Recent Lower Stratospheric Ozone Trends in CCMI-2022 Models: Role of Natural Variability and Transport

Lower stratospheric ozone between 60°S and 60°N has continued to decline since 1998, despite the reduction of ozone-depleting substances following the Montreal Protocol. Previous studies have shown that, while Chemistry Climate Models reproduce the negative ozone trend in the tropical lower stratosphere as a response to increased upwelling, they fail to capture the ozone decline in northern mid-latitudes. This study revisits recent lower stratospheric ozone trends over the period 1998-2018 using two types of simulations from the new Chemistry Climate Model Initiative 2022 (CCMI-2022): REF-D1, with observed sea surface temperatures, and REF-D2, with simulated ocean. The observed negative trend in mid-latitudes falls within the range of model trends, especially when considering simulations with observed boundary conditions. There is a large spread in the simulated mid-latitudes ozone trends, with some simulations showing positive and others negative trends. A multiple linear regression analysis shows that the spread in the trends is not explained by the different linear response to external forcings (solar cycle, global warming and ozone depleting substances) or to the main variability modes (El Niño Southern Oscillation and the Quasi-Biennial Oscillation) but it is attributed to internal atmospheric variability. Moreover, the fact that some models show very different trends across members, while other models show similar trends in all members, suggests fundamental differences in the representation of the internal variability of ozone transport across models. Indeed, we report substantial inter-model differences in the ozone-transport connection on interannual timescales and we find that ozone trends are closely coupled to transport trends.