Tropical cyclone potential hazard in Southeast China and its linkage with the East Asian westerly jet

摘要

A new synthesized index for estimating the hazard of both accumulated strong winds and heavy rainfall from a tropical cyclone (TC) is presented and applied to represent TC potential hazard over Southeast China. Its relationship with the East Asian westerly jet in the upper troposphere is also investigated. The results show that the new TC potential hazard index (PHI) is good at reflecting individual TC hazard and has significantly higher correlation with economic losses. Seasonal variation of TC-PHI shows that the largest TC-PHI on average occurs in July-August, the months when most TCs make landfall over mainland China. The spatial distribution of PHI at site shows that high PHI associated with major landfall TCs occurs along the southeast coast of China. An East Asian westerly jet index (EAWJI), which represents the meridional migration of the westerly jet, is defined based on two regions where significant correlations exist between TC landfall frequency and zonal wind at 200 hPa. Further analyses show that an anomalous easterly steering flow occurred above the tracks of TCs, and favored TCs making landfall along the southeast coast of China, leading to an increase in the landfall TC when the EAWJ was located north of its average latitude. Meanwhile, anomalous easterly wind shear and positive anomaly in low-level relative vorticity along TCs landfall-track favored TC development. In addition, anomalous water vapor transport from westerly wind in the South China Sea resulted in more condensational heating and an enhanced monsoon trough, leading to the maintenance of TC intensity for a longer time. All of these environmental factors increase the TC potential hazard in Southeast China. Furthermore, the EAWJ may affect tropical circulation by exciting meridional propagation of transient eddies. During a low EAWJI phase in July-August, anomalous transient eddy vorticity flux at 200 hPa propagates southward over the exit region of the EAWJ, resulting in eddy vorticity flux convergence and the weakening in the zonal westerly flow to the south of the EAWJ exit region, producing a favorable upper-level circulation for a TC making landfall.