Multi-scale drivers of increasing off-season tropical cyclone landfalls in the western North Pacific

Off-season landfalling tropical cyclones (TCs) in the western North Pacific show evidence of increased frequency between 1981–2022. We identify convectively coupled equatorial Rossby waves (CCERWs) and the Interdecadal Pacific Oscillation (IPO) as key physical modulators of off-season landfalling TC frequency, while long-term asymmetric Pacific warming drives systematic spatial reorganization of cyclogenesis. Our analysis reveals that over 70% of off-season landfalling TCs form during supportive CCERW phases, with fourfold higher daily formation probability—significantly stronger coupling than during the peak TC season. Physical mechanism analysis demonstrates that CCERWs create “windows of opportunity” for cyclogenesis. Positive IPO phases correlate strongly with landfalling TC frequency but show no relationship with non-landfalling systems, suggesting preferential steering-flow modulation. Long-term asymmetric Pacific warming appears to contribute to a systematic westward shift in TC genesis locations, bringing more cyclones coastward. These multi-scale physical mechanisms provide new insights into off-season TC behavior during periods of reduced preparedness.