SALINITY 159 



illustrating the principle already noted that inshore temperatures in the Peru Current are cooler 

 than those offshore, while north of the Peru Current inshore temperatures are warmer than 

 those offshore. The curve at 10-20 miles offshore shows by its progressive warming from 8 to 4° S 

 that the influence of coastal upwelling is less and that the influence of equatorial water is not much 

 felt south of Capo Blanco. The water in the zone 50-100 miles offshore does not show great warmth 

 off Ecuador; its salinity between 2 and 3° S suggests a mixture of the Peru and tropical waters. The 

 curve for greater distances (100-200 miles) has no appreciable temperature rise and indicates that 

 the water is of more purely Peruvian origin. 



SALINITY 



In the preceding section, the cool water at the surface inshore has been shown to be 

 derived from lower layers by upwelling, and in certain localities the distribution of 

 surface isotherms suggest horizontal movement also. But the origin of the water masses 

 participating in this circulation is less easily seen in distribution of temperature because 

 of the regularity of thermal stratification in the deeper water. At the surface, on the 

 other hand, salinity is a less straightforward guide than temperature because it is liable 

 to be altered by precipitation or evaporation. Thus in the south of the region the surface 

 water is diluted by rains of the temperate zone and offshore is less saline than water 

 at a depth of 160-200 m., whereas further north, as a result of the drying action of the 

 south-east trades, the surface offshore is more saline than the lower layers. In conse- 

 quence of this reversal of conditions and of upwelling near the coast, the Peru Coastal 

 Current south of the subtropical convergence is more saline, and north of the conver- 

 gence less saline than the surface of the ocean immediately adjacent. 



Surface salinity in the southern part of the region is moderately low and is probably 

 sub-Antarctic water. At the subtropical convergence in lat. 24-26" S (Fig. 42) it sinks 

 beneath more saline but warmer subtropical water and continues northward beneath 

 it as a subsurface current. The subtropical water at the surface has a depth of about 

 40 m. and continues northwards at the surface until it meets with the still warmer 

 less saline Equatorial Counter-current. The convergence of these two water masses 

 is recognized as the northern boundary of the Peru Current, but the section in Fig. 42 

 indicates that the latter extends some way northwards beneath the counter-current. 

 Beneath these water masses is situated the Antarctic intermediate water whose low 

 salinity is derived mainly from molten ice. 



The origin of water masses in the South Atlantic Ocean corresponding to these four 

 has already been described (Deacon, 1933), but their behaviour in the eastern South 

 Pacific is modified by the peculiar conditions obtaining on the west coast. 



The section illustrated in Fig. 42, which is approximately meridianal, does not cut 

 the subtropical convergence sharply, but at the surface the isohalines of 34-40-34-90 °l^^ 

 are seen to be spread out between the parallels of 21 and 28° S. This shows that the 

 convergence is not crossed at right angles, but that it curves northward near the coast in 

 accordance with the general circulation. The inference receives ample confirmation 

 from Sverdrup's results (193 1). In his fig. 4 illustrating Sts. 50-60 in a north-west to 



