This study investigates the origin of the zonal asymmetry in stratospheric ozone trends at northern high latitudes, identified in satellite limb observations over the past two decades. We use a merged dataset 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 which determine 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 behaviour. The decomposition of TOMCAT ozone time series and of 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, also found in the ozone trend asymmetry. This further stresses the link between changes in the polar vortex position and the identified ozone trend pattern.

We use a 3-D chemical transport model and satellite observations to investigate Arctic ozone depletion in winter/spring 2019/20 and compare with earlier years. Persistently low temperatures caused extensive chlorine activation through to March. March-mean polar-cap-mean modelled chemical column ozone loss reached 78 DU (local maximum loss of ~108 DU in the vortex), similar to that in 2011. However, weak dynamical replenishment of only 59 DU from December to March was key to producing very low (<220 DU) column ozone values. The only other winter to exhibit such weak transport in the past 20 years was 2010/11, so this process is fundamental to causing such low ozone values. A model simulation with peak observed stratospheric total chlorine and bromine loading (from the mid-1990s) shows that gradual recovery of the ozone layer over the past two decades ameliorated the polar cap ozone depletion in March 2020 by ~20 DU.

Satellite observations of relevant trace gases, together with meteorological data from ERA5, were used to describe the dynamics and chemistry of the spectacular Arctic 2019/20 winter/spring season. Exceptionally low total ozone values of slightly less than 220 DU were observed in mid March within an unusually large stratospheric polar vortex. This was associated with very low temperatures and extensive polar stratospheric cloud formation, a prerequisite for substantial springtime ozone depletion. Very high OClO and very low NO2 column amounts observed by GOME-2A are indicative of unusually large active chlorine levels and significant denitrification, which likely contributed to large chemical ozone loss. Using results from the TOMCAT chemical transport model (CTM) and ozone observations from S5P/TROPOMI, GOME-2 (total column), SCIAMACHY and OMPS-LP (vertical profiles) chemical ozone loss was evaluated and compared with the previous record Arctic winter 2010/11. The polar-vortex-averaged total column ozone loss in 2019/20 reached 88 DU (23%) and 106~DU (28%) based upon observations and model, respectively, by the end of March, which was similar to that derived for 2010/11. The total column ozone loss is in agreement with OMPS-LP-derived partial column loss between 350 K and 550 K to within the uncertainty. The maximum ozone loss (~80%) observed by OMPS-LP was near the 450 K potential temperature level (~18 km altitude). Because of the larger polar vortex area in March 2020 compared to March 2011 (about 25% at 450 K), ozone mass loss was larger in Arctic winter 2019/20.