Fig. 7: Influence of the Pacific Decadal Oscillation to tropical cyclone (TC) variability in East Asia during the mature boreal autumn.

The arrows indicate the direction of the circulation while the blue (red) color represents low (high) pressure regions. A negative PDO phase, characterized by warm sea surface temperature (SST) in the North Pacific, is associated with increased rising motion, anomalous upper-level divergence, and anomalous upper-level easterlies. The risen upper air zonally flows westward and descends as dry air, which prompts the strengthening and westward extension of the Western North Pacific subtropical high (WNPSH). In addition, the said upper-level easterlies collides and stalls the upper-level westerlies associated with the subtropical jetstream, which results to further SST warming in the North Pacific. The weakened and stalled subtropical jetstream retreats northward allowing an expanded WNPSH. Meanwhile, the low-level convergence of anomalous westerlies and easterlies to the east of the Philippines prompts the rise of convective anomalies where it meridionally descends as dry air in the midlatitudes, which also strengthens the WNPSH. In the zonal direction, the risen air flows eastward and descends in the cold SST region in the eastern equatorial Pacific. In the meridional direction, the sunken dry air meridionally rises in the warm North Pacific SST. The strengthened WNPSH and northward-displaced jetstream result to a favorable steering environment that allows tropical cyclones to pass in the EA region.