Analysis on typical and atypical spatiotemporal characteristics of Mei-yu precipitation over Zhejiang Province in 2024

This study analyzed the daily precipitation observations from 66 national meteorological stations in Zhejiang Province for June–August during 1973–2024, daily NCEP/NCAR reanalysis data, the CIPAS3.1 western ridge index of the Western Pacific Subtropical High (WPSH), and ERSSTv5 monthly sea surface temperature (SST) data, this study first analyzes the spatial distribution characteristics of precipitation during the Meiyu period in 2024. It then examines the stage-wise evolution of precipitation and its associated circulation patterns and preceding SST background, and finally provides a preliminary discussion on the diversity and complexity of Meiyu conditions in years with near-normal rainfall totals. The results are as follows. (1) The overall statistical characteristics of the 2024 Zhejiang Meiyu were close to the climatological mean, with the onset on 9 June and withdrawal on 4 July. Both the total Meiyu rainfall and the Meiyu intensity index were within the normal range. However, the spatiotemporal distribution of precipitation exhibited atypical features, with an overall pattern of “more rainfall in the west and less in the east.” Precipitation intensity and the number of days with moderate rainfall or above were higher than normal, while some non-Meiyu regions experienced pronounced anomalous rainfall, with non-Meiyu rainfall reaching nearly 90% of the Meiyu rainfall. (2) The Meiyu process in 2024 showed clear stage characteristics. From 9 to 18 June, heavy precipitation mainly occurred in southwestern Zhejiang, while after 19 June it shifted to northwestern Zhejiang. Around 19 June represented a key transition period, during which the strengthening of the East Asian summer monsoon, the intensification and northward shift of the Western Pacific Subtropical High, and the adjustment of the South Asian High acted in concert, leading to enhanced precipitation and a northward shift of the rainfall center over Zhejiang. In terms of SST background, the El Niño event during the preceding winter–spring and SST anomalies in multiple ocean basins jointly influenced the early-summer circulation by enhancing moisture transport while suppressing the southward intrusion of cold air. The interaction between these factors likely contributed to the overall near-normal Meiyu intensity in Zhejiang in 2024. (3) Among the nine near-normal Meiyu years in Zhejiang, precipitation does not necessarily exhibit the typical “central–northern Zhejiang Meiyu” rainfall pattern. The spatial distribution of rainfall varies considerably among different years, and heavy rainfall may also occur in non-Meiyu regions. Therefore, “near-normal Meiyu rainfall” does not necessarily imply a “normal spatial distribution of precipitation.”