Synoptic Flow Patterns Associated with Small and Large Tropical Cyclones over the Western North Pacific

This study investigates the synoptic flow patterns associated with small and large tropical cyclones (TCs) that occurred over the western North Pacific between 1991 and 1996. The size of a TC is defined as the azimuthally averaged radius from the TC center at which the relative vorticity decreases to 1 × 10−5 s−1. Calculation of the relative vorticity is based on the satellite-derived surface winds of the European Remote Sensing Satellites 1 and 2 (ERS-1 and ERS-2). Operational analyses of the U.K. Met Office are employed to identify the synoptic patterns around the TCs. Characteristic synoptic patterns at 850 hPa can be identified with TCs of different sizes. The southwesterly surge and late-season patterns are related to large TCs while the dominant ridge and monsoon-gyre patterns are associated with the occurrence of a small TC. A case study of Typhoon Bart demonstrates the time evolution of the synoptic pattern and its relationship with the TC size change. Bart exhibited a distinct transition from the dominant ridge synoptic pattern to the southwesterly surge synoptic pattern and, correspondingly, the size of Bart increased significantly.