Subtropical and midlatitude ozone trends in the stratosphere: Implications for recovery

Abstract

We present a comprehensive analysis of the trends of stratospheric ozone in the midlatitudes and subtropics. The analysis is performed using ground-based and space-based measurements over the light detection and ranging stations for the period 1985-2012. Also, trends are estimated for the zonal mean data made from a merged satellite data set, Global OZone Chemistry And Related trace gas Data records for the Stratosphere, over 1979-2012. The linear trends in stratospheric ozone are estimated using piecewise linear trend (PWLT) functions. The ozone trends during the increasing phase of halogens (before 1997) range from −0.2±0.08 to −1±0.07% yr −1 in the midlatitudes and −0.2±0.06 to −0.7±0.05 % yr −1 in the subtropics at 15-45 km, depending on altitude. In 1997-2012, the PWLT analyses show a positive trend, significantly different from zero at the 95% confidence intervals, toward ozone recovery in the middle-and low-latitude upper stratosphere (35-45 km), and the trends are about +0.5±0.07% yr −1 at midlatitudes and about +0.3± 0.05% yr −1 at subtropical latitudes. However, negative and insignificant trends are estimated in the lower stratosphere (15-20 km) over 1997-2012 in the midlatitudes, mainly due to the dynamics, as demonstrated by the large (50-60%) contributions from the quasi-biennial oscillation, El Niño-Southern Oscillation, and planetary wave activity to recent ozone changes. This suggests that the ozone changes are governed by the interannual variations in meteorology and dynamics of the regions; these factors will influence the recovery detection time and the behavior of the recovery path to pre-1980 levels.

Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery

“…Most studies consider either piecewise linear trends (PWLTs) or the equivalent effective stratospheric chlorine (EESC) proxy to represent the influence of hODSs on long-term ozone changes (Newman et al, 2007). Chehade et al (2014) and Frith et al (2014) both concluded that total column ozone trends up to 2012 and 2013 estimated from PWLTs or EESC prior to 1997 agree well, but post-1997 the EESC proxy implies significant and positive increases, while PWLTs are generally smaller and non-significant at most non-polar latitudes.…”

“…Concentrations of active stratospheric hODSs reached a maximum in ∼ 1997 (Newman et al, 2007), and vertically resolved satellite measurements show evidence that upper stratospheric ozone (10-1 hPa; ∼ 32-48 km) started recovering soon after (WMO, 2014). Figure 1 presents post-1998 ozone changes from four ozone composites that combine multiple satellite instruments (see Sect.…”

“…DLM analyses have more principled uncertainties than MLR since they are based on a more flexible model and formally integrate over uncertainties in the regression coefficients, (nonstationary) seasonal cycle, autoregressive coefficients, and parameters characterising the degree of non-linearity in the trend. The time-varying background changes are inferred rather than specified by, for example, an estimate of EESC (Newman et al, 2007) or PWLT; there is no need for assumptions about when and where a decline in hODSs occurs.…”

“…Suggestions of a decrease in lower stratospheric ozone have been presented elsewhere Gebhardt et al, 2014;Sioris et al, 2014;Nair et al, 2015;Vigouroux et al, 2015). However, it has been difficult to confirm (WMO, 2014;Harris et al, 2015;Steinbrecht et al, 2017) because (i) ozone is typically integrated over wide latitude bands and/or total column ozone is considered, both of which may lead to cancellation of opposing trends; (ii) large dynamical variability unaccounted for in regression analysis together with shorter time series lead to higher uncertainties (Tegtmeier et al, 2013); (iii) below 20 km there are large ozone gradients, with low ozone concentrations close to the tropopause; and (iv) composite-data merging techniques have hindered identification of robust changes Ball et al, 2017).…”

Interannual Variations in Lower Stratospheric Ozone During the Period 1984–2016

“…WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion (2018) has reported that the upper stratospheric ozone has increased from 1995 to 2016. However, ozone in the lower stratosphere still showed a continuing declining trend (Ball et al, 2017(Ball et al, , 2018Gebhardt et al, 2014;Kyrölä et al, 2013;Nair et al, 2015;Sioris et al, 2014;Vigouroux et al, 2015;Zhang et al, 2018). In addition to trends, the interannual variations in lower stratospheric ozone also require detailed investigations, since the changes in lower stratospheric ozone can significantly influence climate change (Forster et al, 1997;Hu et al, 2015;Polvani et al, 2017).…”