Record-Shattering High Rainfall in the 2024 First Rainy Season and Its Causes

The 2024 first rainy season (FRS) was exceptional, characterized by the highest rainfall since 1961 and recurrent, persistent heavy rainfall in South China. FRS rainfall is predominantly frontal and monsoonal before and after the onset of South China Sea summer monsoon, and it ranked highest and fifth-highest during the two stages of the 2024 FRS, respectively. Diagnostic analyses and numerical experiments suggest that warmer-than-average sea surface temperatures in the tropical North Atlantic and Indian Ocean could excite a lower-tropospheric anticyclonic anomaly over the western North Pacific in both stages, indicating a stronger-than-normal and westward extended western Pacific subtropical high. The latter enhanced the transport of warm, moist air into South China and provided a persistent large-scale background conducive to recurrent heavy rainfall. In contrast, the atmospheric circulation differs between the two stages. During the frontal rainfall stage, the combined effect of the anomalous anticyclone over the Philippines and the one over the Sea of Okhotsk enhanced convergence and ascent over South China. The Philippines anticyclone is driven by active convection near the Maritime Continent and is associated with the Madden–Julian Oscillation (MJO). During the monsoonal rainfall stage, also known as the Dragon Boat Rain stage, a zonally oriented Rossby wave train resembling the British–Okhotsk Corridor pattern facilitated an active Northeast China cold vortex and a resultant southward cold air intrusion, enhancing quasi-stationary dynamic lifting over South China and favoring surplus rainfall. Both the atmospheric circulation and external forcing in 2024 FRS deviate from the conventional knowledge of strong FRS rainfall, posing challenges for seasonal prediction and highlighting the need to better understand and predict record-shattering climate events.