Text S1.
Figure S1 shows the time series of Arctic mean observations of N2O, HNO3, HCl, ClO and O3 from the Microwave Limb Sounder (MLS) from 2004-2020 at 480 K. Also shown are results from the TOMCAT simulations CNTL and ODS95. The equivalent plot of monthly mean anomalies, which removes the large annual variations, is given in the main paper in Figure 1. Note that due to degradation of the MLS 190 GHz receiver, the N2O v4 data shows a drift which becomes apparent in 2010 (N. Livesey, personal communication, 2020). Note that this degradation does not affect the other species used here.
Figure S2 shows the range of observed and modelled seasonal winter/spring variations in N2O, HNO3, HCl, ClO and O3 from MLS and the simulation CNTL in the Arctic at 480 K from 2004-2020. The specific values for the cold years 2010/11 and 2019/20, and the recent warm year 2018/19, are highlighted. The year 2019/20 stands out as extreme in having very low N2O (only model results shown), high ClO in March and low O3. In contrast, 2018/19 shows extreme high values of N2O and HCl, and low values of ClO. It is remarkable how successive years can still cause new and opposite extremes in the data records.
Figure S3 compares the observed OMI total column ozone on March 30, 2020 with TOMCAT model simulations CNTL and ODS95. This is similar to Figure 3 in the main paper but for a later day at the end of the ozone depletion period.
Figure S4 shows OMI column ozone on March 18, 2020, the day of the lowest observed column in this winter (see main paper Figure 3). Also shown in Figure S4 are results from the control simulation CNTL and the ‘world avoided’ simulation WA. As noted in the main text, simulation CNTL (panel b) gives a good representation of the spatial distribution of column ozone and produces larger regions below the 220 DU contour. Panels (c) and (d) show that in the absence of any controls due the Montreal Protocol, and the assumed continued increase in ODS emissions of 3%/year from 1987, Arctic ozone loss in 2020 would have been extremely severe. The minimum column ozone inside the vortex is only around 85 DU. Compared to run CNTL there is additional depletion of over 180 DU in the polar vortex, around 75 DU in northern mid-latitudes and even around 40 DU at low latitudes.