Stratosphere-troposphere coupling under the extreme conditions of the No-Montreal-Protocol scenario

The Montreal Protocol and its amendments (MPA) have been a major success in preventing serious health damage from the destruction of the stratospheric ozone layer by chlorofluorocarbons (CFCs). Additionally, unabated CFC emissions would have significantly accelerated global warming and affected large-scale tropospheric circulation. CFC-induced ozone depletion would have contributed to global climate change through reduced absorption of UV radiation, resulting in weaker stratospheric circulation, affecting the dynamical coupling to the troposphere. With the Earth System Model SOCOLv4, we study an extreme condition where the MPA is absent to disentangle the radiative and chemical (i.e., ozone-mediated) effects of CFCs and their impacts on stratosphere-troposphere coupling. Our results show that at the end of the 21st century, unabated CFC emissions would have largely destroyed the global ozone layer, which would have strongly affected the large-scale stratospheric and tropospheric circulation. In the stratosphere, contrary to historical ozone destruction, the polar vortices severely weaken due to low-latitude ozone depletion. In the Northern Hemisphere (NH), the weakening of the vortex leads to a pronounced negative phase of the North Atlantic Oscillation (NAO) in boreal winter and spring due to the chemical CFC effect. Similarly, the stratosphere also affects the Southern Annular Mode (SAM) to be in a more negative phase in austral winter and spring. However, tropospheric warming from CFCs largely dominates the overall SAM response to be in a more positive phase, whereas in the NH it compensates for the NAO negative phase. Additionally to the circulation changes, uncontrolled CFC emissions would have led to around 2.5 K additional global surface warming, being partially compensated by a cooling of around 0.6 K due to ozone depletion, leading to an overall warming of around 1.9 K. Our study strongly emphasizes the importance of the MPA for our climate and its mitigation of stratospheric circulation changes and their effects on tropospheric variability.