Comparisons of radiation-circulation coupling over the tropical and subtropical ocean between AMIP6 and CMIP6

Abstract

This study compares radiation-circulation coupling over tropical and subtropical oceans by examining ice water path, radiation, low-level wind, and precipitation fields from the uncoupled prescribed sea surface temperature AMIP6 and the fullycoupled CMIP6 historical runs. Ice water path of the CMIP6 ensemble shows a closer agreement than the AMP6 ensemble, particularly in the subtropics. The inclusion of falling ice (snow) radiative effects (FIREs), in general, improves simulation of radiation, low-level wind, and precipitation fields over the northwest Pacific, Southeast Pacific Convergence Zone (SPCZ), equatorial eastern Pacific and Atlantic Intertropical Convergence Zone (ITCZ). When both AMIP6 and CMIP6 models are divided into two groups with inclusion and exclusion of FIREs, the impact of FIREs is most pronounced in ITCZ and the subtropical trade-wind regions in CMIP6 but over the tropical Pacific and SPCZ in AMIP6. This suggests that active ocean plays a significant role in radiation-circulation coupling. The CMIP6 models with FIREs have less over-estimated biases in upward longwave radiation over the convective zones in Pacific and Atlantic and less low-level divergence of anomalous flows over convective zones, i.e., stronger trade winds. The circulation changes stronger in CMIP6 over the trade wind regions than in AMIP6, which suggests that the role of active ocean is important in testing an improved physical process in models. This conclusion is also supported by more systematic improvements in groups of models with inclusion of FIREs in CMIP6 than in AMIP6, hinting the important roles of FIREs in radiationcirculation coupling.