Stratospheric‐tropospheric exchange processes

Qualitative descriptions and quantitative estimates are presented of various transport processes between stratosphere and troposphere. The seasonal changes of tropopause heights account for a flux of about 10% of the mass of the stratosphere in one hemisphere during the course of 1 yr. This flux is balanced approximately by the seasonal shift of stratospheric air masses between the northern and southern hemispheres. Vertical transport through the Hadley cell transfers approximately 38% of the mass equivalent to one hemispheric stratosphere through the tropopause per year. This appears to be the most effective of all transport mechanisms. Large‐scale eddies of the scale of cyclones and anticyclones transfer about 20% of stratospheric air through the tropopause per year. Small‐scale and mesoscale diffusion processes at tropopause level probably account for the transfer of only 1% of stratospheric air. These mass flux estimates are in reasonable agreement with observed residence times of stratospheric pollutants.