Climatology of Extratropical Transition of Tropical Cyclones in the Western North Pacific Defined by Using Cyclone Phase Space

Abstract

Tropical cyclones (TCs) in the western North Pacific are examined to present climatology of their extratropical transition (ET) during the period of 1979–2004. Two parameters of the cyclone phase space (CPS) are calculated using the Japanese 25-year Reanalysis (JRA-25) dataset as indices of ET for the TCs in the best track data from the Japan Meteorological Agency. The onset and the completion of ET are defined as the beginning of an increase in thermal asymmetry and the evolution from the warm to the cold-core structure, respectively. Forty percent of all TCs are assumed to complete ET. The mean transition period from the ET onset until ET completion is estimated to be about 18 hrs, although 16.8% of all ET storms are transformed into the cold-core structure before the increase in thermal asymmetry. Meridional fluctuation in the location of ET completion by season attains its peak in August, while the peak of ET events occurs in September-October with regards to the ratio of ET frequency to all TCs, the transition period, and intensity at the time of ET completion. The background environment of ET events are also examined using the monthly mean of the JRA-25 and sea surface temperature of the centennial in situ observation-based estimates (COBE). Both air-sea thermal contrast and the tropospheric vertical shear are greater in autumn than in the Baiu season and summer. The results suggest that, whereas a TC can translate into higher latitudes with a warm-core structure in an environment of weak vertical shear and small air-sea contrast in summer, it tends to complete ET in lower latitudes in an environment of strong vertical shear and large air-sea contrast in autumn-winter. These characteristics of ET affect the seasonal variation in the structure of landfalling TCs on the main islands of Japan.