Understanding recent trends in lower stratospheric ozone: an update with CCMI-2022 models

Lower stratospheric ozone between 60S and 60N has continued to decline since 1998, despite the reduction of ozone-depleting substances as a consequence of the Montreal Protocol. Previous studies have shown that Chemistry Climate Models are not able to reproduce these negative trends in mid-latitudes, although the reason for this discrepancy between models and observations remains unknown.

In this study, we re-examine recent trends in lower stratospheric ozone using the new simulations from the Chemistry Climate Model Initiative 2022 (CCMI-2022). Historical simulations with observed sea surface temperatures (SSTs) and nudged QBO (ref-D1), and fully-coupled atmosphere-ocean simulations (ref-D2) are available covering the period up to 2018, which allows a better analysis of the role of natural variability in recent ozone trends compared to previous studies.

CCMI-2022 models show a slight improvement in the representation of lower stratospheric ozone trends in mid-latitudes compared to previous studies that used CCMI-1 and CCMVal models. The observational trend now lies inside the 90% confidence interval of the models’ trend distribution. However, the majority of the models are still not able to reproduce the pattern of negative trends in the tropics extending into mid-latitudes. Intermodel differences dominate the spread in the trends, while natural variability from SSTs and QBO are not decisive in explaining the negative mid-latitude trends.  The role of the different ozone transport representations in models, in particular the mixing between the tropics and mid-latitudes, is also explored.