Cross-tropopause ozone flux: diagnosing methods and interannual variations

Ozone is a key chemically reactive greenhouse gas in the atmosphere affecting air quality and climate. Cross-tropopause flux is a major source for tropospheric ozone and thus provides an important pathway for chemistry-climate coupling. However, large uncertainties exist in its estimations based on both models and observations. Here we diagnose the global stratosphere-troposphere exchange (STE) flux of ozone using the Whole Atmosphere Community Climate Model (WACCM) simulation with different, but consistent methods, such as ozone budgets in different regions (i.e., troposphere and lower most stratosphere) and fluxes derived from different vertical velocities (e.g., transformed Eulerian mean circulation and diabatic heating). We analyze the differences between different diagnostics, identify the causes, and evaluate their relative accuracy. We examine the sensitivity of STE ozone flux to major factors in WACCM, such as tropopause definition, nudging height to the offline meteorological field, and interannual meteorological variability (e.g. El Niño vs. La Niña). Based on these results, we recognize key processes driving the interannual variability of stratospheric ozone influx.