Enhancement of Summer Monsoon Rainfall by Tropical Cyclones in Northwestern Philippines

抄録

 The influence of tropical cyclones (TC) on the western North Pacific (WNP) summer monsoon flow and its 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 rainfall in the upper 85th, 95th, and 99th percentile were determined using daily rainfall averaged from eight synoptic stations in northwestern Philippines. More than 90 % of HPE days coincide with TC occurrence in the WNP and landfalling TCs only account for 12.8-15.1 % of HPE days. The present study looks at the non-landfalling TCs coincident with the HPEs. The result shows that these non-landfalling TCs are critical in remotely affecting almost all local HPEs in northwestern Philippines.

 Analysis of the TC tracks and their influence on the southwesterly of the summer monsoon flow in Southeast Asia during HPE days show that most of the TCs moved 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 of the 1005-1007 hPa sea level isobar along 20°N that extends to at least 135°E longitude over the northern half of the Philippines, and a deepening of the monsoon trough in northern South China Sea. The 1005-1007 hPa trough induces an eastward shift of the southwesterly that increased the mean zonal wind along western Luzon by 1.94-4.69 times and water vapor flux by 2.67-6.92 times by way of the ‘moisture conveyor belt’. In addition, significant 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 for the upper 85th percentile daily rainfall. These are attributed to the recent changes in WNP TC tracks.