An emergent constraint on climate sensitivity by seasonal cycle of extratropical low clouds

A large spread in climate model estimates of equilibrium climate sensitivity (ECS), i.e., the global mean surface temperature increase in response to the radiative forcing of a doubling of CO2, can be largely ascribed to model uncertainty in representing low cloud shortwave (SW) radiative feedback. In this study, we show that the estimated ECS in a model is highly related to its depiction of seasonal variations of low cloud fraction (LCF) over the extra-tropics in historical simulations. Marked reduction of extratropical LCF from winter to summer is found in the models with ECS > 4.5 K, but is largely absent in the models with ECS < 3.3 K. The rapid reduction of extratropical LCF with increasing local surface temperature as manifested in its seasonal cycle in the high ECS models is in accord with significantly reduced extratropical LCF under a warming climate, and tends to be in better agreement with satellite observations of both LCF and the top-of-the-atmosphere SW cloud radiation effect. Given the emergent relationship between the seasonal variations of extratropical LCF and ECS derived in this study, the low ECS values in several models are likely underestimated. Further analyses using a cloud regime-based approach suggest that the distinct low cloud feedbacks over the extra-tropics between the high and low ECS models can be ascribed to their different prevailing cloud regimes in governing the low cloud variability. The strong positive low cloud feedback over the extra-tropics in the high ECS models involved with both seasonal cycle and climate trend is resulted from a strong coupling between low clouds and the mid- and high-top clouds through tropospheric vertical motion associated with the storm track variability. In contrast, a low-top cloud regime that largely follows the lower-tropospheric stability tends to dominate low cloud variability in the low ECS models. Considering the uncertainty in depicting the extratropical low cloud variability using satellite retrievals, we call for further investigations to characterize the prevailing cloud regimes and their roles in regulating low cloud variability over the extra-tropics.