The Role of Regional Ocean–Atmosphere Coupling in Simulating the 2020 Extreme Mei-yu Event

The Yangtze–Huaihe River Valley (YHRV) experienced a record-breaking mei-yu season during the summer of 2020, prompting investigation into the role of regional ocean–atmosphere coupling in simulating this extreme event. Through comparative analysis of regional ocean–atmosphere coupled (CP) and atmosphere-only (WRF) simulations, the aim of this study was to elucidate the mechanisms underlying this phenomenon. Results indicate that CP effectively reproduced the observed sea surface temperature spatial distribution and its daily variation during the period from 25 June to 25 July 2020, and notably reduced the wet biases over the YHRV when compared to WRF simulations. This improvement in representation was manifested in the observed rainfall spatial distribution and daily variability. The wet biases simulated by WRF were associated with a stronger 200-hPa westerly jet and a more westward-positioned western North Pacific subtropical high (WNPSH) with more intense 500-hPa winds and a stronger 850-hPa circulation. However, these biases were significantly reduced in CP simulations. Mechanism analysis revealed that regional ocean–atmosphere coupling over the western Northwest Pacific influenced surface turbulent heat fluxes and atmospheric instability, thereby modulating the intensity and position of the WNPSH and associated circulation subsystems at different levels. Moreover, adjustments in land–sea thermal contrast induced by ocean–atmosphere coupling impacted YHRV precipitation by altering East Asian circulation systems. These findings highlight the significant role of regional ocean–atmosphere coupling in enhancing the simulation and understanding of extreme mei-yu events over East Asia.