Quantitative Analysis on Large Scale Circulation System Modulating Landfalling Tropical Cyclone Activities in the Diverse Chinese Regions

The circulation patterns associated with the subtropical high and the monsoon trough over the western North Pacific, which respectively modulate landfalling tropical cyclones (TCs) to the north and south of Xiamen, are systematically explored using the Japanese 25-year Reanalysis (JRA-25) daily wind field and tropical cyclone (TC) dataset derived from the Joint Typhoon Warning Center, USA. Furthermore, the relationships between landfalling TCs and large scale circulations is examined quantitatively by defining the indices of westward-extension and south-northward shift of the subtropical high ridges, as well as those of the monsoon trough slope and intensity. The results reveal that these indices act as key indicators of TC genesis locations, intensity and tracks. The location of subtropical high is responsible for tracks of landfalling TCs to the north of Xiamen. The slope of monsoon trough line impacts the tracks of landfalling TCs to the south of Xiamen, and modulates north-southward shift of TC genesis locations over the western North Pacific. Moreover, the average vorticity of monsoon trough line influences the energy of TC genesis.