Possible influence of western North Pacific monsoon on TC activity in mid-latitudes of East Asia

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This study analyzed the correlation between tropical cyclone (TC) frequency and the Western North Pacific monsoon index (WNPMI), which have both been influential in East Asia’s mid-latitude regions during the summer season over the past 37 years (1977–2013). A high positive correlation existed between these two variables, which was not reduced even if El Nino-Southern Oscillation (ENSO) years were excluded. To determine the cause of this positive correlation, the highest (positive WNPMI phase) and lowest WNPMIs (negative WNPMI phase) during a nine-year period were selected to analyze the mean difference between them, excluding ENSO years. In the positive WNPMI phase, TCs were mainly generated in the eastern seas of the tropical and subtropical western North Pacific, passing through the East China Sea and moving northward toward Korea and Japan. In the negative phase, TCs were mainly generated in the western seas of the tropical and subtropical western North Pacific, passing through the South China Sea and moving westward toward China’s southern regions. Therefore, TC intensity in the positive phase was stronger due to the acquisition of sufficient energy from the sea while moving a long distance up to East Asia’s mid-latitude. Additionally, TCs occurred more in the positive phase. Regarding the difference of the two phases between the 850 and 500-hPa streamlines, anomalous cyclones were strengthened in the tropical and subtropical western North Pacific, whereas anomalous anticyclones were strengthened in East Asia’s mid-latitude regions. Due to these two anomalous pressure systems, anomalous southeasterlies developed in East Asia’s mid-latitude regions, which played a role in the anomalous steering flows that moved TCs into these regions. Furthermore, due to the anomalous cyclones that developed in the tropical and subtropical western North Pacific, more TCs could be generated in the positive phase. Both the lower and upper tropospheric layers had warm anomalies in most regions of the Western North Pacific, while relative humidity in the middle tropospheric layer showed a positive anomaly in the tropical and subtropical western North Pacific, which provided a better environment to strengthen TC intensity in the positive WNPMI phase. Furthermore, a negative anomaly was manifested not only in the tropical and subtropical western North Pacific, but also in East Asia’s mid-latitude regions, with 200–850-hPa vertical wind shear, while a warm sea surface temperature anomaly was shown in East Asia’s mid-latitude seas, which further strengthened TC intensity in the positive phase. The analysis on the global-scale atmospheric circulations showed that converged air in the lower layer of the subtropical western Pacific during the positive phase diverged in the upper layer, which moved westward and converged in the upper layer of the equatorial Indian Ocean and then diverged in its lower layer.