SECT. 3] EASTERN BOUNDARY CURRENTS 267 



A. Ekman Transport Calculations and Upwelling 

 The field of mean wind stress in the world ocean has been computed for five- 

 degree squares by Hidaka (1958), using shipboard observations as summarized 

 in Pilot Charts of the U.S. Navy Hydrographic Office. The accuracy of this 

 representation of the field of mean wind stress suffers because of the paucity of 

 data from large regions of the ocean and because of inadequate knowledge of 

 the drag coefficient at various wind speeds. However, it seems likely that 

 the gross features of the wind-stress field are revealed in these values. 



Using the wind-stress data, we have computed the Ekman transport of 

 surface water away from the coast, according to the following equation : 



M n = T p /f, 

 where M n is the Ekman transport normal to the coast, r p the wind stress 

 parallel to the coast and / the Coriolis parameter. An attempt was made to 

 select the five-degree square lying nearest to the coast, although the squares 

 containing adequate data were not always ideally located. The average orienta- 

 tion of the coastline was determined for each square and values of M n com- 

 puted for each season. These values were then examined as possible crude 

 indices of the seasonal and geographical variation of coastal upwelling. 



An important deficiency of the offshore Ekman transport as an index of 

 upwelling should be noted. The overall transport consists of both Ekman and 

 geostrophic components. The distinction can be visualized by assuming that 

 the Ekman transport first brings about a redistribution of mass, the resulting 

 pressure gradient then being balanced by the Coriolis term to give the geo- 

 strophic transport. Thus the Ekman transport should be considered only the 

 initial step in the process which, at equilibrium, gives an essentially geostrophic 

 boundary current. 



Another difficulty is that the average wind stress in a five-degree square 

 adjacent to the coast may be a poor approximation to the actual stress operating 

 at the boundary. This problem is mentioned by Hart and Currie (1960) who 

 state that on the southwest coast of Africa diurnally variable coastal winds 

 prevail in a belt extending seaward for about 80 miles from the coast. 



Nonetheless, the seasonal and geographical variations in the index do appear 

 to bear some relation to corresponding variations in coastal upwelling. Values 

 of the index are plotted as functions of latitude and season (Fig. 12). Some of 

 the important features of the behavior of the index are summarized below : 



1. In both hemispheres maximum values of the index are usually observed 

 in the spring or summer. Only in the Peru Current north of 30°S is a distinct 

 winter maximum present. 



2. As a rule values of the index are much larger in the Atlantic and Indian 

 Oceans than in the Pacific. Again, winter values off northern South America 

 are exceptional. 



3. Negative values of the index are observed in the North Atlantic (south of 

 10°N and north of 50°N), Indian Ocean (north of 20°S), North Pacific (north of 

 40°N) and South Pacific (south of 45°S). In both the North and South Pacific 



