Investigating zonal asymmetry in stratospheric ozone trends at northern high latitudes using satellite limb observations and CTM simulations

Observations in limb geometry from satellite platforms are very valuable for monitoring the stratospheric ozone layer on a global scale, as they provide information with high spatial and temporal coverage and good vertical resolution. At the University of Bremen, ozone profiles were retrieved from observations from two limb sounders, SCIAMACHY (2002-2012) and OMPS-LP (2012-present), using similar retrieval algorithm setups. These two data sets were merged to obtain a consistent time series of longitudinally resolved global ozone distribution, referred to below as SCIA+OMPS. Recently, the OMPS-LP data set has been re-processed by using improved L1 data produced by the NASA team, with the main aim to mitigate the long-term drift affecting the OMPS-LP time series. The results of this re-processing will be presented; the use of the updated data set gives more confidence in the trend studies from the SCIA+OMPS time series.

The overarching aim of this study is the investigation of vertically consistent patterns in the longitude-resolved trends, particularly at northern mid- and high-latitudes above 30 km altitude, detected in the SCIA+OMPS data set. Large positive trends are found over the Atlantic sector, whereas close-to-zero changes are detected over the Siberian/Pacific sector. To investigate the origin of this behaviour, we performed full chemistry simulations of the TOMCAT global 3-D chemistry transport model (CTM), forced by ERA5 reanalysis, for the 2003-2020 period. We then applied a multi-linear regression model including dynamical proxies to both the satellite observations and TOMCAT simulations. First, we compare the trend resulting from the merged data set with those from the model simulations to check the consistency of the detected zonal and longitudinally resolved patterns. Then, seasonally and monthly resolved trends are studied as they provide valuable insight into the observed zonal asymmetry of the trends. We find the largest variability with longitude occurring in winter- and springtime, and a good consistency between observations and the CTM.

By comparing ozone changes, with trends in temperature and meridional wind fields 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 behaviour 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 and has led to the zonal asymmetry identified in the data and model.