ALBERT

Description: The aim of this study is the investigation of a vertically consistent pattern in longitude-resolved stratospheric ozone trends at northern mid- and high-latitudes, where large positive trends are found over the Atlantic sector, whereas close-to-zero changes are detected over Siberia. This pattern was found in the SCIA+OMPS data set produced at the University of Bremen, which is obtained by merging observations from two limb sounders: SCIAMACHY and OMPS-LP. Recently, the OMPS-LP data set has been re-processed by using a new release of L1 data, which mitigated the long-term drift affecting its time series. To investigate the origin of the zonal asymmetry in trends, we performed full-chemistry simulations of the TOMCAT global 3-D chemistry transport model, forced by ERA5 reanalysis, for the 2003-2020 period. We applied a multi-linear regression model including dynamical proxies to both satellite observations and TOMCAT simulations. After checking the consistency of the trends between the two, we studied seasonally resolved trends, finding the largest variability with longitude occurring in winter- and springtime, and a good consistency between observations and TOMCAT. By comparing ozone changes with trends in temperature and meridional winds from ERA5, we investigated potential mechanisms driving the observed asymmetry. Dedicated TOMCAT simulations showed the negligible role of photochemical processes for the observed pattern. We therefore consider the behavior to be mainly dynamically driven. A composite analysis supports the hypothesis that the long-term change in the position of the polar vortex has influenced the winter- and springtime ozone concentrations, leading to the identified zonal asymmetry.