164 MALKUS [chap. 4 



B. Energy Transformations in the Equatorial Trough Zone 



'Vhe poleward (subsiding) portions of the meridional cell of the tropics has 

 been combed by research ships and airplanes, studying the air-sea boundary 

 fluxes and the eddies and tropical clouds which effect the energy transports. 

 A connective structure has been framed by several large-scale budget studies 

 and the first links between energetics and dynamics forged by introduction of 

 the heat sources into the hydrodynamic equations (Malkus, 1956). Despite 

 deficient data, these studies have been aided by relatively simple conditions : a 

 wdde separation in space and time scales exists between the vital physical 

 processes, namely convective turbulence and the large-scale circulation itself, 

 which is steady and two dimensional. Therefore, to fill in the climatological 

 outline of the region's function (Figs. 13-15), we have at least the framework 

 of the dynamic skeleton and the mechanistic muscles. 



The case is quite different in the equatorial zone, where in many longitudes 

 the trades of opposing hemispheres clash and ascend, lifting and converting the 

 imported water vapor. The physical situation is both more complex and far 

 more fragmentarily explored. Little connective tissue has joined the few 

 widely-separated synoptic, dynamic and strictly oceanographic investigations. 

 Climatology suggests what energy conversions and transports the region must 

 undertake, but the greater unsteadiness of the air circulation does not permit 

 as ready translation of average magnitude into dynamic process. 



As the trade-wind air flows westward and equatorward, away from the 

 steadying influence of the subtropical high -pressure ridges and toward the low 

 pressure trough associated with the thermal equator, the average depth of the 

 moist convective layer grows ; the mean strength of the trade-wind inversion 

 weakens and gradually vanishes. But there is observational evidence that this 

 occurs in bursts, or alternations between intense convective build-ups and un- 

 disturbed trade regime, showing time fluctuations and preferred locations. 

 Quite possibly the ascending portion of the "meridional cell" is a somewhat 

 fictitious average over intermittency. In other words, an intermediate scale of 

 motion, the so-called "synoptic scale" in meteorology, appears to intervene and 

 play a vital role. As we have suggested, such an alteration in atmospheric flow 

 implies heavy consequences for the interaction between sea and air and the 

 feedback between their circulations. 



In 1958, a large-scale budget study of the equatorial region was attempted 

 by Riehl and Malkus. Although even more severely hampered by data deficiency 

 than similar efforts in the trades, it has laid a specific framework for further 

 critical exploration. Moreover, even the qualitative completion of a heat- 

 energy balance in this zone required a new and remarkable mechanistic hypo- 

 thesis which stands to be tested by the more modern observational tools, such 

 as the satellite programs. 



Experience gained in the trade-wind (and rotating dishpan) studies has 

 shown the advantage of natural co-ordinate systems selected for the problem 

 to be treated, rather than the polar co-ordinates customarily used. Since we 



