Why Do Antarctic Ozone Recovery Trends Vary?

Abstract

We use satellite ozone records and Global Modeling Initiative chemistry transport model simulations integrated with Modern Era Retrospective for Research and Analysis 2 meteorology to identify a metric that accurately captures the trend in Antarctic ozone attributable to the decline in ozone depleting substances (ODSs). The GMI CTM Baseline simulation with realistically varying ODS levels closely matches observed interannual to decadal scale variations in Antarctic September ozone over the past four decades. The expected increase or recovery trend is obtained from the differences between the Baseline simulation and one with identical meteorology and fixed 1995 ODS levels. The differences show that vortex-averaged column O 3 has the greatest sensitivity to ODS change from 1 to 20 September. The observed vortex-averaged column O 3 during this period produces a trend consistent with the expected recovery attributable to ODS decline. Trends from dates after 20 September have smaller sensitivity to ODS decline and are more uncertain due to transport variability. Simulations show that the greatest decrease in O 3 loss (i.e., recovery) occurs inside the vortex near the edge. The polar cap metrics have vortex size-dependent bias and do not consistently sample this region. Because the 60-90°S 220 Dobson unit O 3 mass deficit metric does not sample the edge region, its trend is lower than the expected trend; this is improved by area weighting. The 250-Dobson unit O 3 mass deficit metric samples more of the edge region, which increases its trend. Approximately 25% of the September Antarctic O 3 increase is due to higher O 3 levels in June prior to winter depletion.Plain Language Summary Atmospheric levels of ozone depleting substances (ODSs) are decreasing due to the Montreal Protocol and its amendments. The Ozone Hole is beginning to shrink, but trends derived from Antarctic O 3 observations differ widely. We compared model simulations with different ODS levels to reveal where, when, and how much Antarctic O 3 increases in response to ODS decline. We found that trend studies used O 3 measurements from different dates and regions, which usually did not line up with regions where the greatest O 3 recovery occurs or with the time period when O 3 changes are most sensitive to declining ODSs. This explains why reported trends and their uncertainties vary so much. Our simulations found the strongest, clearest signal of increasing O 3 due to ODS change between 1 and 20 September inside the Antarctic polar vortex. The observed vortex O 3 trend from 1 to 20 September is 1.2-1.6 Dobson unit/year and is consistent with the simulations' results, indicating that O 3 is increasing at the rate expected due to ODS decrease. The greatest recovery occurs inside the vortex but just outside the O 3 220 Dobson unit contour, the traditionally defined boundary of the Ozone Hole. Simulations and observations agree that the September Ozone Hole is shrinking at 0.24-0.36 × 10 6 km 2 /year.

COVID‐19 Crisis Reduces Free Tropospheric Ozone Across the Northern Hemisphere

“…The observed 2020 mean anomaly profile is large, −6% to −9%, and statistically significant at the 95% level (more than 99% in fact) from 1 to 8 km (Figure 3b), whereas the corresponding CAMS profile is close to zero (Figure 3d). Figure 3 indicates that Arctic stratospheric springtime ozone depletion did not have a large effect on tropospheric ozone below 8 km in 2011 and 2020 (see also model simulations in Figure S1, based on Gelaro et al, 2017, andStrahan et al, 2019), and that the CAMS "business as usual" simulation does not account for the observed large negative tropospheric anomaly in 2020. Figure 3b also shows a simulated profile of tropospheric ozone reduction from a recent chemistry-climate modeling study of COVID-like emissions decreases by Weber et al (2020).…”

Time evolution of temperature profiles retrieved from 13 years of infrared atmospheric sounding interferometer (IASI) data using an artificial neural network

“…The warming is weaker at 50 • S, 100 hPa (0.6 K/decade) and stronger at 80 • S, 10 hPa (0.8 K/decade). Due to the Montreal Protocol in 1987, the ozone hole has been recovering since the 1990s (WMO, 2018;Weber et al, 2018;Strahan et al, 2019), and warming in the stratospheric South Pole can partly be attributed to this recovery.…”

Did the COVID-19 Crisis Reduce Free Tropospheric Ozone across the Northern Hemisphere?

“…The observed 2020 mean anomaly profile is large, −6% to −9%, and statistically significant at the 95% level (more than 99% in fact) from 1 to 8 km (Figure 3b), whereas the corresponding CAMS profile is close to zero (Figure 3d). Figure 3 indicates that Arctic stratospheric springtime ozone depletion did not have a large effect on tropospheric ozone below 8 km in 2011 and 2020 (see also model simulations in Figure S1, based on Gelaro et al, 2017, andStrahan et al, 2019), and that the CAMS "business as usual" simulation does not account for the observed large negative tropospheric anomaly in 2020.…”