Distinct Effect of the Warming Trend on Extreme Mei-Yu Rainfall in June and July over East Asia: Perspectives from Observation and Simulation

Abstract The record mei-yu rainfall occurred in the Yangtze–Huai River valley (YHRV) and Japan regions in the early summer (June–July) of 2020 and devastated these regions. Whereas several studies had been devoted to the physical processes behind this climate extreme, the impact of the upward trend in the ocean and atmosphere was less investigated. In this study, the potential effect of the linear trend on the mei-yu rainfall extreme was assessed by comparing the 2020 event with a 1998 extreme event. The distinction between June and July was the focus. In June, a significant increasing rainfall trend, accompanied by an enhanced low-level southwesterly, was identified over south China. The linear trend of precipitation also contributed substantially to the total mei-yu rainfall in 2020, which was not observed in 1998. The moisture flux budget indicates that the climatological moisture transport by the linear trend of southwesterly contributes substantially to the extreme rainfall. In contrast to June, the mei-yu rainfall in July over YHRV, accompanied by a low-level easterly trend, showed a decreasing trend. Consequently, the linear trend does not contribute to the extreme rainfall. The numerical experiments, consistent with observations, indicated that the sea surface temperature (SST) warming trend contributed to the extreme mei-yu rainfall in June. This effect was not observed in July.