, however, 

 modifies in turn the results. At present the Ekman theory appears to be a perfectly 

 satisfactory approximation to actual conditions, as long as our knowledge about the 

 vertical distribution of turbulence is not increased. 



Table 119. Deflection angle and wind factor as a function of latitude and wind speed 

 according to the theory of Rossby and Montgomery (1935) 



(d) The '' Element ar'^ Current 



In a homogeneous ocean no currents are possible other than drift and gradient 

 currents; at every point the steady current is made up of a pure wind drift and a pure 

 gradient current. These can be superimposed without mutual interference since each 

 component is entirely independent of the other. If the depth of the sea d is larger than 

 the upper and lower frictional depths D' and D", the resulting current system can be 

 separated into three current layers (see Fig. 177, left-hand side). 



(1) The bottom current from the sea bottom to a height D" (lower frictional depth). 



(2) The deep current from the level D' (the upper frictional depth) to the level D" 

 (the lower frictional depth). 



(3) The surface current which is the resultant of the uniform deep current and the 

 pure drift current generated by the wind. 



'I Su 



rface current 



Deep current 



D"{~ B( 



TTTTTTTTTZ 



Bottom current 



Wind 



Fig. 177. Vertical structure of the "elementar" current (according to Ekman). 



