Different transient climate responses of two versions of an atmosphere-ocean coupled general circulation model

The Model for Interdisciplinary Research on Climate (MIROC), an atmosphere-ocean coupled general circulation model (AOGCM), has two versions with different resolutions, high (Hi-Res) and medium (Mid-Res). While their equilibrium climate sensitivities (ECS) to CO₂ increases are similar, the transient climate response (TCR) of the Hi-Res version is larger than that of the Mid-Res version. The former shows the highest transient response among the Intergovernmental Panel on Climate Change (IPCC) fourth assessment report (AR4) climate models. Our climate feedback analysis indicates that the higher TCR of the Hi-Res version mainly comes from its larger ice-albedo feedback (SFC-SW) and lower ocean heat uptake (OHU). Since the Hi-Res version shows better agreement with observation than the Mid-Res version concerning the factors that affect the SFC-SW and OHU, the TCR of the Hi-Res version is not considered to be unrealistic compared to that of the Mid-Res version. On the other hand, the two versions have similar SFC-SW values and negligible OHU in ECS experiments performed by the atmosphere-slab ocean coupled general circulation model (ASGCM). In the ASGCM, the difference in SFC-SW between the two versions was likely suppressed due to artificial fluxes applied to the ocean and sea-ice system.