Evaluating the Nature and Extent of Changes to Climate Sensitivity Between FGOALS‐g2 and FGOALS‐g3.

Equilibrium climate sensitivity (ECS) and its related feedbacks are important metrics that can be used to measure the global mean surface temperature change in future climate projections, and the cloud feedback (CF) is considered to be the main contributor to ECS uncertainties. However, the use of different CF methods significantly affects the CF amplitudes, and the sign of the CF may also change. Combining the radiative kernel approach with a simplified CF method, the differences in the ECS and associated feedbacks between two versions of the Flexible Global Ocean–Atmosphere–Land System model (i.e., FGOALS‐g2 and FGOALS‐g3) were analyzed. Results show that the ECS of FGOALS‐g3 is smaller than that of FGOALS‐g2 (2.8 vs. 3.3 K). The main feedback processes that contribute to the ECS change in FGOALS‐g3 are the weaker surface albedo feedback and stronger negative shortwave CF. The reduced surface albedo feedback in FGOALS‐g3 can be attributed mainly to the differences in the simulations of sea ice area, surface temperature, and the Atlantic Meridional Overturning Circulation, as well as their interactions, compared with FGOALS‐g2. The enhanced negative shortwave CF in FGOALS‐g3 is directly related to the strengthened feedback of cloud area fraction and liquid water path. Furthermore, these changes can be traced back to the different atmospheric moist processes, parameter tuning, ocean grid, and external forcings used in FGOALS‐g3, as these all affect the mean climate state of the model. Plain Language Summary: Equilibrium climate sensitivity (ECS) is a good proxy for projected warming in CO2 forcing experiments. However, there is some uncertainty regarding ECS‐related feedbacks related to the different methods used to calculate them. Three methods, of differing complexity, that can be used to calculate cloud feedback (CF) are compared here, and the simplest method was selected to analyze the change in ECS between two versions of the Flexible Global Ocean–Atmosphere–Land System model (i.e., FGOALS‐g2 and FGOALS‐g3). The main causes of the ECS difference between FGOALS‐g3 and FGOALS‐g2 are the surface albedo feedback and the shortwave CF. These are further related to the different moist processes, parameter tuning, and ocean grids used in the two models. Key Points: The decrease in equilibrium climate sensitivity in FGOALS‐g3 is directly related to its stronger negative shortwave cloud feedbackThe changes in surface albedo and cloud feedbacks are linked to the changed mean state and feedback in FGOALS‐g3The differences in the feedbacks can be traced back to the upgraded model processes in FGOALS‐g3 [ABSTRACT FROM AUTHOR]