Hemispheric Asymmetry in Stratospheric Trends of HCl and Ozone: Impact of Chemical Feedback on Ozone Recovery

We use trace gas profiles from Atmospheric Chemistry Experiment - Fourier Transform Spectrometer (ACE-FTS) satellite measurements and the TOMCAT three-dimensional chemical transport model to diagnose stratospheric trends in O₃, HCl and N₂O. We find that the 2004–2021 ACE-FTS trends exhibit a clear lower stratosphere (LS) interhemispheric asymmetry with positive (negative) O₃ and N₂O (HCl) trends in the Southern Hemisphere (SH), and trends of opposite sign in the Northern Hemisphere (NH). The trends are larger for the shorter time period of 2004–2018. TOMCAT qualitatively agrees with the ACE-FTS LS N₂O and HCl trends, confirming that transport variability drives such patterns, despite some discrepancies for O₃. An additional model simulation is used to quantify the sensitivity of O₃ to long-term changes in chlorine and bromine and thus determine the chemical contribution of the spatially varying halogen trends to both observed and modeled O₃ trends. Overall, the recent dynamically induced variation in mid-latitude LS halogen abundance has, through chemical feedback, accentuated the O₃ recovery signal in the SH and delayed it in the NH, reflecting the enhanced dynamical variability of the NH. These results further indicate the complexities that exist in the search for the signal of ozone recovery in the mid-latitude LS.