The Influence of Ozone Changes on the Stratospheric Dynamics in 4xCO2 Climate Simulations

Kurzfassung

Ozone modifies the temperature in the stratosphere and thus it is important to quantify the climate impacts exerted by stratospheric ozone in climate models. This could yield new insights into the role of the ozone layer in the climate system. An increase in the CO2 concentrations in this system changes the temperatures and dynamics of the troposphere and stratosphere and thereby also the ozone concentrations. Since ozone absorbs longwave and shortwave radiation, the altered ozone distribution, in turn, affects temperatures and thus also modifies the CO2-induced changes in stratospheric dynamics. The stratospheric dynamics influence the transport of trace gases like ozone and water vapor and can also impact the tropospheric dynamics. This motivates to study these in detail. Therefore, model simulations were performed using a climate-chemistry model in which CO2 concentrations were quadrupled compared to pre-industrial times. For the 4xCO2 simulation, this was done once by prescribing an unchanged (pre-industrial) ozone distribution and once a changed ozone distribution from a previous 4xCO2 simulation. The change in ozone leads to a significant strengthening of the stratospheric easterly winds in summer, as well as a systematic weakening of the polar vortex in both hemispheres. In addition, the duration of the polar vortex period is shorter due to the CO2-induced change in the ozone field. Furthermore, the Brewer-Dobson circulation (BDC) changes as diagnosed by the mass streamfunction. This diagnostic shows that the CO2-induced acceleration of the residual circulation is systematically damped. But the damping is only significant in the summer shallow branch. Another way to analyze the change in the full transport circulation, the BDC, is the diagnostic of the age of air (AoA). Due to ozone changes an AoA increase can be analyzed in the entire stratosphere, although there are no significant changes in the mass streamfunction in the most regions. The ozone induced changes in the stratosphere also affect the troposphere. The tropospheric polar front jet in the Southern Hemisphere (SH) experiences a systematically weaker shift toward the pole due to a changed ozone distribution. These results make clear that CO2-induced ozone changes in the stratosphere have a discernible impact on temperature and dynamics there and in the troposphere. This underline the relevance of realistic representations of ozone distributions in climate models.