Increased Summer European Heatwaves in Recent Decades: Contributions From Greenhouse Gases-Induced Warming and Atlantic Multidecadal Oscillation-Like Variations

Summer heatwaves over Europe, which can cause many deaths and severe damage, have become increasingly frequent over central and eastern Europe and western Russia in recent decades. In this paper, we estimate the contributions of the warming due to increased greenhouse gases (GHG) and nonlinear variations correlated with the Atlantic Multidecadal Oscillation (AMO) to the observed heatwave trend over Europe during 1980-2021, when the GHG-induced warming over Europe exhibits a linear trend. It is found that GHG-induced warming contributes to similar to 57% of the European heatwave trend over 1980-2021, while the cold-to-warm phase shift of the AMO-like variations accounts for similar to 43% of the trend via the intensification of midlatitude North Atlantic jet. The recent trend of heatwaves over western and northern Europe is mainly due to GHG-induced warming, while that over central and eastern Europe and western Russia is primarily related to the combined effect of the AMO-like variations and GHG-induced warming. To some extent, GHG-induced warming is an amplifier of the increasing trend of recent AMO-related European heatwaves. Moreover, European blocking (Ural blocking, UB) is shown to contribute to 55% (42%) of the AMO-related heatwave trend via the influence of midlatitude North Atlantic jet. In the presence of a strong North Atlantic jet during the recent warm AMO phase, UB events concurrent with positive-phase North Atlantic Oscillation can cause intense, persistent and widespread heatwaves over Europe such as that observed in the summer of 2022. Plain Language Summary In recent decades, severe summer heatwaves frequently occurred in Eurasia and North America. However, what leads to the increased European heatwaves in recent decades is still debated. In this paper, we quantify the contributions of greenhouse gases (GHG)-induced warming and the multidecadal variations correlated with the Atlantic Multidecadal Oscillation (AMO) to increased European heatwaves in recent decades by assuming that GHG-induced warming shows a linear upward trend. It is revealed that GHG-induced warming contributes to similar to 57% of the recent European heatwave trend during 1980-2021, whereas the cold-to-warm transition of the AMO-like variations contributes to similar to 43% of the trend via strengthening the midlatitude North Atlantic jet. In particular, European blocking (EB) (Ural blocking, UB) accounts for similar to 55% (42%) of the AMO-related heatwave trend, even though the area extent and intensity of European heat waves depend strongly on whether the EB or UB is concurrent with the positive phase of NAO and whether the midlatitude North Atlantic jet is strong.