Abstract:
The spatial distribution of Meiyu precipitation in Zhejiang Province is highly heterogeneous, with pronounced interannual variability. Investigating both its typical and atypical characteristics is therefore essential for understanding precipitation anomalies. Using daily precipitation observations from 66 national meteorological stations in Zhejiang Province during June-August of 1973-2024, together with the NCEP/NCAR daily reanalysis, the CIPAS3.1 index of the Northwestern Pacific Subtropical High (NWPSH), and ERSSTv5 monthly sea surface temperature (SST) data, this study first analyzes the spatial and temporal characteristics of the 2024 Meiyu precipitation, then examines its intraseasonal evolution, associated atmospheric circulation, and preceding SST background, and finally provides a preliminary discussion of the diversity and complexity of climatologically normal Meiyu years. The results are as follows. (1) The 2024 Meiyu season in Zhejiang Province was statistically close to normal, with the onset on 9 June and withdrawal on 4 July. Both the total Meiyu precipitation and the Meiyu intensity index were within the climatological range, indicating typical overall characteristics. However, the spatial distribution of precipitation was atypical, exhibiting a "more in the west and less in the east" pattern. Precipitation intensity and the number of moderate-to-heavy precipitation days were above normal, while some non-Meiyu regions also experienced remarkable precipitation anomalies. The non-Meiyu precipitation amounted to nearly 90% of the Meiyu precipitation. (2) The 2024 Meiyu process exhibited distinct stage-dependent characteristics. Heavy rainfall was mainly concentrated over southwestern Zhejiang during 9-18 June and shifted to northwestern Zhejiang after 19 June. Around 19 June, a critical transition occurred, during which the strengthening of the East Asian summer monsoon, the development and northward jump of the NWPSH, and the adjustment of the South Asian High jointly enhanced precipitation and caused the precipitation center to migrate northward. In terms of the SST background, the preceding winter-spring El Niño event and SST anomalies over multiple ocean basins influenced the early-summer circulation by enhancing moisture transport and suppressing the southward intrusion of cold air. The combined and competing effects of these SST anomalies likely contributed to the near-normal overall Meiyu intensity in 2024. (3) Among the nine climatologically normal Meiyu years identified in Zhejiang Province, the precipitation pattern did not necessarily conform to the typical "central and northern Zhejiang Meiyu" pattern, and substantial differences existed in the spatial distribution of precipitation among different years. Significant rainfall could also occur in non-Meiyu regions, indicating that a normal total Meiyu precipitation does not necessarily imply a normal spatial distribution of precipitation.