84 



Linear Theories — Viscous 



On the western edge of most of the world's oceans there is a system of strong 

 currents. Now, we may ask ourselves, why is this so ? If the wind system, a 

 broad, widespread phenomenon, drives the ocean currents, how is it that 

 the resulting ocean-current system should be so asymmetrical; in particular, 

 why should the strongest currents be squeezed into a narrow belt on the 

 western edge of the oceans ? It is this question which the theories of Stommel 

 (1948), Munk (1950), and Hidaka (19496) attempt to answer. 



Fig. 52. Schematic wind system (broken lines) and currents (solid lines) 

 which would occur were there no asjTnmetry in the circulation. 



THE THEORY OF WIND-DRIVEN OCEAN CURRENTS 



The physical situation is as follows. Suppose we consider a large ocean 

 basin, such as is depicted in fig. 52, with an anticyclonic wind circulating 

 over it. In both figs. 52 and 53 the streamUnes of ocean current are sho^vn as 

 soHd lines ; those of the wind, as broken lines. Our intuition tells us that this 

 wind will produce an ocean surface current in an anticyclonic sense. From 

 what Httle we know about the distribution of properties in the deeper parts 

 of the ocean it seems reasonable to suppose that the current which the 

 wind induces does not extend to the bottom. Our intuition also tells us that 

 if the ocean- current system has arrived at a state of steady motion under the 

 stress of the wind, the free surface and isopleths of density in the ocean will 

 have adjusted themselves in such a way as to produce the horizontal 

 pressure gradients necessary to balance the CorioUs forces acting upon the 

 moving water. But that is about as far as our intuition takes us, and it 

 gives no hint of the necessary asymmetry in the current system. To pro- 



