89 

 Figure 16 suggests the close relationship between active sea-air fluxes 

 and cvunulus convection. Although a temperature difference of 0.17°C may- 

 appear very small, it amounts to 30-50 percent of the common sea-air 

 temperature difference in the trade-wind region. Therefore, we may presume 

 that, over these warm spots, the flux of latent and sensible heat from sea 

 to air is somewhat greater than in the surrounding regions, as are the 

 vertical fluxes of heat and moisture in the well-mixed layer that character- 

 izes the air below cloud base. 



The important relation between variations in sea-air temperature dif- 

 ference, exchange and cumulus development is illustrated in the semi-diurnal 

 cycle in convection over the tropical oceans. 



A midday minimum in oceanic cumulus convection was long suspected and 

 finally has been substantiated, primarily by the two cruises of Garstang 

 (I96it-) with the Research Vessel CRAWFORD . Oceanic cumuli flourish best near 

 dawn axid sunset and undergo a suppressed period during the noonday hours . 

 At least part of the cause appears to lie in a similar cycle of sensible 

 heat fliix (Figure 17) . There is only a weak variation in latent heat flux 

 (Figure 18). Figure 19 shows that the daily variation in sea-air temperature 

 difference provides the main key to the transfer cycles, although there is 

 also a weak wind minimum in the early afternoon, related by Lavoie (1963) to 

 the S2 component of atmospheric tide. 



Interestingly enough, the convergence - divergence cycle of this tidal 

 wind is quite nicely in phase with the cloudiness ( Figure 20) . Shibata 

 (l96ii-) showed that the amplitude of the convergence ( "^ 2 - 3 x 10~^ sec~^ ) 

 and its attendant vertical motions can be related quantitatively to the 

 cumulus variation. Which is cause and which effect? Does the semi-diurnal 

 cycle in air-sea heat transfer help to drive the atmospheric tide via the 

 cumulus process? Or does the tide, maintained solely in the high atmosphere, 

 add to the air-sea interaction cycle in maintaining that of the cumulus? A 

 very little more research could go a long way toward answering this intriguing 

 question. 



A recurring thread is clearly woven throughout all these examinations 

 of the role played by exchange in tropical meteorology: This role is 

 directed by clouds. How the atmosphere uses what it receives from the sea 

 depends on what sort of clouds its circulation enables it to produce . And 

 this, in turn, depends on large-scale atmospheric dynamics, or the air 

 motions throughout the depth of the troposphere. 



These points are illustrated by a comparison between the operation and 

 energetics of the trade-wind zone and the equatorial trough. The amount 

 of sensible and latent heat that the sea puts into the air does not differ 

 highly between these two regions of the tropics. And yet, its usage is quite 

 different, as is the cloud structure. 



