A Global Analysis of Stratospheric–Tropospheric Exchange during Northern Winter

Abstract

Using a mathematical formulation of stratospheric-tropospheric (ST) exchange, the cross-tropopause mass flux is diagnosed globally for January 1979. Contributions by physical mechanisms including the diabatic transport and the quasi-horizontal adiabatic transport along isentropes that intersect the tropopause surface are evaluated. Both thermal and dynamical definitions of the tropopause are used. Two regions of zonally integrated mass flux into the stratosphere are found, one over tropical latitudes associated with diabatic transports, and a second over subpolar latitudes associated with adiabatic transports. The ingress to the stratosphere in each of the latitude bands 50°?70°N and 40°?70°S is as intense as that occurring over the tropics, a feature of the global budget not previously documented. Compensating mass outflow from the stratosphere occurs mainly over midlatitudes near axes of strong upper-level westerlies. Large zonal asymmetries are found in the regional patterns of ST exchange. Consistent with the concept of a stratosphere fountain, the tropical inflow to the stratosphere is maximized over the Australasian monsoon. The midlatitude mass outflow tends to be concentrated along stationary wave troughs, roughly in the vicinity of cyclogenetic areas. A mass transport into the stratosphere occurs downstream and poleward of the troughs. The extratropical pattern of time-averaged cross-tropopause mass flux thus appears to be interpretable within the framework of simple physical models on three-dimensional airmass trajectories in baroclinic disturbances. While uncertainties concerning quantitative aspects of the global ST exchange remain, qualitative confirmation of the mass-transport diagnostics is found in independent studies of trace atmospheric constituents. In particular, the finding of mass inflow to the stratosphere at subpolar latitudes is consistent with satellite and aircraft measurements of high water vapor mixing ratios in the low stratosphere over these regions.