SECT. 2] t.ak(!K-.S(:alk interac:tions 163 



used to drive the flow. In tlie atmosphere, as in tlie ocean, pressures are pro- 

 duced liydrostatically : warm Huid columns are less dense and exert less pressure 

 near the ground than cold ones, so that horizontal gradients in pressure are 

 created when warm and cold columns exist side-by-side. The dynamically 

 important role of the oceanic heat input to the trades is thus to maintain a 

 downstream-directed pressure force by adding to the heat content of the air as 

 it flows equatorward. 



Physically, then, the trade-wind heat source arises because, in the moist 

 layer, the terms Qs + LP are greater than the radiation sink, Ba. Convection 

 distributes and releases the added energy, so that individual air trajectories 

 move toward higher h = CpT + Agz'^Cpd, or toward higher potential tempera- 

 ture, as illustrated in Fig. 31a. Riehl and Malkus (1957) have shown theoreti- 

 cally that under such conditions the pressure head is simply related to the heat 

 source as follows : 



Jzjs U 



Apo = -K \ —dsdz (31) 



in a unit-width section along the flow, where the externally imposed pressure 

 drop at the top of the heat source is zero, as it is in the trades. In equation (31), 

 Apo is the difference in surface pressure between the end points of integration 

 along a trajectory s, u is the horizontal air velocity, iT is a theoretically 

 determined constant (;^1.4x 10^^ g2 cm~2 sec~2 cal~i in the trades) and the 

 vertical z integration is performed through the depth of the moist layer. The 

 net heat source, H (in cal g~i sec~i), is evaluated from the potential temperature 

 increase following the trajectories (solid arrows. Fig. 31a). In the Pacific trade, 

 with a mean wind speed of about 6 m/sec, the observed pressure drop of about 

 1 mb per 500 km trajectory distance is nicely accounted for using (31) and the 

 heat source, which increases the average potential temperature of the moist 

 layer by about 0.6°C in a day's travel (Fig. 31b), In the Caribbean winter, the 

 slightly greater net warming is more than compensated by a doubly large 

 wind speed. The calculated downstream pressure drop of about 1 mb per 

 1000 km, however, is in good agreement with climatic charts and those of 

 Colon for the selected months. 



Thus the increase in heat content along the lower trades, due to exchange 

 with the sea surface, makes a vital contribution to the balance of forces, 

 permitting the flow-sustaining pressure head to be created locally. This portion 

 of the meridional cell is thus a directly maintained thermal circulation. Above 

 the level where the convection distributes the heating, the internally developed 

 downstream pressure drop disappears and the wind flow becomes more nearly 

 geostrophic. In the very high troposj^here, the pressure head sustaining the 

 antitrade westerlies is created in the equatorial trough zone, from combustion 

 of the water-vapor fuel shipped there by the lower trades. We shall next look 

 into the operation of the "firebox". 



