Enhancement of Summer Monsoon Rainfall by Tropical Cyclones in Northwestern Philippines

The influence of tropical cyclones (TC) on the western North Pacific (WNP) summer monsoon flow-as well as the impact on rainfall in the Philippines during the months of June to September from 1958 to 2017-were investigated. High precipitation event (HPE) days with measured rainfall in the upper 85th, 95th, and 99th percentiles were determined using daily rainfall averages via data acquired from eight synoptic stations in northwestern Philippines. More than 90 % of HPE days coincide with TC occurrence in the WNP, whereas landfalling TCs only account for 12.8-15.1 % of HPE days. The present study looks at the non-landfalling TCs that are coincident with HPEs. The result shows that these non-landfalling TCs remotely play a key role that affects almost all local HPEs in northwestern Philippines. Analysis of the TC tracks and their influence on southwesterly summer monsoon flow in Southeast Asia during HPE days shows that most of the TCs move along a line segment connecting northern Luzon and Okinawa, Japan. The composite low-level flow of all HPE days is characterized by a zonally-oriented eastward trough at the 1005-1007 hPa isobar along 20 degrees N that extends to at least 135 degrees E longitude over the northern half of the Philippines; a deepening of the monsoon trough in northern South China Sea also occurs. The 1005-1007 hPa trough induces an eastward shift on the southwesterly flow causing a 1.94-4.69 times increase in mean zonal wind velocity and 2.67-6.92 times increase in water vapor flux (via moisture conveyor belt) along western Luzon. In addition, increasing trends of 6.0 % per decade in the mean annual number of HPE days per decade and 12.7 % per decade in the annual total HPE precipitation are found to be significant in the upper 85th percentile of daily rainfall. These increases are attributed to the recent changes in WNP TC tracks.