As was the case with several previous sections in the 

 mid-Pacific equatorial region (Sverdrup et al 1946, Cromwell 195 U, 

 there is direct evidence of upwelling immediately south of the Equator 

 with the deeper, cooler, nutrient-rich waters replacing those at the 

 surface„ 



Although a rigorous definition of upwelling is not possible 

 at this time, mention should be made of the present apparent ambi- 

 guity with respect to the term Sverdrup et al (1946, p 501) de- 

 scribe upwelling off the coast of California as the replacement of the 

 light surface water by heavier sub-surface waters. At the Equator, 

 the deeper waters are mixed into the surface layers from depths not 

 exceeding ZOO to 300 meters,, Sverdrup and Fleming (1941, p„ 3.17) 

 propose that the replacement of the surface waters should be dif- 

 ferentiated from the increase (or decrease) in slope of the isotherms 

 and isohalines resulting from current transport, and suggest that 

 once a transport has reached a steady state the vertical displacement 

 of the isopleths will stop and "the process cannot be called upwelling " 

 This same line of reasoning is undoubtedly applicable when consider- 

 ing the upwelling associated with the equatorial divergence and the in- 

 crease or decrease in slope of the isotherms and isohalines associated 

 with the mid-Pacific zonal circulation (see fig D 5). 



We find both of the above-mentioned phenomena in the 

 meridional section described in this reports Near the Equator there 

 is upwelling associated with a wind-induced divergence,, The deeper 

 waters mix with those at the surface;, resulting in a cooling of these 

 surface waters and an increase in nutrients, such as phosphates, and 

 conditions favorable to increased biological productivity ensue (King 

 1953, Murphy 1953, Sette MS)„ 



Throughout the region of the Countercurrent the second 

 phenomenon, that of a vertical displacement of the isopleths associated 

 with zonal flow, is to be found. The sigma-t surfaces (fig 7) slope 

 markedly, the 24 g/1 surface rising from 525 meters at 5° N c to 50 

 meters at 9° N Q The 2 ug at/1 phosphate isopleth (fig„ 9) is near 

 300 meters at 4° N , sloping upward to 100 meters at 9 N c To this 

 situation Dietrich (1935, p. 57) assigns the term "dynamically caused 

 upwelling " Even if there is occasional cooling and enrichment of 

 surface waters at the boundary of a zonal current, such as the Equa- 

 torial Countercurrent, it is believed that any use of the term upwelling 

 is confusing,, 



Although the data on file at POF1 require considerable 

 further study, there is little evidence that there is any mixing of the 

 deeper waters with the surface waters at the northern edge of the 



