Quantifying the Inter-Model Uncertainty of Extreme Extratropical Cyclones in the North Atlantic winter in a Warming Climate

Extratropical cyclones (ETCs) are the primary drivers of severe weather over the North Atlantic, yet projections of changes in the intensity of the most
extreme storms under climate change remain highly uncertain. This study investigates inter-model uncertainty in future climate projections of extreme cyclones in winter, arising from competing processes of reduced midlatitude baroclinicity and enhanced moisture availability. We assess their contributions to projected changes in extreme cyclone intensity.

We analyze the future changes in the most intense 100 ETCs in winter across 13 CMIP6 models under the highest forcing scenario (SSP5-8.5; 2070–2100 vs 1980–2010), using 850 hPa vorticity tracking and cyclone-centered composites of precipitation, near-surface temperature gradients, and surface winds. Our results show that the majority of models project an intensification of the most intense cyclones in the North Atlantic, relative to the historical runs, with an increase in precipitation associated with extratropical cyclones in 11 out of 13 models. Near-surface meridional temperature gradients, however, exhibits a weakening in 9 out of 13 models, reflecting reduced low-level baroclinicity.

Furthermore, surface wind projections reveal no clear consensus, with half of the models projecting strengthening and half projecting weakening of surface winds. In addition, 7 out of 13 models project an eastward shift in peak intensity towards northwestern Europe, while latitudinal changes lack a robust pattern.

Our results show that projected intensification of extreme North Atlantic cyclones in terms of vorticity is accompanied by robust thermodynamic sig-
nals, with intensified precipitation in most models despite weakened near-surface meridional temperature gradient. In contrast, the associated surface wind response shows large inter-model variability, with no consistent change across models, highlighting the need for further assessment of surface wind projections.