268 



WOOSTER AND REID 



[CHAP. 11 



minimal values of the index (at 30°-35°N and 20°-25°S respectively) appear 

 to separate two seasonally different upwelling regimes. 



4. In the Canary, Benguela and California Currents the latitude of the index 

 maximum migrates seasonally, being farther south in spring than in summer. 



Unfortunately no direct measurements of vertical velocity are available, but 

 indirect manifestations of the upwelling process can be used to investigate the 

 validity of the index as an indicator of upwelling. 



SUMMER 

 | WINTER 

 I AUTUMN 



SPRING 



30 25 20 15 10 



35 30 25 20 15 10 5 -5 -10 



M n (kg cm" 1 sec" 1 ) 



Fig. 12. Offshore Ekman transport, M n , computed from mean wind-stress values (Hidaka, 

 1958) and average coastline orientations for five-degree squares along the eastern 

 sides of oceans. 



a. Estimates of the speed of vertical motion 



Various estimates of vertical speed have been based on the observed vertical 

 movement of isopleths or on theoretical analyses of several models of the pro- 

 cess. These estimates range from 10-20 m/month (McEwen, 1929) to 80 m/month 

 (Saito. 1951 ; Hidaka, 1954). It is possible to use the magnitude of offshore 

 Ekman transport to compute the speed of the associated vertical motion at the 

 boundary. If one assumes a steady-state offshore Ekman transport of 10 kg 

 cm -1 sec -1 and a coastal upwelling band 50 km wide, the compensating vertical 

 motion is about 50 m/month, a result compatible with previous estimates. 



b. Annual range of surface temperature 



Since, in the process of upwelling, sub-surface water of relatively low tem- 

 perature ascends to the surface, it can be assumed that, during the season of 



