Impermeability and Constraints on Tropical–Extratropical and Interhemispheric Communication (Invited Presentation)

Predictability is linked to the manner in which remote areas of circulation features communicate. Recent research has indicated a strong interdependency of tropical and extratropical variance on quasi-biweekly and intraseasonal time scales over large scales. It evolves that these latitudinal interactions can be understood in terms of potential vorticity substance (PVS) that is conserved between two isentropical surfaces even in the presence of diabatic heating and dissipative processes. This conservation is expressed in the “impermeability theorem” of Haynes and McIntyre (1987) that states that the zonally averaged PVS between the two isentropes must be zero across a latitude circle. This can only occur with the northward flux of PVS associated with divergent circulations (e.g., the monsoon meridional circulation) located at some longitude band is matched by the equatorward transport of PVS by remnants of the breaking Rossby waves. The recursive nature of the breaking Rossby waves create regions of westerlies, known as the “westerly ducts” that extend towards the equator and exist in the exit regions of the subtropical jet-streams in each hemisphere. Thus, both divergent and Rossby wave dynamics are collaborators in the communication between the tropics and the extratropics and each possess specific geographical locations.

The impermeability theorem, though, is a global construct such that imbalances in one hemisphere may be compensated by interhemispheric PVS transports either through the alternation of divergent circulations or changes in PVS fluxes in the westerly ducts. PVS conservation allows a possible understanding of why the annually averaged top-of-the-atmosphere albedo and outgoing longwave radiation are nearly identical in each hemisphere. It may also explain when the annually averaged precipitation (rate or volume) is essentially identical between the two hemispheres and why on very long time scales a global response occurs when there is asymmetric orbital forcing between the hemispheres.