Moisture sources and isotopic composition of the 2020 extraordinary and persistent Meiyu rainfall in the Yangtze River valley modulated by large-scale circulations

Abstract

Meiyu rainfall is an important climate phenomenon in East Asia, but its interannual variability has amplified in recent decades. To gain a better understanding of the underlying atmospheric processes of torrential Meiyu rainfall, we investigated the moisture sources and isotopic composition of the 2020 extraordinary and persistent Meiyu rainfall in the Yangtze River valley from June 19 to July 10 using an isotopic regional spectral model. During the Meiyu period, significant amounts of Indian Ocean moisture were transported to Yangtze River valley by Indian monsoonal southwesterlies, especially in the middle levels between 850 hPa and 600 hPa. Whereas moderate amounts of East Asian continent moisture were distributed principally within the boundary layer below 850 hPa. Besides, tiny amounts of South China Sea moisture were transported by the western Pacific subtropical high to the middle and lower reaches of Yangtze River valley within the boundary layer below 900 hPa. With respect to isotopic characteristics, the lowest δ2H and the highest d-excess values of the Indian Ocean moisture were attributable to more rainout and below-cloud evaporation during long-distance transport. In contrast, the isotopic signals of the East Asian continent moisture and the South China Sea moisture were relatively preserved. The 2020 Meiyu rainfall exhibited significant intraseasonal variability and was classified into heavy and light Meiyu periods based on rainfall amount. Heavy Meiyu rainfall with northward migration were characterized by enhanced Indian Ocean moisture with lower δ2H and higher d-excess, resulting from the westward expansion of the western Pacific subtropical high and the deepened East Asian mid-latitude trough.