SECT. 2] LARGE-SCALE INTERACTIONS 95 



Study of the energetics of planetary circulations thus logically begins with 

 the exchanges at the tropical ocean surface and from there ascends into the 

 tropical atmosphere. The fluxes and conditions near the interface have received 

 intensive study, using the data provided by the post-war expeditions of the 

 Woods Hole Oceanographic Institution (Wyman et al., 1946; Bunker et al., 

 1949; Bunker, 1955; Malkus, 1958). From the sea surface, the water vapor is 

 stirred by turbulent convection, through a well-mixed layer several hundred 

 meters thick, up to the level of water-vapor condensation. From there, it is 

 pumped aloft by the so-called "trade-wind cumulus" clouds, a picturesque 

 feature of tropical skies at all hours of the day and night (Fig. 2). These pro- 

 cesses create a moist convective layer, gradually deepening along the airflow, 

 from 6-10,000 ft in vertical depth. In the trade-wind zones of high evaporation, 

 most of the moisture is retained in vapor form rather than being rained back 

 into the oceans. In both hemispheres the trade-winds ship their accumulated 

 water vapor equatorward, at a rate of energy export easily two orders of 

 magnitude greater than the rate of kinetic energy consumption by all the 

 global winds and sea currents combined. 



It is in the so-called "equatorial trough zone", a belt about 10°-latitude wide 

 on either side of the thermal equator, that the combustion or condensation of 

 the water-vapor fuel takes place. The "cylinders" in which the vapor is con- 

 verted into liquid water droplets, releasing their 600 calories per gram of water 

 condensed, are a relatively few giant cumulonimbus cloud towers (Fig. 3), over- 

 grown brothers of the trade- wind cumulus mentioned earlier. These "hot 

 towers", though averaging several thousand active at a given instant around 

 the globe, are not uniformly or randomly scattered over the equatorial seas 

 but are highly concentrated into a very few (about 20-50) vortical or wave-like 

 storms, several hundred kilometers in horizontal dimensions, which go through 

 a life cycle of a few days each (Malkus and Ronne, 1960). It is small wonder 

 that the global circulation system operates in fits and starts, with its evanescent 

 cylinders, of transient numbers, whose very existence depends upon the 

 vagaries of the flow itself! 



The giant cloud towers play the role of fuel pump as well as combustion 

 cylinder in the atmospheric heat engine ; their towers commonly reach 35,000 ft 

 and frequently penetrate the tropical tropopause at about 50,000 ft elevation. 

 The water-vapor fuel is thus converted into sensible heat and potential energy, 

 the latter due to the work done against gravity by the cloud buoyancy. At 

 high levels in the tropics, the average air flow is, as it must be, poleward, but it 

 is much less uniform both spatially and temporally than the almost mono- 

 tonous equatorward stream of the steady trade winds. Thus a direct meridional 

 circulation cell exists in the tropics (Palmen, Riehl and Vuorela, 1958 ; Tucker, 

 1959). In the low levels, the equatorward component is a rather uniform 2-3 m/ 

 sec superposed on a steady easterly trade regime; aloft, the jDoleward flow 

 component averages out the same in the long run, although it occurs in bursts 

 and preferred longitudes, due to the irregular and restless character of upper 

 level tropical circulations. 



