A Simplified Model of the Modified Hadley Circulation

A model is developed to find the low-latitude upper-tropospheric atmospheric circulation which would exist in the absence of transient eddies. The forcing in the model is zonally asymmetric to take into account the effect of the distribution of oceans and continents on latent heating. Steady state solutions of the equations of motion in such a case have been termed the “modified Hadley circulation” by Lorenz. The model is the set of steady, nonlinear inviscid shallow-water equations with thermal forcing as introduced by Gill, and is solved semianalytically. The modified Hadley circulation found using forcing derived from observed OLR exhibits some similarities to observed features of the tropical and subtropical time-mean upper-tropospheric flow, particularly with regard to the velocity potential, as well as many dissimilarities. The roles of transients and friction as indicated by the model are discussed.