SECT. 2] LARGE -SCALK INTERACTIONS 99 



within the air-sea system itself for the source of the major instabihties we 

 experience. In doing so, a finger is pointed to air-sea exchange. 



The main roles of the oceans in the global budgets and circulations are, we 

 have seen, as suppliers of the atmosphere's heat (primarily latent) and removers 

 of its momentum. The dishpan experiments (Riehl and Fultz, 1958) suggest 

 that the circulation of a rotating planetary fluid is more critically governed by 

 its energy sources and sinks than by its momentum sinks. The energy sinks of 

 the atmosphere are radiational and to a first order relatively independent of 

 latitude, time and small-scale fluctuations (Rossby, 1959; Budyko, 1956). The 

 direct energy sources for the atmosphere are primarily precipitation, and 

 secondly transport convergence in its own circulation and input of sensible heat 

 from the earth's surface (see Fig. 15, page 135). The indirect source of the first 

 two of these is, as we saw, mainly evaporation from the oceans. The most 

 important demonstration in marine meteorology of the past twenty years 

 (Jacobs, 1951; Montgomery, 1940; Bunker, 1960; Riehl et al., 1951) is that 

 transfer of latent and sensible heat (and momentum) from sea to air is governed 

 largely by just two parameters, the air-sea property difference and the pre- 

 vailing wind speed averaged over about one hour. The clue to the paradox 

 above is that these two parameters at a given place and time are a direct 

 function of the so-called synojjtic pattern, or synoptic scale of motion — the 

 individual circulation features seen on daily weather maps, of sizes 100-1000 km 

 and life-cycle of only several days duration. In middle and high latitudes 

 these are the travelling frontal cyclones and anticyclones, and in the tropics 

 the easterly waves, equatorial vortices, shear lines, and polar troughs whose 

 budgets and dynamics are just beginning to be investigated. Only in the low- 

 level trade-wind region does the average climatological picture bear a close 

 resemblance to the flow one finds on a given weather chart, and even here the 

 synoptic-scale fluctuations are proving more significant than hitherto suspected, 

 particularly with respect to rainfall, which is highly concentrated in limited 

 disturbances. 



Therefore, while the average air-sea fluxes over long periods and large regions 

 are computable from and constrained by planetary budget requirements, the 

 fluctuations which build this picture and give rise to the enormous departures from 

 it, so important to human life, are directly governed by transient atmospheric 

 phenomena, themselves the product of circulation instabilities. 



We shall pursue our discussion of air-sea interaction first by an analysis of 

 exchange calculation methods and their range of validity, leading to a presenta- 

 tion of the overall global climatology of exchange, to the extent this is known 

 today. Using this as a framework, we shall then examine exchange and its 

 fluctuations in some more restricted portions of the system, in which the more 

 detailed operation of the parts have been analyzed, working down finally to 

 the dynamics of exchange in individual synoptic-scale systems which give rise 

 to the variations about the climatic picture they build. In the latter, we shall 

 concentrate primarily upon air-sea interaction in the fuelling and firebox 

 regions of the tropical and equatorial zones. 



