Enhancement of Summer Monsoon Rainfall by Tropical Cyclones in Northwestern Philippines

Abstract

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°N that extends to at least 135°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.

118‐year climate and extreme weather events of Metropolitan Manila in the Philippines

“…TCs moving in the region northeast of Luzon or east of Taiwan are found to enhance the summer monsoonal flow and contribute to rainfall along Western Luzon, including Manila. According to other studies, most extreme rainfall events in Northwestern Philippines are induced by these passing distant TCs (Cayanan et al ., ; Bagtasa, ).…”

“…In addition, peak TC activity in the WNP basin occurs in this season. TCs contribute significant amounts of rainfall to the Philippines, particularly along the western coastal regions of Luzon (Bagtasa, ; ). The Philippines is also within the confluence zone region described by Holland () where southwesterly monsoon flow converges with the easterly trade winds.…”

“…335.8 mm was dumped from August 11 to 16 due to a strong zonal flow induced initially by STS Georgette, then eventually by TY Tip in the Pacific and a tropical depression over Hainan province, China. The TD embedded in the monsoon trough in northern SCS is a characteristic of TC‐monsoon enhancement of rainfall in western Luzon (Bagtasa, ). From August 25 to September 3, TY Wayne (“Miding”) brought 742 mm of precipitation by persisting over northern and northwestern Luzon for more than 3 weeks.…”

“…The influence of TCs on rainfall in Metro Manila is of importance as 76.8 and 86.6% of all rainy days with precipitation in the upper 95th and 99th percentile, respectively, coincides with at least a TC in the vicinity of the Philippines. It was shown (Bagtasa, ) that TCs near the line segment connecting Okinawa and Luzon during JJAS produce westward propagating Rossby waves. The equatorial waves induce low‐level westerlies in SCS that overlap and strengthen the prevailing monsoon westerlies.…”

“…Extreme weather events in the recent years, particularly heavy precipitation from tropical cyclones (TCs), have resulted to thousands of lives lost and damages to properties in the city (Abon et al ., 2011; Lagmay et al ., ). Most heavy precipitation events along western Luzon, including Metro Manila, are due to TCs enhancing the monsoon flow in Southeast Asia (Bagtasa, ). However, despite the impacts of extreme events on the staggering population density of Metro Manila, literature on its climatic condition is limited.…”