486 



Ocean Currents in a Non-homogeneous Ocean 



Laboratory experiments with stratified water have been made by Sandstrom 

 (1908, 1918) in order to demonstrate experimentally the effect of stratification on 

 wind-generated currents. In the experiment, an air flow over the surface of a multiple- 

 stratified water mass in a narrow rectangular basin immediately produces a current in 

 the direction of the wind. The piling up of water at the windward end of the basin 

 gives rise to a counter current in the lower part of the uppermost layer ; there is a 

 closed circulation in this layer. Friction then produces a somewhat weaker circulation 

 with an opposite sense of rotation in the layer immediately beneath the uppermost one. 

 Further circulations are formed in successive layers beneath this, each with the 

 opposite (direct or indirect) rotational sense to that above it. Sandstrom's experimental 

 results for a narrow basin cannot be applied directly to actual conditions in the ocean. 

 In the laboratory experiment, in the first place, boundary conditions at the outer rim 

 of the narrow basin will play a decisive role, and secondly, the deflecting force of 

 earth rotation will have no effect and thus it is precisely that factor which most 

 decisively influences ocean currents in nature that is left out of consideration. The 

 laboratory experiment is thus apphcable in nature only to narrow confined sea basins 

 and to lakes. 



5. Oceanographic Applications of Bjerknes's Circulation Theorem 



The theory of ocean currents in a non-homogeneous sea received a very strong 

 stimulus from the circulation theorem of Bjerknes, since it opened the road for studying 

 in a quantitative way and for the first time the effects of baroclinic mass fields. There 

 are manifold possibilities to apply this theorem in oceanography some of which will 

 be discussed here in more detail. 



{a) The Steady State of Motion 



The most important use of the equation (X.54) is for the steady state in which the 

 circulation accelerations vanish. In this case 



N=f 



dfn 



dt 



(XV. 11) 



(here again A'^ = number of solenoids, / = Coriolis Parameter, F^ = area of the 

 projection of curves on the sea surface). The curve s is now made up of the two 

 station verticals AC and BD and of two isobars AB and CD (Fig. 222). The water 



