The Equilibrium Climate Sensitivity (ECS) is estimated in the Community Earth System Model (CESM) by imposing a doubling of atmospheric CO2 concentrations in the slab ocean model (SOM) configuration. The rationale for using the SOM is that, unlike the fully coupled model that takes centuries to reach equilibrium, the CESM-SOM achieves equilibrium within about 50 years after doubling CO2.

The CESM-SOM configuration relies on the calculation of ocean heat transports (known as the "qflux") from a fully coupled simulation. Currently, computing the qflux is an involved procedure that requires an equilibrated, fully-coupled pre-industrial control simulation with the same model version and resolution. Our objective is to develop a simpler SOM experimental design that can provide a useful indication of the ECS, without the need to compute the qflux from an expensive fully coupled simulation with the same model version.

In this study, we compare the ECS of different model versions: CESM1, CESM2, and a recent configuration in the CESM3 development path. We conduct a series of CESM-SOM experiments in which we swap the qflux of these configurations to estimate the ECS. Our results indicate that the details of the qflux between model versions are important. For example, replacing the CESM2 qflux with CESM1 qflux in CESM2-SOM results in a catastrophic cooling of the surface temperature. The Southern ocean, in particular, has a substantial impact on the behavior of the SOM experiments. Small changes in Southern Ocean qfluxes can lead to large departures in the SOM experiments. We will discuss the role of the Southern Ocean and the factors that determine when a computation of a new qflux version becomes necessary.

Additionally, we explore the use of an idealized qflux to identify the key factors affecting the ECS estimation and to assess the feasibility of constructing an idealized qflux to estimate the ECS. Our preliminary findings indicate promising outcomes (specifically, comparable sensitivities) when swapping the CESM2 global qflux with the CESM2 zonal qflux, suggesting that using idealized qflux could serve as a viable alternative to the current approach.