Abstract
Abstract. In the recent past, the evolution of stratospheric ozone (O3) was affected by both increasing ozone depleting substances (ODSs) and greenhouse gases (GHGs). The impact of the single forcings on O3 is well known. Interactions between the simultaneously increased GHG and ODS concentrations, however, can occur and lead to nonlinear O3 changes. In this study, we investigate if nonlinear processes have affected O3 changes between 1960 and 2000. This is done with an idealised set of time slice simulations with the chemistry-climate model EMAC. Due to nonlinearity the past ozone loss is diminished throughout the stratosphere, with a maximum reduction of 1.2 % at 3 hPa. The total ozone column loss between 1960 and 2000 that is mainly attributed to the ODS increase is mitigated in the extra-polar regions by up to 1.1 % due to nonlinear processes. A separation of the O3 changes into the contribution from chemistry and transport shows that nonlinear interactions occur in both. In the upper stratosphere a reduced efficiency of the ClOx-catalysed O3 loss chiefly causes the nonlinear O3 increase. An enhanced formation of halogen reservoir species through the reaction with methane (CH4) reduces the abundance of halogen radicals significantly. The temperature-induced deceleration of the O3 loss reaction rate in the Chapman cycle is reduced, which leads to a nonlinear O3 decrease and counteracts the increase due to ClOx. Nonlinear effects on the NOx abundance cause hemispheric asymmetric nonlinear changes of the O3 loss. Nonlinear changes in O3 transport occur in particular in the Southern Hemisphere (SH) during the months September to November. Here, the residual circulation is weakened in the lower stratosphere, which goes along with a reduced O3 transport from the tropics to high latitudes. Thus, O3 decreases in the SH polar region but increases in the SH midlatitudes. The existence of nonlinearities implies that future ozone change due to ODS decline slightly depends on the prevailing GHG concentrations. Therefore the future ozone evolution will not simply be a reversal of the past.
Highlights
During the 20th century both the emissions of ozone depleting substances (ODSs) and greenhouse gases (GHGs) increased, which had a large effect on stratospheric ozone (O3) (e.g. WMO, 2007)
The negative GHG-signal in the lower stratosphere, which is found by Jonsson et al (2009), originates from the tropics where a slightly strengthened upwelling reduces the local abundance of ozone
This is caused by an increase of the HOx mixing ratio, which is related to the higher CH4 emissions (e.g. Wuebbles and Hayhoe, 2002)
Summary
During the 20th century both the emissions of ozone depleting substances (ODSs) and greenhouse gases (GHGs) increased, which had a large effect on stratospheric ozone (O3) (e.g. WMO, 2007). Observations show that between 1979 and 2000 the total column ozone decreased by 2– 3 %/decade at midlatitudes in the annual mean and by up to 12 %/decade in the Southern Hemisphere (SH) polar region in spring (e.g. Fioletov et al, 2002). This development was mainly caused by increasing concentrations of ODSs (e.g. WMO, 2007). As these compounds are relatively chemically inert in the troposphere, they are transported into the stratosphere where they are decomposed, releasing reactive chlorine and bromine compounds at levels well above the natural background concentrations. In the polar regions in spring, this catalytic ozone loss is especially effective since the occurrence of polar stratospheric clouds in winter leads to an enhanced conversion of halogen reservoir species to radicals (e.g. Solomon et al, 1986)
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