N₂O as a regression proxy for dynamical variability in stratospheric trace gas trends

Abstract. Trends in stratospheric trace gases like HCl, N₂O, O₃, and NO_y show a hemispheric asymmetry over the last two decades, with trends having opposing signs in the Northern and Southern Hemispheres. Here we use N₂O, a long-lived tracer with a tropospheric source, as a proxy for stratospheric circulation in the multiple linear regression model used to calculate stratospheric trace gas trends. This is done in an effort to isolate trends due to circulation changes from trends due to ozone depleting substances. We use measurements from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) and the Optical Spectrograph and InfraRed Imager System (OSIRIS), and model results from the Whole Atmosphere Community Climate Model (WACCM). Trends in HCl, O₃, and NO_y for 2004–2018 are examined. Using the N₂O regression proxy, we show that observed HCl increases in the Northern Hemisphere are due to changes in the stratospheric circulation. We also show that negative O₃ trends above 30 hPa in the Northern Hemisphere can be explained by change in the circulation, but that negative ozone trends at lower levels cannot. Trends in stratospheric NO_y are found to be largely consistent with trends in N₂O.