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 a structural change from a tropical system to an extratropical system with quite large asymmetric distributions of heavy rainfall and strong wind. This study analyzes the statistical characteristics of ET of TCs in the western North Pacific (WNP) during 1981–2022. The analysis employs the Cyclone Phase Space (CPS) method to evaluate the accuracy of the reanalysis V5 data from the European Centre for Medium-Range Weather Forecasts (ERA5) in identifying ET using different TC center definitions. Results show that the identification based on the TC center defined as the minimum sea level pressure is the most accurate. Subsequently, the study investigates several characteristics of ET events in the WNP. It is found that ETTCs primarily form between 10°N–25°N and 125°E–155°E, with the initiation of ET between 30°N–40°N and completion between 35°N–50°N. ETTCs predominantly occur from April to December, with a peak activity from August to October. Additionally, the average duration of the ET process is 18.5 hours, 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, larger, and experience longer ET durations.