The Forcing of the Meridional Circulation of the Isentropic Zonally Averaged Circumpolar Vortex

Abstract

For the axisymmetric general circulation, Eliassen (1951) showed that stable meridional circulations are controlled by large-scale diabatic heating and friction torque. Because the real atmosphere is longitudinally disturbed, a direct meridional response to large-scale diabatic heating is not readily found by conventional zonal averaging in isobaric coordinates. By introducing, the concept of a zonally averaged general circulation in isentropic coordinates, a direct meridional response to diabatic heating is isolated. Using an approach analogous to Eliassen's, this study has extended the concept of diabatically forced isentropic meridional circulations to include the role of angular momentum torques within the circumpolar vortex. The absolute angular momentum torques comprise three terms: divergence of eddy relative momentum transport, friction and zonal pressure torques. The heating is composed of hemispheric-scale diabatic processes. For a hydrodynamically and statically stable atmosphere, a configuration of tropical heating and higher latitude cooling coupled with a combined upper tropospheric sink and low tropospheric source by the angular momentum torques support a Hadley-type circulation spanning the entire hemisphere. In low latitudes, an upper tropospheric sink by friction torque and divergence of the eddy relative momentum transport combine with a lower tropospheric source by friction torque to force a dominant ageostrophic mode for the meridional branch of the Hadley cell. Within large-scale middle latitude waves, the zonal pressure torque is a principal mechanism for the transfer of angular momentum from upper to lower isentropic layers and therefore is the dominant upper tropospheric sink and lower tropospheric source of angular momentum in high latitudes. Due to the dominance of the pressure torque in extratropical latitudes, the poleward branch of the upper layers and equatorward branch of the lower layers are mainly geostrophic and mask indirect ageostrophic motion that is forced by friction torque and the divergence of eddy relative momentum transport.