Reply on RC2

Abstract

Stratosphere-troposphere exchange (STE) is an important source of tropospheric ozone, affecting all of atmospheric chemistry, climate, and air quality. Observations and the theory of tracer correlations provide only coarse (±20 %) global-mean constraints. For fluxes resolved by latitude and month we rely on global chemistry-transport models (CTMs), and unfortunately, these results diverge greatly. Overall, we lack guidance from model-measurement metrics that inform us about processes and patterns related to the STE flux of ozone. In this work, we use modeled tracers (N2O, CFCl3) whose distributions and budgets can be constrained by satellite and surface observations, allowing us to follow stratospheric signals across the tropopause. The satellite derived photochemical loss of N2O on annual and quasi-biennial cycles can be matched by the models. The STE flux of N2O-depleted air in our CTM drives surface variability that closely matches observed fluctuations on both annual and quasi-biennial cycles, confirming the modeled flux. The observed tracer correlations between N2O and O3 in the lowermost stratosphere provide a seasonal, hemispheric scaling of the N2O flux to that of O3. For N2O and CFCl3, we model greater southern hemispheric STE fluxes, a result supported by some metrics, but counter to prevailing theory of wave-driven stratospheric circulation. The STE flux of O3, however, is predominantly northern hemispheric, but observational constraints show that this is only caused by the Antarctic ozone hole. Here we show that metrics founded on observations can better constrain the STE O3 flux which will help guide future model assessments.