Spatial Decomposition of Climate Feedbacks in the Community

4 An ensemble of simulations from different versions of the Community Atmosphere Model 5 (CAM) in the Community Earth System Model (CESM) is used to investigate the processes 6 responsible for the inter-model spread in climate sensitivity. In the CESM simulations, 7 the climate sensitivity spread is primarily explained by shortwave cloud feedbacks on the 8 equator-ward flank of the mid-latitude storm tracks. While shortwave cloud feedbacks have 9 been found to explain climate sensitivity spread in previous studies, the location of feedback 10 differences was in the sub-tropics, not the storm tracks as identified in CESM. The cloud 11 feedback relationships are slightly stronger in the winter hemisphere. The spread in climate 12 sensitivity in this study is related both to the cloud base state and to the cloud feedbacks. 13 Simulated climate sensitivity is correlated with cloud fraction changes on the equator-ward 14 side of the storm tracks, cloud condensate in the storm tracks, and cloud microphysical 15 state on the poleward side of the storm tracks. Changes in the extent and water content 16 of stratiform clouds (that make up cloud feedback) are regulated by the base state vertical 17 velocity, humidity, and deep convective mass fluxes. Within the storm tracks, the cloud base 18 state affects the cloud response to CO2 induced temperature changes and alters the cloud 19 feedbacks, contributing to climate sensitivity spread within the CESM ensemble. 20