Cloud feedback overestimated in coupled CESM2 simulations relative to AMIP simulations and satellite observations: importance of the pattern effect and implications for climate sensitivity

The community earth system model 2 (CESM2) has a high equilibrium climate sensitivity due to a large cloud feedback. Understanding whether its cloud feedback is realistic is thus of scientific and societal importance. We show that when CESM2 is prescribed observed sea surface temperatures (SSTs) and sea ice during 2002–2022 (atmospheric model intercomparison project (AMIP) conditions), it reproduces satellite observations within uncertainty. When CESM2 is allowed to freely evolve with a coupled atmosphere-ocean during the same period, the modelled cloud feedback becomes much larger than the observations and associated uncertainty. This larger cloud feedback in the coupled simulations compared to AMIP simulations is due to the SST pattern effect, particularly the reduced east-west temperature gradient in the tropical Pacific. The cloud feedback overestimation bias largely arises from the East Pacific. Using a quantitative process-based decomposition, we found that the thick low cloud amount decreases much more in the model than observations thereby reflecting less sunlight and hence warming the planet. These cloud changes are tied to coupled atmosphere-ocean processes such as excessive surface warming in the East Pacific, an enhanced weakening of the Walker circulation, and reduced lower tropospheric stability in the East Pacific relative to observations. Finally, the present-day coupled simulations show several similarities to the future equilibrium warming simulation’s SST pattern and cloud feedback. Should such a warming pattern indeed manifest at equilibrium, our results suggest that the model’s high climate sensitivity may be realized.