UPWELLING 185 



to explain the Californian upwelling by Eckman's theory of currents. He showed that the upwelling 

 is a direct effect of the coastal winds which on the assumptions of Eckman's theory need not blow 

 offshore to produce an offshore transport of surface water. A wind blowing parallel to the coast would 

 be sufficient to induce such a transport with consequent upwelling. 



Sverdrup (1938), from a detailed examination of the Californian current, supports this view, and 

 concludes that the upwelling is a direct effect of the local winds transporting surface-water away from 

 the coast. Gunther (1936) found that on the Peru Coast the 'William Scoresby's' observations indi- 

 cated that the upwelling was brought about ' as a result of wind acting in conjunction with forces due 

 to the earth's rotation'. 



The present work indicates that while the trade wind in the open ocean must maintain the denser 

 water nearer the surface inshore, the periodic and local intense upwelling is probably mainly dependent 

 upon the local coastal winds producing a northwards, longshore or offshore displacement of the 

 surface water, thus initiating a vertical compensation flow from the subsurface layers. 



Ekman's theory (1905) provided the basis of our present-day understanding of the effect of wind 

 stress on the sea-surface. It shows us qualitatively that, owing to the effect of the earth's rotation and 

 frictional forces, the drift produced by a wind blowing over the ocean deviates at an angle of 45 ° to the 

 left of the wind direction in the southern hemisphere. Owing to the viscosity of the water, the velocity 

 in this drift current will decrease regularly with depth. There will also be an increased deflection with 

 depth, until a point is reached where the current is directed against the surface drift. At this point 

 the velocity of the current is about i/23rd of that at the surface, and Ekman has termed this depth (D) 

 the ' depth of frictional influence '. While the current vectors at different levels in the wind current 

 vary, the total transport remains directed normal to the wind direction (that is 90 to the left in the 

 southern hemisphere). 



These results apply to an ocean of which the bottom is very deep, and where the influence of coast- 

 line and varying density of the water is not considered. In this latter respect, the quantitative applica- 

 tion of the theory is hindered, as the magnitude of the reaction between the different density layers 

 in the sea is not known. Eddy viscosity is, however, a measure of this reaction, and more recently 

 Rosby and Montgomery (1935) have introduced the conception of a 'mixing length' which varies 

 with depth and upon which the eddy viscosity is dependent. On this basis they find that the deviation 

 of the drift is not constant as Ekman postulated, but varies with the strength of wind and with the 

 latitude. Even these improved assumptions, however, still appear to fall short of giving a realistic 

 picture. 



Ekman further studied the problem of wind drift in the presence of coastlines and where the sea 

 bottom was shallow. Where the bottom is greater than twice the depth of frictional influence it does 

 not have much influence, but in shallower water there is a restriction of the deflection and slowing of 

 the turning with depth, until in very shallow water the whole movement follows the direction of the 

 wind (the effect of the earth's rotation in this case being negligible). Thus in coastal regions three 

 main factors affect the wind drift. 



1. The position of the coastline in relation to the water in question. 



2. The relation between the depth D and the sea bottom. 



3. The direction of the wind stress, in relation to the direction of the coastline. 



The effect of winds on the South-west African coast 

 We can find the approximate magnitude of D in metres from the relationship : 



D = 7-6 W/Jsin 6, 

 where the wind stress Wis expressed in m./sec, and 6 is the latitude. (For wind stresses <6 m./sec. 



