TEMPERATURE 135 



In the following pages the isotherms in vertical section and in surface plan will be 

 examined with a view to tracing correlations between them and such factors as wind, 

 surface drift, salinity, phosphate and plankton distribution, and the colour of the water. 



45-35° S: CAPE CARRANZA 



The climate is temperate and the southerly regions are drenched in heavy rainfall, 

 the surface salinity being thereby reduced (see Fig. 18 and p. 159). At the surface the 

 isotherm of 12° C. followed the ship's track from south to north for as much as 600 

 miles ; and beneath the surface, similar thermal uniformity is shown, isotherms being 

 spaced widely apart and the temperature sinking but 5° through the depth of 400 m. : 

 salinity shows, however, that the water is distinctly layered (pp. 159-163). 



Off Cape Carranza the water was, in appearance, actively welling up, but in view of 

 the fact that no drift was noticed at the surface, and of the fact that the ship enjoyed calm 

 weather, the wind being in the north, this appearance of upwelling may be a relic of earlier 

 activity. Comparable conditions in other localities will later be seen to suggest a reversal 

 of upwelling, and the possibility should be borne in mind that subsidence of the cool 

 surface water may have been in progress off Cape Carranza at the time of our visit. 

 Upwelling is shown to have been extensive for some considerable time in the past by 

 the high content of phosphate and of plankton of the upper layers (pp. 182 and 184). 



The surface temperature rose steadily from 11-45° ^- inshore to 13-57° C. at 58 miles 

 offshore, after which the rise was less, reaching i3"65° C. at 83 miles offshore (Fig. 29). 

 If the inshore water at this time were really subsiding as suggested, the lowermost of the 

 isotherms and isohalines formerly showing upward movement might at the time of our 

 visit have regained horizontal stratification. If this were so, the sections illustrated in 

 Figs. 18 and 19 would no longer indicate the depth previously involved in upwelling. 



After the light airs that had prevailed up to St. WS 596, a north-north-east wind arose reaching 

 force 4, and this coincided with warm water at the surface at St. WS 597. This suggests at first that 

 wind from the north had driven warmer water southwards, but the same wind persisted at St. WS 598 

 where cooler water was again met with. 



35-30° S: PICHIDANQUE BAY 



Over this 300-mile stretch of coast the water becomes rapidly warmer and the weather 

 as recorded at Valparaiso was calm ; surface isotherms instead of running parallel to the 

 coast slant steeply towards it. 



At the end of May, Pichidanque Bay was still enjoying a period of calm that had been 

 in existence for some considerable time. Warming up of the surface layers had led to a 

 mean inshore temperature of about 14° C, while a temperature of if45° C., found at 

 the surface at Cape Carranza, was here at 40-70 m. depth. A poverty of phytoplankton 

 and zooplankton such as was not found at any other locality examined and extreme 

 depletion of phosphate are signs that for many weeks past the upper layers cannot have 

 been replenished with nutrient salts, or at any rate only on a very modest scale. 



Sections (Figs. 20 and 21) indicate that mild upwelling had been taking place, for the 

 isotherms and isohalines curve slightly upwards near the coast, yet on our arrival on 



