Extratropical Transition of Tropical Cyclones in the Western North Pacific: Part Ⅰ. Statistical Characteristics

Extratropical transition (ET) is one of the last phases of tropical cyclones (TCs) and corresponds to the structural change from a tropical system to an extratropical system characterized by pronounced asymmetric distributions of heavy rainfall and strong wind. This study analyzes the statistical characteristics of ET events involving TCs over the western North Pacific (WNP) during 1981–2022. The analysis employs the Cyclone Phase Space (CPS) method to evaluate the accuracy of the fifth-generation reanalysis from the European Centre for Medium-Range Weather Forecasts (ERA5) in identifying ET based on different TC center definitions. Results show that defining the TC center by the minimum sea level pressure yields the most accurate ET identification. Subsequently, the study investigates several characteristics of ET events in the WNP. It is found that TCs undergoing ET (ETTCs) primarily form in the region of 125°–155°E, 10°–25°N, with ET typically initiating between 30°–40°N and completing between 35°–50°N. These ETTCs predominantly occur from April to December, with peak activity observed from August to October. Additionally, the average duration of the ET process is 18.5 h, with longer durations observed from August to October, displaying a roughly 6-year cycle. Spatially, ET events with longer durations tend to occur at lower latitudes. Correspondingly, TCs initiating their ET phase at lower latitudes are typically stronger and larger, and they also experience longer ET durations.