Causes and impacts of changes in the stratospheric meridional circulation in a chemistry-climate model

The climate of the stratosphere is known to be subject to long-term changes induced by anthropogenic emissions of long-lived greenhouse gases (GHGs) as well as by emissions of ozone depleting substances (primarily chlorofluorocarbons, CFCs). Enhanced concentrations of CFCs have led to strong ozone depletion over the last decades. Thanks to the Montreal Protocol and its amendments and adjustments, the stratospheric halogen loading is expected to retreat again in the future. Emissions of GHGs, on the other hand, are not yet controlled sufficiently, and concentrations of GHGs are projected to increase further in the future. The effects of enhanced GHG concentrations on the stratosphere include decreasing temperatures, as well as changes in the dynamical balances and interactions with the troposphere, and thus changes in the large-scale circulation. In particular, the stratospheric meridional circulation is projected to be subject to changes. These GHG-induced changes will also affect stratospheric ozone chemistry and transport of ozone. Therefore, the expected recovery of ozone due to decreasing CFC concentrations will coincide with alterations of the ozone layer by climate change. This study aims to diagnose and explain long-term changes in the stratospheric meridional circulation using the chemistry-climate model E39CA. The dynamical causes for these changes are identified, and the impact of changes in the meridional circulation on the future development of ozone is quantified. In a changing climate, the meridional circulation is found to strengthen in the tropical lower stratosphere. In particular, tropical upwelling in the lowermost stratosphere intensifies at a rate of about 3% per decade over the analysed period of 1960 to 2049. This enhanced upwelling is balanced by downwelling in the subtropics at latitudes around 20-40°N/S. The increase in tropical upwelling can be explained by stronger local forcing by large scale waves. It is shown that enhanced tropical upwelling is driven by processes induced by increases in tropical sea surface temperatures (SSTs). Higher tropical SSTs cause both a) a strengthening of the subtropical jets and b) modifications of deep convection, leading to changes in the strength and location of latent heat release. While the former (a) can modify wave propagation and dissipation, the latter (b) affects tropical wave generation. Evidence is presented that the dominating mechanism leading to enhanced vertical wave propagation into the lower stratosphere is an upward shift of the easterly shear zone due to the strengthening and upward and equatorward shifts of the subtropical jets. In addition to the increase in tropical upwelling caused by climate change, the changes in CFC concentrations also affect the dynamical forcing of the meridional circulation. The CFC-induced depletion of ozone in the past has led to changes in the background wind field in the southern hemisphere in summer, which cause enhanced wave propagation into the middle stratosphere and thus a strengthening of the meridional circulation. This effect is reversed in the future, when CFC concentrations decline. The future development of ozone is found to be dominated by changes in local chemistry in most regions of the stratosphere. Both decreasing CFC concentrations and stratospheric cooling due to enhanced GHG concentrations lead to less efficient ozone destruction, and thus increasing ozone concentrations. However, changes in transport of ozone due to the strengthening of the meridional circulation play an important role in the tropical lower stratosphere, where ozone concentrations decrease due to more export of ozone. Furthermore, it is found that the chemically induced positive ozone trend in southern high latitudes in the future is counteracted by decreased ozone transport from middle to high latitudes. This decrease in transport is due to the weakening of the meridional circulation in the southern hemisphere in summer, which, in turn, is induced by ozone changes.