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

Off-season tropical cyclones in the western North Pacific, occurring between December and April, are rare, but their impacts can be catastrophic. Unlike peak-season storms, they strike when disaster preparedness is reduced, public awareness is limited, and forecasting efforts are deprioritized. Recent decades have seen a troubling rise in these events, with Typhoon Bopha (2012), Typhoon Nock-ten (2016), and record-breaking Typhoon Wutip (2019) serving as stark reminders of their destructive potential. Understanding what is driving this increase is therefore an urgent scientific and societal priority.

This study examines the multi-scale physical mechanisms behind the observed increase in off-season landfalling tropical cyclone frequency in the western North Pacific over 1981 to 2022. We find that genesis locations have shifted significantly westward at a rate of 0.60 degrees per year, bringing more storms into coastal-proximate regions where landfall probability is higher, while the overall number of off-season cyclones has remained stable.

Three interacting mechanisms drive this spatial reorganization. Convectively coupled equatorial Rossby waves act as short-timescale triggers, creating windows of opportunity for cyclogenesis through reduced vertical wind shear, enhanced low-level moisture and increased convective ascent. Over 70% of landfalling off-season tropical cyclones form during dynamically supportive wave phases, with a fourfold increase in daily formation probability compared to inactive phases. On decadal timescales, positive phases of the Interdecadal Pacific Oscillation correlate strongly with landfalling cyclone frequency through steering flow modifications that direct storms toward Asian coastlines. At the longest timescale, persistent asymmetric Pacific warming has expanded the Western Pacific Warm Pool northwestward, systematically shifting the most favorable cyclogenesis regions toward densely populated coastlines.

These findings highlight a compounding risk scenario: more frequent landfalls, reduced lead times for preparation, and populations caught off-guard during a period of historically low vigilance. As climate change continues to reshape tropical ocean temperatures, off-season coastal communities across Southeast and East Asia face growing exposure to high-impact events that fall outside traditional risk frameworks.

This work is published open access: https://doi.org/10.1038/s41612-026-01349-0