WIND CURRENTS AND WIND WAVES 131 



runs in the direction of the wind. It has not been possible as yet to 

 examine theoretically the velocity distribution within this relative cur- 

 rent, but it is a priori probable that this current becomes more and more 

 prominent the longer the wind blows. As it increases in speed, eddies will 

 probably develop, and these may limit the velocities that can be attained 

 under any given circumstances. Also, it is probable that a vertical 

 circulation will be present that will consume energy and tend to limit 

 the velocities which can be reached. 



Consider next a wind in the Northern Hemisphere which blows parallel 

 to the coast, with the coast on the left-hand side. In this case the light 

 surface water will be transported away from the coast and, owing to the 

 continuity of the system, must be replaced near the coast by heavier 

 subsurface water. This process is known as upwelling, and is a con- 

 spicuous phenomenon along the coasts of northwest and southwest 

 Africa, California, and Peru. The upwelling also leads to changes in the 

 distribution of mass, but now the denser upwelled water accumulates 

 along the coast and the light surface water is transported away from 

 the coast. This distribution of mass will again give rise to a current that 

 flows in the direction of the wind. 



The qualitative explanation of the upwelling was first suggested by 

 Thorade and was developed by McEwen. Recent investigations have 

 added to the knowledge of the phenomenon and have especially shown 

 that water is not drawn to the surface from depths exceeding 200 to 300 m. 

 Deep water does not rise to the surface, but an overturning of the upper 

 layer takes place. 



In spite of the added knowledge, it is as j^et not possible to discuss 

 quantitatively the process of upwelling or to predict theoretically the 

 velocity and width of the coastal current that develops. It is probable 

 that the forced vertical circulation and the tendency of the current to 

 break up in eddies limit the development of the current. Also, the wind 

 that causes the upwelling as a rule does not blow with a steady velocity, 

 and variations of the wind may greatly further the formation of eddies. 



Fig. 34 demonstrates the effect of winds from different directions on 

 the currents off the coast of southern California in 1938. The charts 

 show the geopotential topography of the sea surface relative to the 

 500-decibar surface, which, in this case, can be considered as nearly 

 coinciding with a level surface. Arrows have been entered on the isolines, 

 indicating that these are approximately stream lines of the surface 

 currents. 



In the absence of wind, one should expect a flow to the south or 

 southeast, more or less parallel to the coast. In February, 1938, winds 

 from the south or southwest had prevailed. The light surface water 

 had been carried toward the coast, and, consequently, a coastal current 

 running north was present and was separated from the general flow to 



