On the Changes in the Connection Between SSWs and Surface Extreme Weather in Response to an Abrupt 4xCO ₂ Increase

Abstract Sudden Stratospheric Warmings (SSWs) are the most important variability phenomena of the wintertime polar stratosphere. SSWs impact tropospheric circulation and surface temperature, even influencing the occurrence of surface extreme events. Still, there is a lack of consensus on how SSWs will change in the future. In this study, we investigate how the SSW impacts on temperature surface extreme events are affected by an abrupt quadrupling of CO 2 , compared to pre‐industrial conditions, in CMIP6 models. We analyze two subsets of models regarding their changes in SSWs frequency in response to 4xCO 2 (models with decreasing SSWs frequency (NEG), and models with an increase frequency (POS)). Under 4xCO 2 conditions, NEG models project a strengthened polar vortex, have more persistent SSW signals in the lower stratosphere and have larger surface impacts than POS models. NEG models also show a higher probability of days affected by SSW‐related cold spells under a 4xCO 2 increase with longer cold spells over Scandinavia (and to a lesser extent, Siberia). This would be related to the larger persistence of the SSW signal in the lower stratosphere and the intensified stratosphere‐troposphere coupling in response to 4xCO 2 in this group of models. SSWs seem to have less influence on warm extreme events over Greenland and so, they are not affected by changes in the downward coupling after SSWs under a 4xCO 2 increase. These results improve the current understanding of how the stratosphere‐troposphere coupling is modified under 4xCO 2 conditions.