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 The second version of Chinese Academy of Sciences Earth System Model (CAS-ESM2) is described.  Strength and weakness of the model simulations from the CMIP6 DECK experiments are described along possible causes.  The model has an equilibrium climate sensitivity of 3.4 K with a positive cloud feedback from the shortwave radiation.

Responses of tropical Pacific air–sea CO2 flux (fCO2) to El Niño–Southern Oscillation (ENSO) events in 14 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are examined. The contributions of sea surface temperature (SST), dissolved inorganic carbon in surface seawater (DIC), and total alkalinity of surface seawater (TALK) to interannual variation of ln(pCO2sea) (instead of partial pressure of CO2 in surface seawater pCO2sea) are quantified based on standardized empirical orthogonal function (EOF) results. Results show that six of the models have poor responses because they fail to reproduce observed interannual variation of pCO2sea in the central-eastern tropical Pacific. These six models underestimate the contribution of DIC interannual variation to interannual variation of pCO2sea in the central-eastern tropical Pacific due to a weak relation between interannual variation of upwelling and ENSO events or a weak relation (including no relation) between interannual variation of upwelling and that of DIC. Furthermore, some models have biases in interannual variation of DIC, in terms of both location and period, that are associated with interannual variation of modeled precipitation. It is also found that two models produce unreasonable interannual variation of bioproductivity, which enlarges interannual variation of DIC in the central-eastern tropical Pacific; this may partly explain why the influence of upwelling on interannual variation of DIC is weak in these models, even when the relationship between interannual variation of DIC and ENSO index is reasonable.