Analysis of recent lower-stratospheric ozone trends in chemistry climate models

Simone Dietmüller,Hella Garny,William T Ball,Roland Eichinger

doi:10.5194/acp-21-6811-2021

Abstract

Abstract. Recent observations show a significant decrease in lower-stratospheric (LS) ozone concentrations in tropical and mid-latitude regions since 1998. By analysing 31 chemistry climate model (CCM) simulations performed for the Chemistry Climate Model Initiative (CCMI; Morgenstern et al., 2017), 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 reproduce the observed ozone trend structure of coherent negative trends in the LS. In contrast to the observations, most models show an LS trend pattern with negative trends in the tropics (20∘ S–20∘ N) and positive trends in the northern mid-latitudes (30–50∘ N) or vice versa. To investigate the influence of natural variability on recent LS ozone trends, we analyse the sensitivity of observational trends and the models' trend probability distributions for varying periods with start dates from 1995 to 2001 and end dates from 2013 to 2019. Generally, modelled and observed LS trends remain robust for these different periods; however observational data show a change towards weaker mid-latitude trends for certain periods, likely forced by natural variability. Moreover we show that in the tropics the observed trends agree well with the models' trend distribution, whereas in the mid-latitudes the observational trend is typically an 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 positive ozone trend at mid-latitudes, and the trends converge to constant values by the period that spans 1998–2060. Inter-model correlations between ozone trends and transport-circulation trends confirm the dominant role of greenhouse gas (GHG)-driven tropical upwelling enhancement on the tropical LS ozone decrease. Mid-latitude ozone, on the other hand, appears to be influenced by multiple competing factors: an enhancement in the shallow branch decreases ozone, while an enhancement in the deep branch increases ozone, and, furthermore, mixing plays a role here too. Sensitivity simulations with fixed forcing of GHGs or ozone-depleting substances (ODSs) reveal that the GHG-driven increase in circulation strength does not lead to a net trend in LS mid-latitude column ozone. Rather, the positive ozone trends simulated consistently in the models in this region emerge from the decline in ODSs, i.e. the ozone recovery. Therefore, we hypothesize that next to the influence of natural variability, the disagreement of modelled and observed LS mid-latitude ozone trends could indicate a mismatch in the relative role of the response of ozone to ODS versus GHG forcing in the models.

Highlights

Stratospheric ozone is essential for protecting the Earth’s surface from ultraviolet radiation, which is harmful for plants, animals, and humans
By analysing 31 chemistry climate model (CCM) simulations performed for the Chemistry Climate Model Initiative (CCMI; Morgenstern et al, 2017), we find a large spread in the 1998–2018 trend patterns between different CCMs and between different realizations performed with the same CCM
We refrain from excluding sources of variability such as quasi-biennial oscillation (QBO), ENSO (El Niño– Southern Oscillation), solar cycle, or volcanic eruptions in the regression analysis to capture the full range of variability in ozone trends over the given period

Summary

Introduction

Stratospheric ozone is essential for protecting the Earth’s surface from ultraviolet radiation, which is harmful for plants, animals, and humans. Human-made ozone-depletingsubstance (ODS) emissions significantly reduced ozone concentrations for some decades after 1960. Dhomse et al (2018) have analysed the recovery of stratospheric ozone mixing ratios of the CCMI-1 (Chemistry Climate Model Intercomparison project part 1) climate projection simulations. They found that the ozone layer is simulated to return to a pre-1980 ODS level between 2030 and 2060, depending on the region. They discovered a large spread among the individual models, which shows that there are many uncertainties in these projections. On the other hand, increasing GHG concentrations slows down ozone depletion through GHG-induced stratospheric cooling (e.g. Jonsson et al, 2004; Oman et al, 2010; Bekki et al, 2013; Dietmüller et al, 2014; Marsh et al, 2016), and emissions of CH4 and N2O impact ozone through chemical processes (e.g. Ravishankara et al, 2009; Kirner et al, 2015; Revell et al, 2012; Winterstein et al, 2019)

Objectives

Methods

Results

Discussion

Conclusion