Investigating Zonal Asymmetries in Stratospheric Ozone Trends From Satellite Limb Observations and a Chemical Transport Model

This study investigates the origin of a zonal asymmetry in stratospheric ozone trends at northern high latitudes, identified in satellite limb observations over the past two decades. We use a merged data set consisting of ozone profiles retrieved at the University of Bremen from SCIAMACHY and OMPS‐LP measurements to derive ozone trends. We also use TOMCAT chemical transport model (CTM) simulations, forced by ERA5 reanalyses, to investigate the factors that drive the asymmetry observed in the long‐term changes. By studying seasonally and longitudinally resolved observation‐based ozone trends, we find, especially during spring, a well‐pronounced asymmetry at polar latitudes with values up to +6 % per decade over Greenland and 5 % per decade over western Russia. The control CTM simulation agrees well with these observed trends, whereas sensitivity simulations indicate that chemical mechanisms involved in the production and removal of ozone, or their changes, are unlikely to explain the observed behavior. The decomposition of TOMCAT ozone time series and ERA5 geopotential height into the first two wavenumber components shows a clear correlation between the two variables in the middle stratosphere and demonstrates a weakening and a shift in the wavenumber‐1 planetary wave activity over the past two decades. Finally, the analysis of the polar vortex position and strength points to a decadal oscillation with a reversal pattern at the beginning of the century. The same is found in the ozone trend asymmetry. This further stresses the link between changes in the polar vortex position and the identified ozone trend pattern. Plain Language Summary Monitoring long‐term ozone changes in the stratosphere is important and aims to assess the ozone layer's evolution in response to the Montreal Protocol and climate change. In this study, we investigate the origin of a zonal asymmetry in stratospheric ozone trends over the past two decades, which was identified at northern polar latitudes by analyzing satellite observations. To this aim, we use a merged data set consisting of ozone profiles retrieved at the University of Bremen from SCIAMACHY and OMPS‐LP measurements to derive ozone trends. We also use TOMCAT chemical transport model (CTM) simulations to investigate the factors that determine the asymmetry observed in long‐term ozone changes. The asymmetry is largest in springtime, and the CTM simulation agrees well with the observation‐ based trends. Sensitivity simulations indicate that chemical mechanisms involved in the production and destruction of ozone are unlikely to explain the observed pattern. In contrast, changes in atmospheric dynamics are found to be relevant. Our analysis of the polar vortex position and strength shows a cyclical pattern that reverses its phase near the year 2000. We observe this same pattern in the ozone trend asymmetry.