Extratropical Transition of Tropical Cyclones in the Western North Pacific: Part II. Duration and Possible Affecting Factors

The duration of the extratropical transition (ET) of tropical cyclones (TCs) is often closely associated with disaster intensity. Therefore, it is essential to understand the key factors that influence ET duration. Using the TC best-track data and reanalysis data, this study investigates the structural characteristics and key influencing factors of ET durations in the western North Pacific (WNP) during 1981–2022. Results show that, compared to TCs that undergo ET rapidly (≤12 h), TCs that experience ET slowly (≥24 h) undergo a more prolonged and complex ET process, maintaining a warm core and an upright structure for extended periods. By contrast, TCs undergoing rapid transition experience structural disintegration within 6–12 hours after ET onset. Further analysis reveals that TCs with longer ET durations are characterized by stronger warm cores, more abundant moisture, lower environmental vertical wind shear, slower translation speed and a longer residence time over warmer sea surface temperatures (SSTs), all of which support the maintenance of axisymmetric structures with strong warm cores. Environmental and structural factors, including cold-air intensity, vertical wind shear, TC translation speed, SST, relative humidity, TC intensity, the radius of 17 m s-1 wind, mean sea level pressure, and the radius of maximum wind, were found to correlate significantly with ET duration. Cold-air intensity exhibited the highest correlation (r = -0.35), followed by TC translation speed and SST. A relative importance analysis shows that the environmental factors play a more substantial role than the internal TC structure, accounting for about 19.23% of the variance, with TC translation speed and cold-air effects explaining11.68%. Based on the nine factors, a statistical forecasting model was developed, which shows considerable skill in predicting ET duration.