Interannual variation of the Philippines affecting tropical cyclone intensity and its possible causes

Abstract

This study analyzed changes in tropical cyclone (TC) intensity for the past 62 years (1951–2012) by calculating annual average value of central pressure (CP) of TCs that affected the Philippines from July to September. Although TC intensity slightly weakened for the last 62 years, it was not statistically significant. In order to examine the causes of changes in intensity of TCs that influenced the Philippines, nine low CP years and nine high CP years among the 62 years were selected to analyze differences between the two groups. TCs largely occurred in the southeastern quadrant of tropical and subtropical western North Pacific during low CP years and tended to move to the Philippines from the far sea in the southeast of the nation. In differences in deep-layer-mean wind between the two groups, western North Pacific subtropical high (WNPSH) did not develop toward the middle latitudes of East Asia but toward the low latitudes of the region during low CP years. Therefore, TCs occurred in the southeastern quadrant of tropical and subtropical western North Pacific, triggering its movement of long distance westward toward the Philippines. As positive anomalies in precipitable water and at 600 hPa relative humidity, 850 hPa air temperature, and sea surface temperature continued to the Philippines from the southeastern quadrant, a favorable environment where intensity of TCs can be strengthened while they move to the Philippines during low CP years has been formed. In the end, because WNPSH developed toward the low latitudes during low CP years, anomalous easterlies that strengthened in the southern regions of WNPSH blew to the Philippines, and SST in the sea near the nation was heightened. Therefore, because TCs were able to obtain sufficient energy from the relatively warm sea while TCs moved a long distance to the Philippines, TC intensity was able to be strengthened.