A Changing North Atlantic Background Amplifies Western European Heatwaves

Houwen Wang , Fanghe Zhao , Yuhang Wang, Hsiang-Jui Wang, Guanru Wu, Shengjun Xi, Weichen Liu

Western European summer heatwaves have increased markedly since the 1990s, culminating in the record warmth of May–July 2026 since 1950. We show that this rise is closely associated with the slowly evolving North Atlantic background. We combine two leading April sea surface temperature modes, representing basin-wide warming and a tripole-like pattern, into an Atlantic Two-Mode Index (ATMI). The index closely tracks the low-frequency increase in heatwave days and peaked in 2026. Under closely matched heatwave-favoring atmospheric circulation, the post-1990 events were 1.3 °C warmer than historical counterparts despite little change in atmospheric ridge amplitude; removing the ATMI-linked low-frequency temperature component reduced this difference to 0.1 °C. Persistent heatwaves occurred in 90% of cases under high-ATMI conditions but were absent under low-ATMI conditions. These results indicate that western European heatwaves have become more frequent mainly because a warmer Atlantic–European background allows similar circulation patterns to produce substantially greater heat extremes.