The mass flux across the tropopause: Quasi-geostrophic theory

The mean meridional circulation in the troposphere is driven by eddy momentum fluxes associated with baroclinic waves. The dynamics of these waves can be described qualitatively by quasi-geostrophic theory. Here, the flow in the vicinity of the storm tracks is modelled using f-plane quasi-geostrophic theory with the tropopause represented as a nearly horizontal surface separating a troposphere of uniform potential vorticity from a similarly uniform stratosphere. Combining an analysis of the mean meridional mass-nux associated with decaying eddies and the cross-tropopause mass-flux generated in the same process, it is possible to estimate the net flux both in the zonal mean and in a quasi-Lagrangian mean framework. The quasi-Lagrangian mean is here defined as an average along potential temperature contours on the tropopause. Consistent with earlier numerical and observational studies, it is found that the zonal mean mass-flux is upwards poleward of the jet, whereas the quasi-Lagrangian mean is downwards.