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

Abstract

Abstract. The Montreal Protocol, and its subsequent amendments, has successfully prevented catastrophic losses of stratospheric ozone, and signs of recovery are now evident. Nevertheless, recent work has suggested that ozone in the lower stratosphere (< 24 km) continued to decline over the 1998–2016 period, 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 offset 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 interannual changes in the past, and was restricted to Southern Hemisphere (SH) midlatitudes (60–30∘ S). In the SH midlatitude lower stratosphere, the data suggest that by the end of 2018 ozone is still likely lower than in 1998 (probability ∼80 %). In contrast, tropical and Northern 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 ozone decrease, with 95 % probability. These decreases do not reveal an inefficacy of the Montreal Protocol; rather, they suggest that other effects are at work, mainly dynamical variability on long or short timescales, counteracting the positive effects of the Montreal Protocol on stratospheric ozone recovery. We demonstrate that large interannual midlatitude (30–60∘) variations, such as the 2017 resurgence, are driven by non-linear quasi-biennial oscillation (QBO) phase-dependent seasonal variability. However, this variability is not represented in current regression analyses. To understand if observed lower stratospheric ozone decreases are a transient or long-term phenomenon, progress needs to be made in accounting for this dynamically driven variability.

Transport of short-lived halocarbons to the stratosphere over the Pacific Ocean

“…The NAME-modelled results presented here ( Fig. 9, Table 5) are thus in good agreement with the values reported by Navarro et al (2015), Butler et al (2018 and Wales et al (2018).…”

“…The approach using NAME trajectories and boundary layer measurements produces Br-VSL org estimates of 3.5 ± 0.4 (3.3 ± 0.4) ppt in the lower eastern (western) Pacific TTL (14-15 km) and 2.5 ± 0.2 (2.4 ± 0.4) ppt in the upper eastern (western) Pacific TTL (17-18 km). These lie within the range of the recent literature findings (Tegtmeier et al, 2012;Carpenter et al, 2014;Liang et al, 2014;Navarro et al, 2015;Butler et al, 2018;Wales et al 2018). The validation with the ATTREX measurements provides confidence that a similar approach could be used for years when high-altitude measurements are not available, assuming that realistic estimates of the background tropospheric contributions can be obtained from either models or measurements.…”

“…The short-lived bromocarbons, chiefly CHBr 3 and CH 2 Br 2 , have been identified as the missing source for stratospheric bromine (the sum of bromine atoms in long-lived brominated organic and inorganic substances; Pfeilsticker et al, 2000;Dessens et al, 2009). The current estimate of the contribution of the short-lived bromocarbons to the active bromine (Br y ) in the stratosphere is ∼ 5 (3-7) ppt , which is slightly narrower than the previous range of 3-8 ppt (Liang et al, 2010(Liang et al, , 2014Carpenter et al, 2014;Fernandez et al, 2014;Sala et al, 2014;Tegtmeier et al, 2015;Navarro et al, 2015Navarro et al, , 2017Hossaini et al, 2016;Butler et al, 2018;Fiehn et al, 2017). Much of the uncertainty is linked to the contribution of CHBr 3 , which has both the shortest lifetime and the largest emissions of the commonly observed bromocarbons.…”

“…The minor short-lived organic bromine substances were included in the analysis of Navarro et al (2015), accounting for the higher Br-VSL org . Butler et al (2018), report a mean mole fraction and range of 0.46 (0.13-0.72) ppt and 0.88 (0.71-1.01) ppt of CHBr 3 and CH 2 Br 2 , respectively, being transported to the TTL during January and February 2014. This is consistent with a contribution of 3.14 (1.81-4.18) ppt of organic bromine to the TTL over the region of the campaign.…”

“…The successful implementation of the Montreal Protocol with its adjustments and amendments has led to reductions in stratospheric chlorine and bromine amounts since the late 1990s (Carpenter et al, 2014). These reductions have halted the ozone decrease Chipperfield et al, 2017;Steinbrecht et al, 2017) with the exception of the possible reduction in the lower stratosphere (Ball et al, 2017(Ball et al, , 2019Chipperfield et al, 2017). Recently, the importance of very short-lived (VSL) chlorine-and bromine-containing compounds has received a great deal of attention (e.g.…”