The Evaporation-Precipitation Cycle of the Trades

Abstract

The area distribution of surface wind strengths between 30° S and 30° N is expressed as a function of the median velocity, on the basis of climatological data. This is used to obtain estimates of evaporation and energy dissipation over the same area.

The main energy supply of the trade circulation is due to condensation in the equatorial trough. It depends partly on evaporation into the trade winds at some earlier time. A study of the energy balance shows that this may lead to oscillations with a period of about 3 weeks; corresponding to the mode of the time which elapses between the evaporation and the precipitation of the same vapour molecules.

Disturbances or fluctuations in the direct meridional circulation of the sub-tropics may be initiated by changes in the infra-red cooling rate and by changes in the height of the tropopause. It is suggested tentatively that this could be caused by changes in the ozone distribution above the tropical tropopause.