Stratospheric ozone trends for 1985–2018: sensitivity to recent large variability

Abstract

The Montreal Protocol has successfully prevented catastrophic losses of stratosphericozone, and signs of recovery are now evident. Nevertheless, recent work suggests that ozone in thelower stratosphere (<24 km) continued to decline over 1998–2016, offsetting recovery at higher altitudes and preventing a statistically significant increase in quasi-global (60◦S – 60◦N) total column ozone. In 2017, a large lower stratospheric ozone resurgence over less than 12 months was estimated (using a chemistry-transport model; CTM) to have wiped out the long-term decline in the quasi-global integrated lower stratospheric ozone column. Here, we extend the analysis of space-based ozone observations to December 2018 using the BASICSG ozone composite. We find that the observed 2017 resurgence was only around half that modelled by the CTM, was of comparable magnitude to other strong inter-annual changes in the past, and restricted to southern hemispheric mid-latitudes (SH; 60◦S–30◦S). In the SH mid-latitude lower stratosphere, the data suggest that by the end of 2018 ozone is still likely lower than in 1998 (probability∼80%). In contrast, tropical andnorthern hemisphere (NH) ozone continue to display ongoing decreases, exceeding 90% probability. Robust tropical (>95%, 30◦S–30◦N) decreases dominate the quasi-global integrated decrease (99% probability); the integrated tropical stratospheric column (1–100 hPa, 30◦S–30◦N) displays a significant overall decrease, with 95% probability. These decreases do not reveal an inefficacy of the Montreal Protocol. Rather, they suggest other effects to be at work, mainly dynamical variabilityon long or short timescales, counteracting the protocol’s regulation of halogenated ozone depleting substances (hODS). We demonstrate that large inter-annual mid-latitude variations (30◦–60◦),such as the 2017 resurgence, are driven by non-linear QBO phase-dependent seasonal variability. However, this variability is not represented in current regression analyses. To understand if observed lower stratospheric decreases are a transient or long-term phenomenon, progress needs to be made in accounting for this dynamically-driven variability.