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The impact of atmospheric blocking on European heat waves (HWs) and cold spells (CSs) is investigated for present and future conditions . A 50-member ensemble of the second generation Canadian Earth System Model is used to quantify the role of internal variability in the response to blocking. We find that the present blocking-extreme temperature link is well represented compared to ERA-Interim, despite a significant underestimation of blocking frequency in most ensemble members. Our results show a strong correlation of blocking with northern European HWs in summer, spring, and fall. However, we also find a strong anticorrelation between blocking and HW occurrence in southern Europe in all seasons. Blocking increases the CS frequency particularly in southern Europe in fall, winter, and spring but reduces it in summer. For the future we find that blocking will continue to play an important role in the development of both CSs and HWs in all seasons.

Plain Language Summary Atmospheric blocking describes strong and stationary high-pressure systems at midlatitudes. It is frequently connected to surface extremes such as cold spells (CSs) and heat waves (HWs). We use a global climate model (second generation Canadian Earth System Model) with multiple realizations and investigate its performance in representing blocking and the impact on temperature extremes compared to the ERA-Interim reanalysis for present-day (1979-2010) conditions. We also look into the development in the future (2070-2099) under a high-emission scenario (Representative Concentration Pathway 8.5). We find the link between blocking and temperature extremes in Europe well represented in the second generation Canadian Earth System Model compared to ERA-Interim, despite a significant underestimation of blocking occurrence in almost all realizations. Our results show a strong impact of blocking on HWs in northern Europe in summer, spring, and fall, increasing the HW occurrence by a factor 2 and more. However, in southern Europe we find a strong anticorrelation between blocking and HW occurrence in all seasons. Blocking increases the CS frequency in all of Europe in fall, winter, and spring but reduces CSs in summer. For the future we find that blocking will continue to play an important role in the development of both CSs and HWs in all seasons.