Stratospheric-Tropospheric Exchange Based on Radioactivity, Ozone and Potential Vorticity

Vertical cross sections of potential vorticity, computed and contoured by band, and hemispheric distributions of potential vorticity, computed and contoured by machine, are compared to both discrete and continuous measurements of radioactivity made during Project Springfield. Comparisons made in the upper troposphere and lower stratosphere show a positive correlation between potential vorticity and radioactivity of stratospheric origin. The radioactivity measurements and machine-computed isentropic trajectories prove that stratospheric air from high latitudes is transported southward and downward into the troposphere as the tropopause folds. Changes in potential vorticity and the concentration of radioactivity along the trajectories also prove that subsynoptic-scale mixing destroys the extruded stratospheric layer in the troposphere. The mass outflow from stratosphere to troposphere associated with tropopause folding is compensated by a quasi-steady inflow along the tropopause. This inflow is necessary to maintain the large radioactivity gradients observed at the tropopause and to amount for the observed mixing of stratosphere and tropospheric tracers on the stratospheric side of the tropopause. Tropospheric air entering the stratosphere at low latitudes with large values of potential temperature acquires radioactivity, ozone and potential vorticity by mixing in the stratosphere. Radiative cooling at high latitudes systematically moves the mixture to lower potential temperature levels in the cyclonic stratosphere, where the large-scale baroclinic waves transport it southward and the amplifying vortices fold it into the troposphere.