Flux distributions as robust diagnostics of stratosphere‐troposphere exchange

Abstract

We perform the first analysis of stratosphere‐troposphere exchange in terms of distributions that partition the one‐way flux across the thermal tropopause according to stratospheric residence time τ and the regions where air enters and exits the stratosphere. These distributions robustly quantify one‐way flux without being rendered ill defined by the short‐τ eddy‐diffusive singularity. Diagnostics are computed with an idealized circulation model with topography only in the Northern Hemisphere (NH) run under perpetual NH winter conditions. Suitable integrations of the flux distribution are used to determine the stratospheric mean residence time and the mass fraction of the stratosphere in any given residence time interval. We find that the largest mass fraction is destined for isentropic cross‐tropopause transport, with one‐way fluxes that are sustained over a broad range of residence times. Air exiting the stratosphere in the winter hemisphere has significantly longer mean residence times than air exiting in the summer hemisphere because the winter hemisphere has a deeper circulation and stronger eddy diffusion. We also explore the sensitivity of the stratosphere‐troposphere exchange to changes in the circulation by increasing the amplitude of the topography. The resulting more vigorous residual mean circulation dominates over increased eddy diffusion, leading to decreased except for air exiting at high NH latitudes, for which increases. These findings underline that the flux distributions diagnose the integrated advective‐diffusive tropopause‐to‐tropopause transport and not merely advection by the residual mean circulation.