Improvements and Persistent Biases in the Ocean Climatology Simulated by the Beijing Climate Center Climate System Models from CMIP5 to CMIP6

This study evaluates the ocean climatology simulated by the Beijing Climate Center Climate System Models (BCC-CSMs) participating in phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). CMIP6 BCC models generally outperform CMIP5 ones in reproducing ocean states. The CMIP6 high-resolution model, BCC-CSM2-HR, with an enhanced ocean component, exhibits the best simulation performance overall. Specifically, only BCC-CSM2-HR can accurately reproduce the southern equatorial current in the Pacific Ocean, implying the benefits of an enhanced ocean component. Persistent biases are also identified in BCC models across CMIP5 to CMIP6, including substantial biases in sea surface salinity in the Arctic Ocean, warm biases in the intermediate and deep ocean, and notable salinity biases in the northern Indian Ocean. These biases are also commonly presented in other CMIP5 and CMIP6 models. Furthermore, this study evaluates how BCC models simulate modes of climate variability, such as ENSO (El Ni & ntilde;o-Southern Oscillation), PDO (Pacific Decadal Oscillation), and NPGO (North Pacific Gyre Oscillation). Future plans are also outlined, including the online integration of an ocean surface wave model and the refinement of model resolution, for development efforts aimed at bolstering the accuracy and reliability of BCC model simulations of ocean climatology.