Ocean Currents in a Non-homogeneous Ocean 



483 



X, cm/sec 



Fig. 218. Velocity profiles in density currents for shallow ocean depths (according to 

 Ekman). The unit of the velocity scale is U:5. 



Table 134. Frictional influence on density currents in different depth of the ocean 



direction and in addition the velocity of the surface current as a percentage of the 

 geostrophic current U. The vertical velocity decrease is at first very slow and then 

 becomes almost linear. By this it is shown that the law of parallel fields also applies 

 to a close approximation when frictional effects are present. 



Simple mass distributions such as these rarely occur in nature. In addition Ekman has 

 also investigated cases in which the eflFect of a homogeneous solenoid field is superim- 

 posed on a gradient current. A lighter stratified top layer spreads out over a homogeneous 

 deep water. The lighter water body may, for instance, be coastal water lying in a wedge- 

 form off a long coast and can be regarded as a mixed layer of fresh water from the land 

 and of deep water. External forces are not taken into account ; at the boundary surface 

 between the top and the deep layer the water movement of the upper density current 

 exerts a shearing force on the deep water which gives rise to an "internal drift current". 

 A closer examination of the case of a boundary layer at a depth d, parallel to a straight 

 coast between a homogeneous upper and lower layer, gives the velocity profiles 

 for different values of dID presented in Fig. 219. The points on each curve refer again 

 to the depths 00, 0-1 D, 0-2 D . . . , below the sea surface. The part of the curve re- 

 ferring to the top layer is shown by a thick line ; the points on the thin part of the curve 

 (deep water) have been omitted for clarity. The unit of velocity is the same as in Fig. 218. 

 If the depth of the top layer is small as compared with D, there will be a strong deflec- 

 tion of the upper current away from the coast. The effect of the deep water lying just 

 underneath the top layer varies according to variations in the depth of the top layer. 

 U d < hD the deep water will in part be dragged out to sea by the water of the top 

 layer so that underneath this there will be a current directed away from the coast and 



