Relation of summer AsiaPacific oscillation to tropi cal cyclone activities over the coastal waters of China

Using data from the Joint Typhoon Warning Center (JTWC) optimal typhoon tracks a nd the NCEPNCAR reanalysis, an investigation is made on the summer Asia-Pacif ic Oscillation (APO) in relation to synchronous atmospheric circulation from the coastal waters of East Asia to the western North Pacific (WNP), along with a further exploration on the relationship between the APO and the tropical cyclone (TC) activities over the coastal waters of China. The results show that there exists a close relationship between the APO intensity and the TC activities over the WNP and the coastal waters of eastern China (CWEC) during summer. When the APO is stronger (weaker), the TC activities over the WNP are located in a more west ward (eastward) and northward (southward) position and the TC number over the CW EC greatly increases (decreases). Meanwhile, the atmospheric circulation over th e CWEC is manifested by a low-level anomalous cyclonic (anticyclonic) circulati on, a decreasing (an increasing) vertical shear of zonal wind between the high a nd low levels of the troposphere, and strengthened (weakened) convection. These features are favorable (unfavorable) for the maintenance and development of the TC. The APO also modulates the direction of the steering current that affects th e movement of the TC. Corresponding to a stronger (weaker) APO, the WNP subtropi cal high appears more northward (southward) and eastward (westward), with the we akening (strengthening) of the easterly wind south of the high. This feature fav ors the TC to move northwestwards or westwards at more northern (southern) latit udes, leading to an increase (a decrease) of the numer of the TC that enters int o the CWEC. Moreover, the APO also affects atmospheric circulation over the Sout h China Sea and the tropical WNP from which the TC originates. Corresponding to a stronger (weaker) APO, the monsoon trough over these areas appears northwards (southwards) and westwards (eastwards) and the TC activities are located in a mo re northward (southward) and westward (eastward) position, which favors an incre ase (a decrease) of the TC number over the CWEC.