Zonal mean circulations for two conserved meridional coordinates

Abstract

While many investigations of the atmospheric zonalmean circulation have been published using various vertical coordinates, this article concentrates on meridional coordinates other than latitude. Potential vorticity q and potential temperature θ are selected, with height as the vertical coordinate. Both q and θ are conserved in adiabatic frictionless flow. Although this helps in the interpretation of the results, both choices are problematic because of contortions in the contours. The related problems are solved by integrating over suitable zonal tubes. The isertelic mean circulation exhibits global cells in the upper troposphere and separate shallow hemispheric cells near the ground. Mean q fluxes are derived from the mass circulation. The mean circulation in (θ, z) coordinates is similar to that in (ϕ, θ) coordinates and has hemispheric direct cells. Mean θ fluxes follow from that. Both circulations are forced, in the sense that they can be derived and understood if the zonal mean heating is known. This is demonstrated by explaining, for example, the equatorward surface flow in (θ, z) coordinates and the shallow boundary-layer circulation in the isertelic system.