Ocean Currents in a Non-homogeneous Ocean 



485 



to the frictional depth and in the lower layer is assumed as equal to double the fric- 

 tional depth. The thin hnes refer to the lower layer and the thick lines to the top layer. 

 The two arrow-heads at the right-hand edge connected with the + sign represent the 

 vector of the surface current in the case of frictionless motion. For sharper discon- 

 tinuity surfaces and a greater thickness of the top layer the velocity profile, as before, is 

 made up of two Ekman spirals. If the top layer is stratified there is in both cases a 



Fig. 221. Density currents in a top layer considering friction and for motionless deep 



water (according to Ekman). 



current of almost uniform direction and the current velocity will decrease almost 

 linearly with depth. Due to the stratification of the current, intensity in the lower 

 layer (internal drift current) will be strongly reduced, and for a deeper top layer this 

 current will disappear almost entirely. The transport in the deep current will then be 

 insignificant. This leads to the important conclusion that: the sea surface under the 

 influence of external disturbances will adjust itself in such a way that the pressure gradient 

 arising from density dijferences in the top layer has a inaximum value at the sea surface, 

 decreases with depth and will largely or entirely vanish at the lower boundary of the 

 top layer; the deep water will remain practically motionless. 



The "elementar" current in a vertically comphcated stratified ocean consisting of a 

 stratified top layer and an almost homogeneous deep water will thus, according to 

 Ekman, have the following three current constituents. 



(1) The physical sea level and the isobaric surfaces of the top layer will be turned in 

 such a way that the pressure gradient has the same direction everywhere and will be 

 proportional at every level to the density; in the homogeneous deep water, however, 

 this pressure gradient will remain constant. The current produced by this mass structure 

 will be a simple gradient current. 



(2) If the physical sea level and the isosteric surfaces are brought back to the initial 

 position, then an additional current resulting from this mass displacement adds to 

 the gradient current described above. This is called the density current. 



(3) In addition, the effect of the wind on the sea surface generates a pure drift 

 current. This current will differ only slightly from that in a homogeneous sea if the top 

 layer is sufficiently thick. However, the density current will not be confined to the top 

 layer alone, but when this is reasonably thick, the influence on the homogeneous deep 

 water from above remains small. 



