Analysis of recent lower stratospheric ozone trends in chemistry climate models

Abstract. Recent observations show a significant decrease of lower stratospheric (LS) ozone concentrations in tropical and mid-latitude regions since 1998. By analyzing 31 chemistry climate model (CCM) simulations performed for the Chemistry Climate Model Initiative (CCMI), we find a large spread in the 1998–2018 trend patterns between different CCMs and between different realizations performed with the same CCM. The latter, in particular, indicates that natural variability strongly influences LS ozone trends. However none of the model simulations reproduces the observed ozone trend structure of coherent negative trends in the LS. In contrast to the observations, most models show a dipole trend pattern in the LS with negative trends in the tropics and positive trends in the northern mid-latitudes or vice versa. To investigate the influence of natural variability on the LS ozone trends we analyze the observational trends and the models' trend probability distributions for slightly varied post-ODS (ozone depleting substances) periods. Generally, modeled and observed LS trends remain robust for different post-ODS periods, however observational data show a systematic change towards weaker mid-latitude trends forced by natural variability for certain periods. Moreover we can show that in the tropics the observed trends agree quite well with the models' trend distribution, whereas in the mid-latitudes the observational trend is a rather extreme value of the models' distribution. We further investigate the LS ozone trends for extended periods reaching into the future and find that all models develop a dipole trend pattern in the future, i.e. in almost all models the trends converge to constant values for the entire period 1998–2060. An investigation of interannual ozone variability also reveals a clear dipole pattern in ozone variability in all CCMs and in observational data, however it is more pronounced in the models. Thus, although the LS ozone variability pattern is similar, the probability of overall negative LS ozone trends simultaneously in the tropics and mid-latitudes is higher in observations. To access the dynamical influence on LS ozone trends, the models' tropical upwelling trends are correlated against their LS ozone trends. For the period 1998–2018 tropical ozone trends are negatively correlated (−0.83) and mid-latitude trends are positively correlated (+0.49). However, the correlation in the mid-latitudes is rather weak and not robust for slightly varied time periods, which indicates that other processes like two-way mixing play an important role here, too.