Track-Pattern-Based Characteristics of Extratropical Transitioning Tropical Cyclones in the Western North Pacific

Based on the Regional Specialized Meteorological Center (RSMC) Tokyo-Typhoon Center best-track data and the NCEP-NCAR reanalysis dataset, extratropical transitioning (ET) tropical cyclones (ETCs) over the western North Pacific (WNP) during 1951–2021 are classified into six clusters using the fuzzy c-means clustering method (FCM) according to their track patterns. The characteristics of the six hard-clustered ETCs with the highest membership coefficient are shown. Most tropical cyclones (TCs) that were assigned to clusters C2, C5, and C6 made landfall over eastern Asian countries, which severely threatened these regions. Among landfalling TCs, 93.2% completed their ET after landfall, whereas 39.8% of ETCs completed their transition within one day. The frequency of ETCs over the WNP has decreased in the past four decades, wherein cluster C5 demonstrated a significant decrease on both interannual and interdecadal timescales with the expansion and intensification of the western Pacific subtropical high (WPSH). This large-scale circulation pattern is favorable for C2 and causes it to become the dominant track pattern, owning to it containing the largest number of intensifying ETCs among the six clusters, a number that has increased insignificantly over the past four decades. The surface roughness variation and three-dimensional background circulation led to C5 containing the maximum number of landfalling TCs and a minimum number of intensifying ETCs. Our results will facilitate a better understanding of the spatiotemporal distributions of ET events and associated environment background fields, which will benefit the effective monitoring of these events over the WNP.