COASTAL AND OCEANIC CURRENT 223 



currents the surface layers of the open ocean are cut off sharply from those below by 

 a well-defined discontinuity layer which checks vertical mixing. Above the dis- 

 continuity layer a depletion of nutrient salts brought about by the phytoplankton is 

 followed further west by a decrease of the latter and of the zooplankton : in this en- 

 vironment, species are oceanic. Near the coast, on the other hand, where upwelling 

 provides a constant supply of nutrient salts at the surface, a dense growth of phyto- 

 plankton is possible, and this leads to a wealthy plankton fauna and to immense numbers 

 of animals of economic importance. The species here are littoral. Temperature and 

 salinity of the surface inshore and offshore are to some extent symptomatic of these 

 changes. 



Beneath the surface, a northerly current of sub-Antarctic origin, and a southerly warm 

 highly saline return current wedged between sub-Antarctic water and the Antarctic 

 intermediate water, appear to be features characteristic of the coastal water. They 

 appear to be drawn towards the upwelling region in compensation for the upwelling 

 water and thus may not feature in the open ocean. 



Thus in the upper 400 m., biological, chemical, and physical characteristics, both of 

 the surface and of the deeper layers, distinguish the inshore from the offshore waters. 

 Meteorological differences also exist, and among them the condensation of cloud over 

 the cool upwelling zone should be mentioned. As a result of this lessened illumination, 

 phytoplankton probably comes close to the surface inshore, but in the open ocean 

 either affects systrophe or sinks deeper, with the consequence that inshore waters are 

 normally coloured green, whereas waters of the open ocean are ultramarine. 



In view of the desirability of keeping the distinct identity of the two regions in mind, 

 the oceanic drift will be distinguished from the coastal current by the name Peru 

 Oceanic Current. The Peru Coastal Current will be kept for the system of inshore cur- 

 rents with which the name Humboldt Current has often been associated. The name 

 Peru Oceanic Current will be kept exclusively for the waters offshore, different in 

 composition but sharing, to some extent, the northerly movement of the anticyclonic 

 circulation. 



Salinity in the upwelling region is lower than values offshore north of the subtropical 

 convergence, but higher inshore than offshore south of the convergence (pp. 159-62). 

 On account of this reversal north and south, and because salinity at the surface is liable 

 to be altered by precipitation and evaporation, temperature is perhaps a better guide to 

 the boundary of the Coastal Current. 



WESTERN BOUNDARY 



The effect of upwelling upon the surface isotherms is shown in Figs. 16-17 ^^^ 

 29-30, where it is seen that the water inshore is some 2-5° cooler than the outermost 

 of the observations on the same parallel, with the consequence that isotherms run in 

 the same direction as the coast but converge with it slightly towards the lower latitudes. 



The normal oceanic trend of isotherms in an east and west direction, reflecting the 

 increase of surface temperature with decrease of latitude which occurs over the major 



