Cross-equatorial flow and the Hadley Cell

Abstract:

The usual picture of solsticial Hadley Cells gives the impression of air moving from one hemisphere to the other in a smooth manner. The classic theories of the Hadley Cell are based on angular momentum conservation with the additional consideration of some mixing. However potential vorticity (PV) conservation, which includes potential temperature conservation as well as angular momentum/vorticity constraints, tells us that air cannot move easily from one hemisphere to the other. Near the surface, topographic and frictional processes, aided by diabatic processes, are able to overcome the PV constraint. However in the upper troposphere there are no equivalent frictional processes. The cross equatorial mass flux occurs in PV streamers that are mixed in in the winter subtropics or reach and amplify the anticyclonic side of the winter subtropical jets. The cross equatorial motion in the upper troposphere takes place in westerly waves on the oceanic ITCZs and in convective events in monsoon heating regions. The extent to which these aspects are important for weather and climate will be discussed.