Robust Expansion of Extreme Midlatitude Storms Under Global Warming

Extreme extratropical storms are among the most impact-relevant weather events in the midlatitudes. Under global warming, extreme storms are expected to intensify; however, little is known about the response of their spatial structure. Here, we show that with warming, extreme storms not only become stronger, but also grow larger. By employing multi-model projections from the sixth Coupled Model Intercomparison Project and an idealized aquaplanet simulation, we demonstrate that global warming leads to a robust increase in extreme storm size that is relatively spatially uniform in the midlatitudes and consistent among the models. The physical mechanism for the size increase is the increase in the Rossby deformation radius due to the increase in low-level dry static stability with warming. The storm expansion adds a substantial contribution (2.6%/K), along with the increase in precipitation intensity (3.2%/K), to the increase in storm total precipitation. The results improve our understanding and have significant implications for climate mitigation.