Clustering Study on Key Circulation Pattern of Flood-Waterlogging Rainfall Events in Middle and Lower Reaches of Yangtze River

Prolonged and widespread heavy rainfall events can significantly impact hydrological conditions, leading to devastating floods. This study focuses on individual cases of flood-waterlogging rainfall events in the middle and lower reaches of the Yangtze River since the 1960s. We employed a combination of SAN (simulated annealing and diversified randomization) clustering method and perturbed ensemble analog method to investigate the key circulation patterns associated with these flood-waterlogging rainfall events and quantify their contributions to heavy rainfall. Results indicate that these flood-waterlogging rainfall events in the middle and lower reaches of the Yangtze River typically occur during the Meiyu season. Averaged over all events, the daily peak precipitation intensity reaches the level of a rainstorm. The 500 hPa circulation patterns associated with extreme rainfall were categorized into four classes: East Asian dipole mode (Class 1), East Asian Sandwich mode (Class 2), South Branch Trough (Class 3), and High-Latitude Double Block (Class 4). The key circulation features of each class are distributed in the Western Pacific Subtropical High, South Branch Trough, mid-latitude westerly trough, and high-latitude blocking activity areas, contributing to 40% to 70% of flood-waterlogging rainfall events. The Western Pacific Subtropical High and South Branch Trough contribute relatively consistently in all four categories, accounting for approximately 30% and 15%, respectively. However, the influence of mid-high latitude systems is less stable, with the Northeast Asian circulation anomaly contributing to an average of nearly 20% to Classes 1, 3, and 4, and the Baikal Lake blocking anomaly making a weak contribution to Class 2. The mid-latitude westerly trough anomaly contributes to approximately 20% of Class 4 events. The significant anomaly regions of low-frequency circulation at 10-30 days and 30-60 days in the four classes are generally consistent with the observed circulation anomaly key regions. Low-frequency circulation in the Western Pacific Subtropical High region has a positive contribution to all events, ranging from about 20% to 70%. Among these, the 10-30 day low-frequency activity also has a notable impact on Class 1 and 2 events. The 30-60 day low-frequency circulation in the South Branch Trough region contributes 27% and 16% to Classes 3 and 4 events, respectively. The key high-latitude low-frequency circulation relating to the flood-waterlogging rainfall events located in Lake Baikal and the Okhotsk Sea (Class 1), the Ural Mountains and the westerly trough region (Class 2), the cold vortex region in Northeast China (Class 3), with their contribution varying among different classes (12%-31%). The findings of this study on key circulation patterns and their quantitative contributions provide valuable insights for a deeper understanding of the formation and prediction of flood-waterlogging rainfall events.