Understanding equilibrium climate sensitivity changes from CMIP5 to CMIP6: Feedback, AMOC, and precipitation responses

Abstract

To unravel the equilibrium climate sensitivity (ECS) changes of the models of the Coupled Model Intercomparison Project (CMIP) during the upgrade, 10 pairs of CMIP phase 5 (CMIP5) and phase 6 (CMIP6) models from different centers were categorized into high and low ECS groups according to their ECS and surface air temperature response to CO2. Results showed that the higher ECS of the CMIP6 multimodel mean is derived primarily from five models of the high group, and is contributed by both stronger positive cloud feedback (CF) and stronger albedo feedback relative to the corresponding values in the CMIP5 models, and the spread of CF is associated with that of the ECS. Positive albedo feedback in the Arctic may be related to the relationship between weakening of the Atlantic Meridional Overturning Circulation (AMOC) and diminishing sea ice area (SIA). For example, the stronger albedo feedback in the CMIP6 high group is linked to the strongly weakening AMOC and sharply reducing SIA, further associated with their mean states compared with those of the CMIP5 models. The higher CF in the CMIP6 high group results from the greater reduction in both cloud area fraction (CAF) and ice water path (IWP) and the weaker increase in the liquid water path (LWP), leading to enhanced shortwave CF and reduced longwave CF. Furthermore, when the total precipitation response is dominated by only the convective or stratiform component, it is prone to substantial increase by the model upgrade, thereby notably affecting the changes in CAF, IWP, and LWP and the variation in CF and ECS in the high group.