Abstract/Summary

The global perspective presented here is built on earlier papers discussing the dynamics of the upper branch of the Hadley Cell in the two solsticial seasons. The role of the tropics is made explicit in a conceptual model that is presented and evaluated. The fluctuation of deep tropical convection in longitude and time is seen as crucial. The filamentary outflows from such convective events move westwards and across the equator deep into the winter hemisphere. The horizontal tilt of the cross-equatorial flow implies a significant upper tropospheric flux of westerly momentum from the winter tropics to the summer hemisphere. These properties are related to the cross-equatorial propagation of wave activity triggered by deep tropical convection in the summer hemisphere. The filaments carry with them near-equatorial values of absolute vorticity and potential vorticity. After turning anticyclonically, some filaments move eastwards and polewards to the equatorial edge of the winter subtropical jet. There is strong evidence they can interact with the eddies on this jet and enhance their poleward westerly momentum flux. In the global perspective, tropical and extra-tropical systems and the interaction between them are all important for the dynamics of the upper branch of the Hadley Cell.