202 MALKUS [chap. 4 



7. Exchange Mechanisms and Fluctuations 



How is sea-air exchange brought about? What physical processes put sea- 

 water into the air and impart the winds' momentum to the ocean? To find out, 

 we must come down from the broad view of planetary energetics and climato- 

 logy to scrutinize the small-scale chaos near the sea surface ; we must look 

 closely at the interaction between atmospheric eddy and the foaming wave 

 crest it strikes as it swirls through its evanescent life in the sub-cloud layer. 



Ocean and atmosphere are turbulent fluids and exchange between them is 

 primarily turbulent transfer. If the boundary turbulence is suppressed, for all 

 practical purposes exchange ceases. Fluid turbulence is one of the most chal- 

 lenging and complex problems at the frontier of physical science. The form of 

 the transfer formulas, derived in the 1930's, was based on then-existing models 

 of turbulent flow, largely carried over from the engineering studies in wind 

 timnels led by Prandtl and Von Karman. The formulas themselves, and the 

 laws upon which they were based, contain empirical hypotheses and constants 

 which, as we saw, must still rely on experiment for their justiflcation and 

 evaluation. 



In the decade 1950-1960, exciting theoretical developments have been made 

 (W. V. R. Malkus, 1954, 1956) in modelling fluid turbulence starting from funda- 

 mental physical principles. With one energetic hypothesis and no disposable 

 constants, this formulation permits prediction of the transports, gradients and 

 motion spectra in very simplified situations in which the turbulence is produced 

 by either imposed heating or shear flow alone under uniform, specified boundary 

 conditions. In the real atmosphere and ocean, the eddying motion is most 

 often driven by a combination of heating and shear flow, neither of which can 

 be quite regarded as externally imposed in a fixed fashion. The boundary 

 conditions are generally neither uniform nor determined independently of the 

 flow. Thus, although the new models have not yet been extended to treat the 

 complex geophysical situation, they do establish a language of inquiry, suggest 

 possibly isolatable prototype problems and point to critical measurements to 

 be made. Within this framework, we shall attempt a physical, mechanistic 

 description of the air-sea boundary. We shall now examine the sizes, structure 

 and behavior of the elements effecting the exchanges of heat, water and 

 momentum, the climatology and dynamic importance of which have been 

 outlined. 



Again, the trade-wind region is used as the starting point. This globally 

 important segment of the interface is fortunately in a relatively steady state. 

 The overall turbulent structure is reproducible when sampled over wide regions 

 and time intervals, and the tropical ocean surface provides as nearly uniform 

 a boundary as the atmosphere ever experiences. Numerous expeditions have 

 provided quantitative information on the important scales of motion and their 

 operation. After describing exchange mechanisms in the normal trade regime, 

 we shall inquire into the nature and causes of their variations. In particular, 

 we shall seek to learn how the exchange processes are organized and altered by 



