Mechanisms and Subseasonal Predictability of Unprecedented Multiple Tropical Cyclone Event in Autumn 2024

In autumn 2024 (late October to mid-November), an unprecedented multiple tropical cyclone (MTC) event occurred over the western North Pacific (WNP). Six storms formed within a month, surpassing the climatological mean by 4 standard deviations. Four storms were simultaneously active—an event unmatched in 45 years. This anomaly was linked to exceptionally strong Madden–Julian Oscillation (MJO)-related suppressed convection over the Indo-Pacific warm pool, triggering a Kelvin wave response that enhanced cyclonic circulation and moisture convergence, and reduced vertical wind shear over the WNP. All subseasonal-to-seasonal models, including the ECMWF, deep learning FuXi-S2S, and WNP TC hybrid models, predicted the active TC period beyond 4 weeks. Although ECMWF model struggled to predict the inactive-to-active transition, the other two models maintained predictive skill up to 4–5 weeks. A comparison of the ECMWF ensemble members that did and did not capture the MTC event showed that accurate MJO prediction is key to subseasonal MTC forecasting.