2o6 DISCOVERY REPORTS 



from January to April, when the trough of low pressure usually associated with regions 

 north of the Equator shifts southwards, there follows a complete change of wind: 

 the south-east trades give place to winds from the north; while north-east trades 

 from the Atlantic blow with increased strength over the Gulf of Panama and enter the 

 South American Continent as north-west winds. These are monsoon winds and bring 

 torrential rains to a district whose mean annual rainfall is normally less than half an 

 inch. A southerly flow of equatorial water from the Gulf of Panama converges with 

 the Peruvian coast, sometimes reaching as far south as Callao and Pisco, and it raises 

 the inshore temperature by as much as io°. The consequences of this to marine 

 life have frequently been described. But of immediate interest is the simultaneous 

 appearance of upwelling off the inner part of the Gulf of Panama. During these few 

 weeks the coast in the Gulf of Panama has the characteristics of a windward shore and 

 upwelling results. Thus in a region usually bathed by the light hot water of the Equa- 

 torial Counter-current, a region characterized by conditions which are usually the 

 antithesis of upwelling on account of the convergence of the current with the coast and 

 the steep thermocline in the upper layers, a cold current is found welling up and flowing 

 southwards in the wake of El Nino, and reaching sometimes at least as far south as 

 the Equator. Schott has already emphasized the interest of this in dynamic oceano- 

 graphy by pointing out the correspondence between periodical rises and falls of tem- 

 perature in the Gulf of Panama with inverse falling and rising of temperature in the 

 Nino region, and the fall and rise of temperature in this region with barometric oscilla- 

 tions at Puerto Chicama. 



Serial records over a number of years at two or three coastal stations make these 

 correlations possible. Such opportunity is denied to a ship on a brief cruise, but we 

 were able to record changes in hydrological conditions apparently following changes of 

 wind in three localities. At all of them upwelling was increased in the presence of winds 

 from the east and south but diminished in the presence of winds from the west and 

 north. Other factors were diff^erent in each of the localities, which must therefore be 

 examined individually. 



Antofagasta 



The change in temperature conditions at Antofagasta, both at the surface and be- 

 neath it, has been noted on pp. 141-5, and parallel changes in the sections illustrating 

 phosphate content on pp. 182 and 185. It was shown that on steaming out from the 

 shore strong easterly and southerly winds were blowing, and the temperature indicated an 

 active state of upwelling: phosphates at the surface were rich (Figs. 26-28 and 58, 59). 

 On the return journey the wind had changed to the north, and both cool water and rich 

 phosphates had vanished from the surface ; that is to say, surface isotherms were found 

 closer to the coast (Fig. 28). 



Several mechanisms may have been acting in this change. Such a shift in the position 

 of surface isotherms would result if the cool inshore water and the warm offshore water 

 oecame thoroughly mixed together. Mixing is of course a feature in any region of 

 turbulence, but such extensive mixing is unlikely in so short a period as two days. Then 



