In the recent past, the evolution of stratospheric ozone (O) wasaffected by both increasing ozone depleting substances (ODSs) and greenhousegases (GHGs). The impact of the single forcings on O is well known.Interactions between the simultaneously increased GHG and ODS concentrations,however, can occur and lead to nonlinear O changes. In this study, weinvestigate if nonlinear processes have affected O changes between 1960and 2000. This is done with an idealised set of time slice simulations withthe chemistry-climate model EMAC. Due to nonlinearity the past ozoneloss is diminished throughout the stratosphere, with a maximum reduction of1.2 % at 3 hPa. The total ozone column loss between 1960 and 2000 that ismainly attributed to the ODS increase is mitigated in the extra-polar regionsby up to 1.1 % due to nonlinear processes. A separation of the Ochanges into the contribution from chemistry and transport shows thatnonlinear interactions occur in both. In the upper stratosphere a reducedefficiency of the ClO-catalysed O loss chiefly causes thenonlinear O increase. An enhanced formation of halogen reservoirspecies through the reaction with methane (CH) reduces the abundance ofhalogen radicals significantly. The temperature-induced deceleration of theO loss reaction rate in the Chapman cycle is reduced, which leads to anonlinear O decrease and counteracts the increase due to ClO.Nonlinear effects on the NO abundance cause hemispheric asymmetricnonlinear changes of the O loss. Nonlinear changes in O transportoccur in particular in the Southern Hemisphere (SH) during the monthsSeptember to November. Here, the residual circulation is weakened in thelower stratosphere, which goes along with a reduced O transport fromthe tropics to high latitudes. Thus, O decreases in the SH polarregion but increases in the SH midlatitudes. The existence of nonlinearitiesimplies that future ozone change due to ODS decline slightly depends on theprevailing GHG concentrations. Therefore the future ozone evolution will notsimply be a reversal of the past.
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