228 DISCOVERY REPORTS 



NORTHERN BOUNDARY 



From the wedge-shaped area of cool water over the tract extending westwards from 

 the Piura coast towards the Equator where the Peru Coastal Current is on its way to 

 the South Equatorial Current, Schott (193 1) deduces a divergence line along which 

 upwelling occurs far out to sea. Water north of the line remains in the South Equatorial 

 Current, water deflected to the south mixes with the warmer South Pacific. 



This interpretation may explain the data recorded in Fig. 16, which are themselves too 

 few to illustrate the conditions described by Schott. The bulge in the isotherms off 

 northern Peru may represent the base of a wedge of cool water extending westwards, and 

 it is possible that some of the water in this area has come to the surface along a di- 

 vergence line extending west-north-west into the open sea. The results obtained on a 

 line of stations cutting across this region in a north-east by north and south-west by 

 south direction do not, however, support this (Figs. 49 and 69). At the northern end, 

 at 54 miles off Santa Elena, the hot poorly saline Equatorial Counter-current overlays 

 water which is essentially similar to subtropical water in the Peru Current. Most of the 

 surface water over the rest of the section is cool water of 19° C. and less: the warm- 

 water wedge is entered between lat. 5° S and lat. 7° S, and here temperature is higher 

 than 20° C. The isotherm of 18° C. maintains a more or less constant depth between 

 40 and 50 m. If the cooler water in the middle of the section was welling up to the 

 surface, it is natural to expect the water at 40 m. to be involved, and the isotherms of 

 18 and 17° C. would betray upward movement in this region, and since this does not 

 occur it may be doubted if upwelling is in progress except close to the shore. Conditions 

 may be different farther to the west where divergence may be more active. 



The northern boundary of the Peru Coastal Current during the present survey as 

 shown by the S -shaped line of convergence between it and the Equatorial Counter- 

 current is noted in Figs. 70 and 71. Like the southern boundary, this is liable to vary 

 (Schott, 1931). 



