SECT. 3] 



EQUATORIAL CURRENT SYSTEMS 



243 



100 

 200 

 300 

 400 

 500 





VELOCITY (cm/sec): 



Fig. 7. Composite cross-sections of temperature, salinity, dissolved oxygen, inorganic 

 phosphate and velocity for the central Pacific. Curves based on data taken by the 

 U.S. Fish and Wildlife Pacific Oceanic Fisheries Investigation cruises 5 and 35 of the 

 Hugh M. Smith, summer 1950 and 1956, respectively. Velocities are geostrophic 

 velocities except near the equator where the values are based on direct observation 

 (Knauss, 1960). One thousand meters was assumed for the "level of no motion". 

 Darker areas indicate eastward velocity, lighter areas westward velocity. 



Wooster and Cromwell, 1958). The shallow thermocline is a permanent feature 

 in the tropics. Primary productivity is probably dependent upon mixing 

 nutrients through the thermocline into the surface zone, where photosynthesis 

 can take place. Regions of high productivity in the tropics are limited to zones 

 like the equator where there is upwelling, or areas such as those off Costa Rica 

 where the thermocline is very close to the surface. 



The oxygen minimum, found immediately below the thermocline, is a 



