ENSO simulation in the coupled Korean Integrated Model

Abstract. This study presents a comprehensive evaluation of the El Niño–Southern Oscillation (ENSO) simulation characteristics in the newly established coupled Korea Integrated Model (coupled KIM), which is designed for extended-range prediction. To robustly diagnose ENSO behavior, a 171-year coupled simulation was conducted and systematically evaluated against eight CMIP6 models that share the Nucleus for European Modelling of the Ocean (NEMO) as their oceanic component. Benchmarking the CLIVAR 2020 ENSO metrics framework, we evaluate diverse aspects of ENSO, including its spatial pattern, intensity, skewness, seasonality, and teleconnections, within the context of the tropical Pacific mean state. The coupled KIM exhibits moderate to above-average performance across most key metrics. Notably, the model successfully mitigates the common systematic bias of an excessive westward extension of the ENSO sea surface temperature (SST) pattern. This improvement is linked to a warmer eastern Pacific mean state compared to other CMIP6 models. However, this warm bias leads to a weak zonal SST gradient and a relatively deeper-than-normal thermocline, which consequently underestimates the overall ENSO intensity. Furthermore, this eastern Pacific warm bias undergoes pronounced seasonal expansion during January–February, intimately synchronized with an amplified double ITCZ bias. This seasonal coupling further flattens the simulated ENSO seasonality, although the ratio of ENSO amplitude between winter and spring remains one of the most realistic among the evaluated models. The model also demonstrates outstanding skill in reproducing ENSO skewness, which is plausibly attributed to recent advancements in the vertical mixing processes of the NEMO component. Additionally, the coupled KIM realistically captures tropical precipitation anomalies and their teleconnection patterns, including the extratropical atmospheric responses in the Northern Hemisphere. These findings validate the current developmental trajectory of the coupled KIM and highlight its robust potential for reliable extended-range forecasting.