Quantifying isentropic stratosphere-troposphere exchange (STE) of ozone

There is increasing evidence showing that stratosphere-troposphere exchange (STE) of ozone can have a significant impact on the interannual variability and long- term trend of the tropospheric chemistry and radiation budget. Traditional diagnostics of STE ozone flux consider the ozone budget of the lowermost stratosphere by coupling the residual circulation and ozone. However, this method can only provide information of the hemispheric mean ozone flux, and therefore it does not distinguish the exchange of ozone into the tropics from the exchange of ozone into the midlatitudes that may have different tropospheric impacts.

This present study extends the traditional approach from the entire lowermost stratosphere to individual isentropic layers in the lower stratosphere, and therefore distinguishes the meridional location of STE. The specified dynamics (SD) version of the Whole Atmosphere Community Climate Model (WACCM) is used for the estimate of isentropic STE flux. The diagnosed meridional structure of ozone flux is generally consistent with studies with other methods (e.g., tracer trajectories or the budget of tropospheric ozone). Different seasonal cycles of ozone STE are found at different isentropic surfaces, emphasizing different tropospheric impacts from ozone STE over different meridional regions. For isentropes between 350K and 380K, net troposphere-to-stratosphere ozone STE flux peaks in summer. For isentropes between 330K and 350K, the net ozone STE flux peaks in summer too, but it is from stratosphere to troposphere. For isentropes between 280K and 330K, larger net stratosphere-to-troposphere ozone STE flux is found in the Northern Hemisphere and peaks in spring, whereas little seasonal variability is detected in the Southern Hemisphere. Furthermore, the diagnostic enables a partition that links the variability in the STE flux to specific dynamic processes. In particular, the air mass STE flux component associated with the isentropic mixing is found temporally consistent with the variation of the effective diffusivity, along with the migration of the subtropical jet. At the edge of the jet where zonal winds are weaker, effective diffusivity is larger suggesting a larger air mass stirring by mixing, which results in the simulated larger air mass STE flux due to isentropic mixing. As for the ozone STE flux, its seasonal variability is influenced by both the seasonal cycle of the air mass STE flux and the seasonal variation in the ozone mixing ratio.