Influence of stratosphere-troposphere exchange on long-term trends of surface ozone in CMIP6

Surface ozone, which is a major air pollutant, has gained widespread public attention due to its significant harm to human health and ecosystems. Here we find that long-term trends of surface ozone are largely influenced by stratosphere-troposphere exchange (STE) using the historical simulations in the CMIP6 multi-model ensemble. It is found that STE contributes up to 39.5% of climatological surface ozone in the extratropics of the winter hemisphere. In the Southern Hemisphere (SH), prominent stratospheric ozone depletion tends to reduce tropospheric and surface ozone due to STE over 1960-1997, which has been reversed during 1997-2014. In the Northern Hemisphere (NH), STE enhances surface ozone during both ozone depletion and recovery periods. About 29.5% of the positive trends of surface ozone over 1960-2014 can be attributed to STE processes in boreal winter in the NH extratropics. The enhancement of STE is related to the changes in atmospheric circulation associated with global warming. Our results suggest that STE will further worsen surface ozone pollution in the future associated with the stratospheric ozone recovery and global warming.