A study of the relationship between the AsianPacific oscillation and tropical cyclone activities over the coastal waters of China during autumn

Using the datasets from the JTWC optimal typhoon tracks and the NCEP/NCAR reanalysis data, we investigate the interannual variability of the autumn (September and October) AsianPacific Oscillation (APO) and its relationships with the atmospheric circulation over the AsianPacific region and tropical cyclone (TC) activities over the western North Pacific and the coastal waters of China. The results show that the interannual variability of the APO is closely related to the TC activity over the western North Pacific and the coastal waters of China during autumn. Corresponding to stronger (weaker) APO, the TC often appears in a more westward (eastward) position and there is a more (less) TC number in the coastal waters of China. The APO may affect the vertical shear of the zonal wind between high and low levels of the troposphere, the lowertropospheric convergence, and the midtropospheric steering current over the coastal waters of China and thus the TC activity over the western North Pacific and the coastal waters of China. When the APO is stronger (weaker), the longwave trough over the extratropics of East Asia is weaker (stronger) and the East Asian winter monsoon is weaker (stronger), accompanying the invasion of weaker (stronger) cold masses into the tropical western North Pacific and the coastal waters of China, favoring (not favoring) the occurrence and development of the TC over these seas. Moreover, corresponding to stronger APO, the subtropical ridge over the western North Pacific is located in a more westward position and the easterly current south of the ridge is stronger, which favors the TC to move westwards along the stronger easterly steering current or most likely causes the TC to turn its moving direction in a more westward position. Corresponding to weaker APO, the ridge is weaker and located in a more eastward position, the easterly steering current is weaker, both of which does not favor the TC to move westwards or favors the TC to change the direction in a more eastern position.