A first-order understanding of the large-scale tropical circulation is often founded on the idea of a mean meridional circulation (MMC), the Hadley circulation, which is characterized by thermally-driven overturning within hemispheric Hadley cells. Axisymmetric models in which the Hadley circulation is driven by zonally-uniform atmospheric heating provide a theoretical basic for our understanding of the MMC in the tropics (e.g., Held and Hou, 1980; Lindzen and Hou,1988; Hou and Lindzen, 1992; Schubert et al.,1991; Fang and Tung, 1997). These models simulate changes in the circulation in association with changes in the magnitude, position and shape of an imposed heating function, and these results have been used to explain the seasonal behavior of the Hadley circulation.

But an examination of heating fields shows that the tropical heating that drives the Hadley circulation is not zonally uniform. Instead, the heating is concentrated into three centers - one over Africa, one over South America, and a third over the maritime continent/western warm pool of the Pacific. The purpose of this work is to explore the implications of this zonal variation in heating for the structure and seasonal behavior of the Hadley circulation.

Four multi-year GCM simulations with different boundary conditions are discussed. One has zonally-uniform observed SSTs and no continents, a second adds flat featureless continents with the same zonal SSTs, and a third includes topography and realistic land surface characteristics. The fourth simulation mimics the zonally-uniform forcing used in the axisymmetric models. An examination of the differences in the Hadley circulation and horizontal momentum balances shows that the continents play different roles in modifying the Hadley cells in the winter and summer hemispheres. In the winter cell, for example, enhanced surface friction and boundary layer development associated with land surfaces play a major role in strengthening the Hadley circulation. In the summer hemisphere, the presence of land causes a decrease in the meridional height gradient and a weakening of the Hadley circulation. This suggests that zonal asymmetry of the tropical heating is an important feature of the circulation system. The Hadley circulation in the GCM simulation that mimics the axisymmetric models does not control the seasonal evolution of the Hadley circulation by the same mechanisms.

Axisymmetric model results suggest that the Hadley circulation becomes stronger when the heating is farther off the equator, and when the heating is more concentrated. This behavior is duplicated in the GCM simulation that is designed to mimic the forcing imposed in the axisymmetric models. However, the other three simulations, which have boundary conditions based on observations, as well as the NCEP reanalysis, do not show similar correlations between the strength of the Hadley circulation and the structure of the heating field. The physical mechanisms that lead to the observed and simulated seasonal variations are analyzed, and compared with those captured in the axisymmetric framework.