Studies of axisymmetric circulations have provided many insights on the behavior of the Hadley circulation. However, most of these studies have focused on dry atmospheres and do not discuss the dynamical feedbacks between the large-scale flow and convective activity. The present paper investigates how such interactions play a major role in determining both the structure of the Hadley circulation and distribution of precipitation in the tropical regions.

In particular, it is shown that the cross-equatorial Hadley circulation can exist in two different regimes. A first regime corresponds to the classic description of the Hadley circulation, with air rising in the summer hemisphere, crossing the equator at the tropopause, subsiding in the subtropics of the winter hemisphere, and returning to the the summer hemisphere near the surface. The second regime differs in that an additional region of large-scale ascent is present near the Equator in the winter hemisphere and in that the return flow crosses the equator in the middle troposphere instead of occurring near the surface. These two regimes also differ significantly in the distribution of precipitation. In the first case, a single maximum of precipitation is present over the warmer regions of the summer hemisphere. In the second case, a secondary maximum of precipitation is located on the winter side of the equator.

The existence of these two regimes can be explained from dynamical constraints on the flow in the planetary boundary layer (PBL). These constraints strongly limit the cross-equatorial mass transport within the PBL, and it is shown that the transition between the two regimes can be obtained by changing the depth of the PBL. In numerical experiments, this transition is found to occur for a PBL thikcness of approximately 1500m, which indicates that the same dynamical constraints can play an important role in the distribution of precipitation in the Earth's atmosphere.