UPWELLING 195 



in nutrient salts, and the process of mixture with the other layers may be a factor of 

 importance in the high productivity of these waters. 



ORIGIN OF THE COOL WATER 



The probability that cool water reaches the surface close inshore by upwelling was 

 inferred by de Tessan (1844), by Dinklage in 1874, by Witte (1880), HoUmann (1882), 

 Hoffmann (1884) and Buchanan (1886), because the explanation advanced by Humboldt 

 (181 1 ) of a cool surface current from high latitudes was obviously untenable. As a result 

 of Duperrey's observations (183 1), the adherents to Humboldt's theory claimed for the 

 current a polar origin, but Bougainville (1837) showed that the water would warm up 

 if it flowed through so many degrees of latitude. Though the upwelling explanation 

 has hitherto rested on indirect evidence it is accepted by the majority of modern writers, 

 butThoulet (1928), working upon data collected by the Challenger Expedition on her 

 passage from Tahiti to Valparaiso, attributes the cool water to melted snow carried down 

 by rivers from the Cordillera; and Wiist (1935), apparently viewing the subject from 

 a nationalistic angle, makes enhanced claims for Humboldt's theory. 



Three principal views have been expressed on the causes of upwelling. Dinklage 

 (Schott, 1891, p. 215) maintained that the inshore waters were drawn away from the 

 coast by aspiration as a result of the action of the trade winds in the open ocean {vide his 

 observation on current at Callao); and Witte (1880), on theoretical grounds, maintained 

 that upwelling would result either by the action of the earth's rotation upon meridianal 

 current such as this, or by the action of winds blowing off the coast. Buchanan (1886) 

 tried to establish the latter of these views on the theory of "Trade Belts". On this 

 coast, however, winds do not blow off the shore but parallel to it ; and the theory of 

 "Trade Belts" has long given place to the theory of high-pressure centres. Thus 

 Buchanan's explanation is easily disproved. 



The present work confirms the views already expressed by Schott, and reference to 

 Figs. 18-61 can leave little doubt that, as a result of wind acting in conjunction with 

 forces due to the earth's rotation, the subsurface layers were upwelling or had been 

 upwelling in every one of the twelve localities examined between Cape Carranza and 

 Punta Aguja. 



Widespread lowering of temperature in the Peru Current is to be expected as a result 

 of its flow from cooler to warmer latitudes ; and cooling by this means must happen in 

 accordance with oceanographical principles : but consideration of the temperature and 

 current data suggest that this process is secondary in importance to cooling by upwelled 

 water. 



The charts of surface temperature (Figs. 16 and 17) and the temperature curves in 

 Figs. 34 and 66 reveal isolated patches of cool water off Payta, Puerto Chicama, San 

 Juan, Iquique, Antofagasta and probably Caldera, which cannot easily be explained in 

 terms of northerly transport. These are clearly instances where upwelling has modified 

 the orientation of surface isotherms. The influence of upwelling may be supposed to be 



