On the Maintenance of the Trade Winds

Abstract

A relation between the large-scale features of the lower trade stream and the effects of small-scale convective motions is investigated. The small-scale processes impose constraints upon the over-all circulation through non-adiabatic heating and the production of shearing stresses. These constraints may be formulated in terms of a theoretical model describing the mean flow in a section along the northern portion of the Pacific trade, which has previously been studied observationally in some detail. In a simplified frictionless model with non-adiabatic heating the flow exhibits qualitatively many of the important features of the trades, such as subsidence, and a downstream acceleration, increased vertical wind shear, and pressure drop. The motion and pressure fields become numerically related to the heating as observed, however, only upon the introduction of frictional stresses. It is found possible to describe the frictional stress distribution in the lower trades without resorting to direct measurements, which thus provide a later independent check of the model. The last sections of the paper consider possible quasi-steady rearrangements between the fields of motion, heating, and stress which might be regarded as steps in the slow fluctuations of the system about its average condition. This is done to inquire whether changes in any one of these properties will produce readjustments in the others acting to restore the mean or to accelerate the departure from it. Although such an analysis is handicapped by lack of knowledge concerning the interaction between the heating and stress distributions, one sample calculation suggests that a reduction of heating acts to relieve the subsidence and might thus be expected to restore the convection and thereby the heating. A stable coupling between small- and large-scale motions in the low-level trades is thus indicated, which may play a role in maintaining the high steadiness of the flow. The fact that the wind steadiness vanishes rapidly above the convective layer is clarified if this model is acceptable.