The frequency of autumn western North Pacific tropical cyclone linking to spring western Pacific SST anomalies

Autumn typhoons, despite their lower occurrence rate, impose significant, and at times, greater economic impacts on Asia than summer typhoons. Accurately predicting the interannual variations in autumn typhoon frequency remains a persistent challenge. Our finding discovers a pattern of sea surface temperature (SST) anomaly in the western Pacific, termed the horseshoe-shaped mode (HWP), and finds a strong interannual correlation between the February–March SST anomalies in the western Pacific and September–October tropical cyclones (TCs) frequency in the western North Pacific. The February–March warming HWP triggers enhanced easterlies over the equatorial Pacific as a Kelvin wave response, strengthening the east-west SST gradient and, in turn, further intensifying the September–October HWP through positive feedback. The intensified September–October HWP could boost upwelling in the northwestern and southwestern Pacific and induce dynamic subsidence in the equatorial western Pacific, mirroring a localized reversal of Hadley-like circulation. This is accompanied by higher relative humidity, cloud cover, and longwave radiation over the western North Pacific, warming local SST and fostering TC formation. An SST-based statistical linear model could reproduce September–October TCs for both training and testing periods, demonstrating the reliability and stability of this linear model. Our results indicate that HWP could be an important indicator for improving TC prediction level.