The Effects of Increasing the Coupling Frequency and Considering the Sublayer Temperature on the Simulation by CAS-ESM2

The diurnal variation in sea surface temperature (SST) plays a critical role in the simulation of oceanic and climate changes. Only increasing the air-sea coupling frequency from daily to hourly will result in a small SST diurnal amplitude. To accurately simulate this diurnal characteristic, a parameterization scheme for the diagnostic sublayer, which is shallower than the top model layer, and its temperature (sublayer temperature) was applied to models with coarse vertical resolution. In this study, we considered the effect of diurnal variation in SST on the performance of the second version of the Chinese Academy of Sciences Earth System Model (CAS-ESM2). Upon increasing the coupling frequency (EXP1), the diurnal variation in the global SST was reasonably reproduced with an underestimated amplitude. The observed amplitude was reasonably captured when applying sublayer temperature parameterization (EXP2). We compared these findings to a simulation of no diurnal variation in the SST by focusing on climatology, interannual variation, and ENSO asymmetry in a tropical Pacific region. In EXP1, the climatological mean state bias was not improved, but the discrepancy in the overestimation of interannual variability was largely improved. Compared with EXP1, in EXP2, the climatological cold tongue bias was reduced, and the skewness bias was less underestimated. Both diurnal coupling and the sublayer temperature should be considered to improve the performance of climate models.