The radiation bias over the Southern Ocean was terribly bad in MRI-CGCM3 that was used for CMIP5 simulations. However, the bias is significantly reduced in MRI-ESM2 (Yukimoto et al. 2019) that is used for CMIP6 simulations by various modifications related to clouds (Kawai et al. 2019). On the other hand, the double-ITCZ problem is also alleviated in the MRI-ESM2 (Tian and Dong, 2020). Is the reduction in the Southern Ocean radiation bias the cause of alleviation of the double-ITCZ problem? Each modification that contributed to the reduction of the Southern Ocean radiation bias in the MRI-ESM2 was progressively reverted to the corresponding older treatment in order to examine their individual impacts on the ITCZ problem. Results show the double-ITCZ problem worsens almost monotonically when the excessive shortwave insolation over the Southern Ocean increases (Kawai et al. 2020, 2021).