Nonlinear response of modelled stratospheric ozone to changes in greenhouse gases and ozone depleting substances in the recent past

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 ClO_x-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 ClO_x. Nonlinear effects on the NO_x 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.