Abstract
Abstract. Total column ozone values from an ensemble of UM-UKCA model simulations are examined to investigate different definitions of progress on the road to ozone recovery. The impacts of modelled internal atmospheric variability are accounted for by applying a multiple linear regression model to modelled total column ozone values, and ozone trend analysis is performed on the resulting ozone residuals. Three definitions of recovery are investigated: (i) a slowed rate of decline and the date of minimum column ozone, (ii) the identification of significant positive trends and (iii) a return to historic values. A return to past thresholds is the last state to be achieved. Minimum column ozone values, averaged from 60° S to 60° N, occur between 1990 and 1995 for each ensemble member, driven in part by the solar minimum conditions during the 1990s. When natural cycles are accounted for, identification of the year of minimum ozone in the resulting ozone residuals is uncertain, with minimum values for each ensemble member occurring at different times between 1992 and 2000. As a result of this large variability, identification of the date of minimum ozone constitutes a poor measure of ozone recovery. Trends for the 2000–2017 period are positive at most latitudes and are statistically significant in the mid-latitudes in both hemispheres when natural cycles are accounted for. This significance results largely from the large sample size of the multi-member ensemble. Significant trends cannot be identified by 2017 at the highest latitudes, due to the large interannual variability in the data, nor in the tropics, due to the small trend magnitude, although it is projected that significant trends may be identified in these regions soon thereafter. While significant positive trends in total column ozone could be identified at all latitudes by ∼ 2030, column ozone values which are lower than the 1980 annual mean can occur in the mid-latitudes until ∼ 2050, and in the tropics and high latitudes deep into the second half of the 21st century.
Highlights
The year 2017 marked the 30th anniversary of the Montreal Protocol, which was implemented to protect the stratospheric ozone layer from the harmful effects of ozone depleting substances (ODSs)
Identifying an increase in total column ozone resulting from reductions in stratospheric chlorine requires removing the effects of natural processes from the modelled total column ozone data, as these cycles may impose short-term trends in the data which are wrongly interpreted as signs of recovery
This statistical model can be used to remove the component of total column ozone variations related to the quasi-biennial oscillation (QBO), solar cycle and volcanic aerosol changes from the raw model data to leave a set of ozone residuals, RO3, which retain the long-term trend and any interannual variability not explained by the multiple linear regression (MLR): RO3e,l,t = TO3e,l,t − αeQ,Bl O50 · QBO50e,t + αeQ,Bl O30
Summary
The year 2017 marked the 30th anniversary of the Montreal Protocol, which was implemented to protect the stratospheric ozone layer from the harmful effects of ozone depleting substances (ODSs). Detecting recovery of the stratospheric ozone layer is complicated by a number of additional factors which affect the year-to-year variability in total column ozone values. These factors include volcanic eruptions, such as the eruption of Mt. Pinatubo in 1991 Pawson et al, 2014; Harris et al, 2015; Steinbrecht et al, 2017; Ball et al, 2018; Weber et al, 2018) These studies have indicated that statistically significant recovery of column ozone values can be identified in some datasets at some latitudes, but that this is not true for all datasets
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