Multiscale Driving Mechanisms of the Genesis of Rare Extreme Multityphoons over the Northwestern Pacific Ocean in November 2024—Based on the Dynamic Genesis Potential Index Decomposition

In November 2024, a rare extreme phenomenon of four typhoons coexisting simultaneously occurred over the northwestern Pacific Ocean. As the typhoon tracks approached China’s coastal areas, many regions experienced strong gusts and were threatened by persistent heavy rainfall. To unravel its formation mechanism, this study systematically investigates the external forcing factors driving this rare extreme phenomenon and the characteristics of large-scale circulation anomalies induced by these factors using the best-track typhoon dataset of the China Meteorological Administration and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) atmospheric reanalysis data. Results show that the synergistic effect between the sustained accumulation of upper-ocean heat on seasonal-to-interannual scales and the subseasonal signals of the Madden–Julian Oscillation (MJO) was the main mechanism responsible for this extreme phenomenon. On the one hand, throughout 2024, the upper-ocean temperature over the western Pacific was abnormally high, and the Pacific Meridional Mode was in a positive phase. These two factors led to a stronger intensity and a westward–northward shift of the western Pacific subtropical high as well as an intensified Australian high. This anomaly pattern further induced strong cross-equatorial flows, transporting abundant moisture and convergent air currents. On the other hand, a quantitative diagnosis based on the Dynamic Genesis Potential Index (DGPI) indicates that the contribution of the vertical motion term (55.0%) was considerably higher than that of wind shear (27.3%) and absolute vorticity (24.7%), making it the core dynamic factor dominating the genesis of these rare extreme multityphoons. Further analysis indicated that this anomalous ascent was primarily driven by the active MJO on the subseasonal scale. MJO-induced Kelvin waves and their associated circulation anomalies unusually maintained and extended the monsoon trough eastward, providing a large-scale, persistent background of cyclonic shear and deep convection conducive to typhoon genesis, ultimately leading to the rare extreme phenomenon of four concurrent typhoons of “four typhoons coexisting”.