Are SSW impacts on surface temperature extremes changing due to increasing CO2 concentrations?

Sudden Stratospheric Warmings (SSWs) represent the dominant mode of variability in the winter polar stratosphere and play a key role in modulating tropospheric circulation and surface climate. SSWs can influence surface temperature extremes, with important implications for regional climate variability. However, how SSW characteristics and their surface impacts may change under strong greenhouse gas forcing remains an open question, and current projections show substantial inter-model uncertainty. In this study, we examine the response of SSW-related surface temperature extremes in the Northern Hemisphere to an abrupt quadrupling of CO₂ concentrations (abrupt-4xCO₂) relative to preindustrial conditions, using simulations from CMIP6 models. To better characterize the model uncertainty, we separate the models into two groups according to their projected change in SSW frequency under 4xCO₂ forcing: models exhibiting a decrease in SSW frequency and models showing an increase.

Models with a decrease in SSW frequency project a strengthened polar vortex and a more persistent SSW signal in the lower stratosphere under 4xCO₂ with respect to preindustrial conditions. Consequently, they show a stronger stratosphere–troposphere coupling and a more pronounced surface response following SSW events. SSWs for this group of models are associated with an increased probability and longer duration of cold spells over Scandinavia, and to a lesser extent over northern Siberia, under 4xCO₂ conditions. In contrast, models with increasing SSW frequency exhibit weaker persistence of the stratospheric signal and reduced surface impacts as a response to 4xCO₂. The results highlight contrasting responses between the two model groups, suggesting that projected changes in polar vortex strength and stratosphere-troposphere coupling play an important role in shaping future SSW impacts at surface extreme events.