SECT. 2] LARGE-SCALE INTERACTIONS 279 



year in the Southern Hemisphere off Peru (Rodewald, 1959; Wooster, 1960). 

 El Nino is an interruption in the normal cold water upwelling off the South 

 American coast ( ^ 20°-3°S), and a substitution of warmer and less saline waters 

 flowing down from equatorial regions lying to the north. These waters contain 

 fewer nutrients to support the plankton and small fish population serving as 

 food for larger fishes and the guano birds, which emigrate or starve in great 

 numbers, leading to disastrous consequences for the fisheries and fertilizer 

 industries. There is evidence that El Niiio occurs in years when the trades of 

 both hemispheres are weak^ and the equatorial trough goes farther to the 

 south in winter than usual, relieving the southerly winds (southeast trades of 

 the Southern Hemisphere) which maintain the upwelling, and permitting the 

 invasion of warmer water from the equatorial counter-current (see Fig. 22a). 



Although they are too severely hampered by data limitations to prove any- 

 thing definite as yet, these studies suggest lines of important inquiry to pursue, 

 and further emphasize the need for joint oceanographic-meteorological measure- 

 ments in the tropics on a continuing network basis, particularly in the meteoro-, 

 logically blank southeastern Pacific. Furthermore, they focus attention upon 

 a key question in long-period interaction, namely that concerning the nature 

 and time interval of the response of the upper ocean layers to specified air- 

 circulation changes. It may now be possible to make models or to seek natural 

 situations where the air's input and the sea's reaction can be studied under 

 more rigorously specified or controlled conditions. Namias' work also causes 

 one to wonder how, in such cases, the system goes back to normal. His results 

 suggest that the very existence of a definable "normal" state implies a nearly 

 invariant large outside forcing of the system and a high internal restoring or 

 damping influence which is rather slowly brought to bear. The former is plainly 

 the solar seasonal cycle, while the damping role must be played by the vast 

 reservoir of abyssal waters and their reaction with the surface layers. That one 

 or both of these stabilizing influences is not quite perfect is implied by the 

 existence of still longer period variations in sea and air resulting in slow 

 climatic change. Our final discussion concerns an oceanic warming over several 

 decades, which is about the limiting time interval amenable to even a semi- 

 quantitative description of what actually changed and by how much. 



b. Variations in interaction over several decades 



In trying to approach interaction and exchange variations and their con- 

 sequences over decades, we are in an even more precarious position. For 

 example, in equation (1) for the energy budget of an ocean column, let us 

 suppose that, in a tropical situation, all terms except Qe and S are unchanged. 

 We find that if a 200-m deep layer is considered, an increased evaporation of 

 about one per cent, or an increased heat loss of roughly 3 cal per cm^ per day 



1 It is interesting to note that 195.3, in which occurred the "weak trade" of the western 

 North Atlantic discussed in Section 7, page 202, was an El Nino year (only moderately 

 intense). 



