Development and evaluation of the GRIMs–NEMO fully coupled model v1.0 for simulating the East Asian Summer Monsoon

Abstract. This study introduces the newly developed GRIMs–NEMO fully coupled model version 1.0 for the first time and assesses its capability to simulate the boreal summer climate, with a particular focus on the East Asian summer monsoon (EASM). The model couples the GRIMs atmospheric model, NEMO ocean, and SI3 sea ice models through OASIS3-MCT, providing a computationally efficient framework for climate simulation. Benchmark experiments show that GRIMs–NEMO requires substantially fewer computational resources than does CESM2, suggesting its potential utility for long-term integrations and ensemble-based studies. The model reasonably reproduces the large-scale features of precipitation, atmospheric circulation, and temperature, and its precipitation skill is broadly comparable to those of CMIP5 and CMIP6 models. Nevertheless, several systematic biases are identified during boreal summer, including excessive rainfall over the equatorial western Pacific, cold surface temperature biases, weakened Tibetan Plateau heating, and circulation errors over the western North Pacific. The model also captures key modes of climate variability associated with the EASM, including the El Niño–Southern Oscillation, the Pacific–Japan pattern, and the boreal summer intraseasonal oscillation, despite remaining biases in their spatial structure, amplitude, and periodicity. A comparison between the coupled and atmosphere-only simulations indicates that air–sea coupling improves sea surface temperature–precipitation feedbacks and the seasonal evolution of the EASM rainband. Overall, GRIMs–NEMO shows reasonable capability as a computationally efficient, fully coupled climate model, while the identified biases highlight the need for further improvements in atmospheric physical parameterizations and the ocean model component.