Relation of Summer Asian-Pacific Oscillation to Tropical Cyclone Activities over the Coastal Waters of China

Using the Joint Typhoon Warning Center (JTWC) optimal typhoon tracks data and the NCEP/NCAR reanalysis, an investigation is made on the summer Asian-Pacific Oscillation (APO) in relation to synchronous air circulation from the coastal waters of East Asia to 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 strong correlation between the APO intensity and TC activities over the WNP and 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 westward (eastward) and northward (southward) position and the TC number over the CWEC greatly increases (decreases).Meanwhile, in stronger (weaker) APO years, the atmospheric circulation over the CWEC is manifested by a low-level anomalous cyclonic (anticyclonic) circulation, a decreasing (an increasing) vertical shear of zonal wind 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 air current that steers the movement of the TC. Corresponding to a stronger (weaker) APO, the WNP subtropical high appears more northward (southward) and eastward (westward), with the weakening (strengthening) of the easterly wind south of the high. This feature favors the TC moving at more northern (southern) latitudes, leading to an increase (a decrease) of the number of the TCs into the CWEC. Moreover, the APO also affects the atmospheric circulation over the South China Sea and the tropical WNP where the TC is originated. Corresponding to a stronger (weaker) APO, the monsoon trough over these areas appears northward (southward) and westward (eastward), which favors an increase (a decrease) of the TC number over the CWEC.