Dual drivers of European severe weather: how the NAO and low-frequency climate variability modulate the seasonal transition from shear- to CAPE-dominated tornadogenesis

The climatology of severe convective storms is well documented in the United States, but the large-scale atmospheric drivers of European tornadoes remain less well understood. This study investigates the interannual variability and environmental forcing of tornadoes across the Euro-Atlantic domain using observational reports and ERA5 reanalysis over the 1950–2022 period. Statistical analysis reveals a moderate multidecadal increase in tornado frequency. Sensitivity tests restricting data to significant events (F2 and above on the Fujita scale) confirm this trend is not merely an observational artifact: it displays robust links with background climate change and low-frequency variability, such as the Atlantic Multidecadal Oscillation (AMO), which provide the essential thermodynamic backdrop for deep convection. A sharp seasonal dichotomy is additionally identified in synoptic forcing controlled by the North Atlantic Oscillation (NAO). Winter events prevail during positive NAO phases, with strong zonal flow driving a high-shear, low-CAPE (convective available potential energy) kinematic regime. Summer events, conversely, are tied to negative NAO phases, where atmospheric blocking fosters a low-shear, high-CAPE thermodynamic regime. Spring exhibits a distinct continental susceptibility to NAO-related blocking. Previously assumed Mediterranean multidecadal teleconnection (WeMO) were also examined, but are shown to be spurious artifacts linked to tornado reporting trends. These findings demonstrate that European tornadogenesis hinges on seasonally dependent atmospheric environments modulated by low-frequency, AMO-related climate variability with NAO-driven overprints. Recognising these distinct teleconnection signatures provides a potentially fruitful framework for anticipating interannual to decadal fluctuations in European severe weather risk.