144 MALKUS [chap. 4 



maintains the meridional circulation, which we have said is the outstanding 

 feature of this part of the heat engine. To understand how the energy sources 

 are utilized to drive air and sea currents and how, in turn, these distribute and 

 constrain the sources, we must incorporate results of energy-budget studies 

 into models relating forces and motions. In the next section of this chapter, 

 two examples build the bridge between energetics and dynamics in the vital 

 fuelling (trade-wind) and firebox (equatorial) regions of the atmosphere. 



5. Heat and Water Exchange and its Role in Tropical Circulations 



The first step toward connecting energy exchange and the dynamics of 

 large-scale circulations have been taken in tropical meteorology, where neces- 

 sity, chance and invention have combined to permit isolation of tractable 

 problems. The ultimate goal is to couple energy source to motion in a predictive 

 manner through the hydrodynamic equations, the key link usually lying in the 

 first law of thermodynamics, and to learn physically how the released heat 

 produces the pressure gradients which drive the wind systems. 



Commonly, the foundation for such approaches has been application of the 

 methods outlined herein to balance the heat-energy conservation law (27) for a 

 given portion of the atmosphere or along the trajectory of a circulation branch. 

 The crucial new step is that Qva, the atmospheric flux divergence of sensible 

 heat plus potential energy {CpT -\-Agz, henceforth often simply called h or 

 "heat energy") is now formulated explicitly so that it may be evaluated 

 directly from meteorological data and broken down into its contributions from 

 the various scales of motion in space and time. We thereby use and extend our 

 insight into the role of the different-sized physical phenomena, such as eddy, 

 cloud or tropical storm as energy transporters or transformers within a region ; 

 we are able to inquire mechanistically how that portion of the heat engine 

 performs its climatologically deduced function in the general circulation 

 (Figs. 13-15). 



Usually the data available are not adequate to evaluate all terms in (27) with 

 independent certainty, so that other equations, j)articularly (1) for the heat 

 balance of the ocean surface, that for water conservation, and the transfer 

 formulas, are introduced. As we shall see in the examples, the added labor is 

 well repaid by the further insight afforded (and critical questions raised) 

 concerning physical processes in sea and air, and their mutual interaction. 



A . Studies of the Energy Transactions in the Trade- Wind Zone 



Sea-air exchange is at its maximum in the trade-wind region and so are its 

 direct dynamic consequences. As we saw, the latent heat acquired there is the 

 major energy source for atmospheric motions and, as we shall see, even the four 

 times smaller sensible heat accumulation is vital in driving this portion of the 

 low-latitude meridional cell. 



