a. Model and experiments
We use the High-Resolution Atmospheric Model (HiRAM; Zhao et al. (2009)) developed at the Geophysical Fluid Dynamics Laboratory (GFDL). All simulations are conducted with prescribed climatological monthly means of sea surface temperature (SST) and sea ice from Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) dataset (Rayner et al., 2003) based on the 20-year period from 1986 to 2005, and are integrated for 50 years with constant atmospheric CO2, greenhouse gases and aerosol concentrations (at 1990 levels). The first simulation (referred to as the Control run) follows the default model configuration and thus has fully interactive radiation. The second simulation (referred to as the ClimRad run) overwrites the model-generated atmospheric radiative cooling rates with its monthly-varying climatological values computed from the Control run. Specifically, the overwriting process is implemented as follows: (a) monthly atmospheric radiative cooling rates are retrieved from the last 20 years of the Control run; (b) a multiyear average is applied to the 20-year data to get monthly-varying climatological radiative cooling rates; and (c) each time when the radiation code is called in the ClimRad run, the atmospheric radiative cooling rates are overwritten by its monthly-varying climatological values that are temporally interpolated to the current time step. A summary of these simulations is listed in Table 1. These simulations are also used in Zhang et al. (2021).