Analysis of the Monthly-Scale Air-Sea Causes for the 2020 Summer Rainfall over the Middle-Lower Yangtze River

From a month-to-month evolution perspective, this study analyzes changes in atmospheric circulation and the forcing effects of three‑dimensional thermal structures such as oceanic heat content during the summer of 2020, using ERA5, OISST, and SODA reanalysis datasets. The results show that in June, the double warm‑core structure of the warm pool drove adjustments in atmospheric circulation and moisture transport, leading to anomalously high Meiyu rainfall in the area north of the Yangtze River. In July, as the thermal forcing of the warm pool peaked, the atmospheric circulation further intensified and generated strong low‑level wind convergence, resulting in a precipitation increase of approximately 90% in the middle and lower reaches of the basin. Entering August, the thermal structure of the western Pacific warm pool underwent reorganization. The weakening of its heat source induced an eastward retreat of the subtropical high and a decay in moisture transport, ultimately causing reduced precipitation over the middle and lower reaches of the Yangtze River. In summary, this study reveals the key “three-dimensional oceanic thermal forcing–atmospheric circulation response–regional precipitation” mechanism responsible for precipitation anomalies in the region. This provides a novel and diagnostic perspective for a deeper understanding of extreme precipitation events under Western Pacific Warm Pool thermal conditions.