The Hadley Circulation as a Radiative–Convective Instability

Abstract

Hadley cell simulations over a tropical ocean are reported that suggest that the emission and absorption of thermal radiation by clouds plays an important role in the dynamics of the Hadley circulation. In particular, inclusion of interactions between clouds and radiation causes the tropical atmosphere to become unstable to large-scale convective circulations driven by differential radiative heating between cloudy and clear regions, even when the sea surface temperature and the solar forcing do not vary with latitude or time. Zonally symmetric circulations take the form of an equatorially asymmetric convective cell, with ascent in one hemisphere and descent in the other. This may explain why there is usually only one intertropical convergence zone that is located away from the equator even when the sea surface temperature maximum is on or near the equator. The conclusions of this paper are tentative because the treatments of convection, radiation, and cloudiness are highly simplified. However, if the simulated radiative?convective instability exists in nature, it also may explain the tendency of the tropical atmosphere to divide into large areas that are convectively suppressed, punctuated by smaller areas of intense convection.