Analysis of Meiyu precipitation characteristics and influencing factors over the Yangtze-Huai River Basin

Abstract

[Objective] The Meiyu phenomenon stands as a distinctive weather occurrence in East Asia, with its precipitation levels during this period exerting significant influence on the emergence of droughts and floods during the summer, thereby incurring substantial socio-economic losses. Therefore, it is necessary to analyze the precipitation characteristics and influencing factors during the Meiyu period over the Yangtze-Huai River Basin, which will help to provide reference for the early warning of drought and flooding. [Methods] This study used observed daily precipitation data from 239 meteorological stations over the Yangtze-Huai River Basin from 1961 to 2020 combined with NOAA and ERA5 reanalysis data to analyze the frequency of the number of days without rainfall and heavy downpour and through the application of Empirical Orthogonal Functions(EOF), it examines the precipitation patterns during the Meiyu period, alongside its correlation with sea surface temperature(SST) and water vapor flux. [Results] Over the Yangtze-Huai River Basin, the average number of days without rainfall during the Meiyu period is distributed incrementally from south to north, and there is a significant increasing trend in Jianghuai area(area Ⅲ) from 1961 to 2020. Over the past six decades, the basin has witnessed 47 heavy downpour events, and the number of stations with daily precipitation up to the level of exceptionally heavy rainfall on each date during the Meiyu period had a significant upward trend. Two primary spatial precipitation distribution patterns emerge during the Meiyu period within the basin: one demonstrating uniform basin-wide trends, showcasing an overall upward trend in precipitation, while the other manifests a north-south antiphase distribution. [Conclusion] The increase in the number of days without rain and the number of stations with precipitation up to the level of exceptionally heavy rainfall during the Meiyu period over the Yangtze-Huai River Basin need to be of some concern. A notable correlation between the Nino3.4 index and Meiyu period precipitation, with higher(lower) Nino3.4 values indicative of increased(decreased) Meiyu precipitation in the Yangtze-Huai River Basin. Furthermore, precipitation fluctuations during this period exhibit significant positive correlations with SST anomalies across tropical Indian Ocean, tropical central-eastern Pacific, and tropical Atlantic regions during preceding winter, spring and concurrent periods. Specifically, positive SST anomalies in these regions correspond to increased Meiyu precipitation, and vice versa. Water vapor transport influencing Meiyu period precipitation primarily originates from the Bay of Bengal, South China Sea, and the western periphery of the western Pacific subtropical high. By June, the convergence of water vapor transport from the South China Sea and western Pacific fosters a robust water vapor flux belt. Subsequently, in July, intensified water vapor transport from the South China Sea and Bay of Bengal further fortifies this flux belt.

Keywords

Yangtze-Huai River Basin / Meiyu / EOF analysis / SST / water vapor transport / precipitation / climate change / influence factor