Extratropical stratosphere‐troposphere mass exchange

The net mass flux from the stratosphere to the troposphere can be computed from the heating rate along the 380 K isentropic surface and the time rate of change of the mass of the lowermost stratosphere (the region between the tropopause and the 380 K isentrope) following Appenzeller et al. [1996]. Given this net mass flux and the cross‐tropopause diabatic mass flux, the residual adiabatic mass flux across the tropopause can also be estimated. These fluxes have been computed using meteorological fields from a free‐running general circulation model (the finite volume general circulation model) and two assimilated data sets, finite volume data assimilation system (FVDAS), and UK Met Office (UKMO). All of the calculations agree that the annual average net mass flux for the Northern Hemisphere is about 1010 kg/s. There is less agreement on the Southern Hemisphere flux that might be half as large. For all three data sets the adiabatic mass flux is from the upper troposphere into the lowermost stratosphere. This adiabatic flux into the lowermost stratosphere is roughly 5 times larger than the diabatic mass flux into the stratosphere across the tropical tropopause. For the FVDAS the midlatitude tropopause diabatic flux is smaller than UKMO, apparently due to a systematically colder, higher FVDAS tropopause. Both data assimilation systems have a warmer, lower midlatitude tropopause compared to radiosondes, so the mass flux estimates can be considered upper bounds. Finally, we note that the difference in the diabatic mass fluxes between the two assimilated meteorological analyses is much larger than the interannual variability in either.