JournalOctober of2019 the Meteorological Society of Japan, 97(5),G. 967−976, BAGTASA 2019. doi:10.2151/jmsj.2019-052 967

Enhancement of Summer Monsoon Rainfall by Tropical Cyclones in Northwestern

Gerry BAGTASA

Institute of Environmental Science & Meteorology, University of the Philippines, Philippines

(Manuscript received 23 November 2018, in final form 25 May 2019)

Abstract

The influence of tropical cyclones (TC) on the western North Pacific (WNP) summer monsoon flow—aswell as the impact on rainfall in the Philippines during the months of June to September from 1958 to 2017—were in- vestigated. High precipitation event (HPE) days with measured rainfall in the upper 85th, 95th, and 99th percen- tiles 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 and Okinawa, Japan. The composite low-level flow of all HPE days is characterized by a zonally-oriented eastward trough atthe 1005 – 1007 hPa isobar along 20°N that extends to at least 135°E longitude over the northern half of the Philip- pines; a deepening of the monsoon trough in northern South 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.

Keywords summer monsoon; ; heavy precipitation events; the Philippines

Citation BAGTASA, G., 2019: Enhancement of summer monsoon rainfall by tropical cyclones in northwestern Philippines. J. Meteor. Soc. Japan, 97, 967–976, doi:10.2151/jmsj.2019-052.

Astronomical Services Administration 2018; Wang 1. Introduction 2002). The southwest monsoon, or in the local lan- In the Philippines, the rainy season is officially guage “Haba-gat”, brings in warm moist air from the declared at the onset of the tropical western North South China Sea (SCS; locally known as West Phil- Pacific (WNP; 10 – 20°N, 110 – 160°E) summer mon- ippine Sea) along the western coastal regions of the soon period (Philippine Atmospheric, Geophysical and Philippines. The southwest monsoon regime usually starts by the end of May or June and ends in Septem- Corresponding author: Gerry Bagtasa, Institute of Environ- ber. A study by Asuncion and Jose (1980) found that mental Science & Meteorology, University of the Philip- pines, Diliman , Philippines approximately 43 % of the country’s total rainfall E-mail: [email protected] occurs during this season. The onset of Habagat is also J-stage Advance Published Date: 14 June 2019 the basis used for planting calendars in most regions ©The Author(s) 2019. This is an open access article published by the Meteorological Society of Japan under a Creative Commons Attribution 4.0 International (CC BY 4.0) license (https://creativecommons.org/licenses/by/4.0). 968 Journal of the Meteorological Society of Japan Vol. 97, No. 5 of the country, hence its importance for the manage- events. ment of national water resources and agriculture. The interaction between TCs and the WNP south- Previous studies found that there is a small but sig- west monsoon flow can exhibit a significant effect on nificant decreasing trend in rainfall during the summer rainfall in certain parts of the Philippines. A researcher monsoon period from 1960 to early 2010. This is previously reported that the northwestern Philip- likely caused by the weakening of westerly winds pines—the region where the highest TC-induced rain (Cruz et al. 2013; Villafuerte II et al. 2014). Simul- and TC rain contribution is found—is most affected taneously, the numbers of extreme events associated by this TC-monsoon interaction (Bagtasa 2017). Fur- with the same monsoonal flow have been shown to be thermore, 44.8 % of the Philippine population resides on the rise (Cinco et al. 2014). Other than the influ- in this region (Philippine Statistics Authority 2017). ence of the WNP monsoon on Philippine rainfall, TCs This remote TC precipitation effect is usually dis- also play a major role in modulating summer monsoon cussed on national weather reports by the Philippine rainfall in the Philippines. Kubota et al. (2017) found Weather Bureau, Philippine Atmospheric, Geophysical the summer monsoon onset tended to start earlier and Astronomical Services Administration (PAGASA). in May after the 1990s due to the influence of TC However, there is limited literature characterizing activity in the Philippine Sea and SCS. The variability this underlying mechanism. The aim of this paper is of TC-induced rainfall is significantly correlated with to investigate the characteristics of TCs as well as the total annual rainfall variability in northwestern Phil- synoptic environment that induces heavy precipitation ippines. Also, the observed increasing trend in TC- events along parts of the northwestern Philippines induced rainfall since early 2000 led to the reversal during the southwest monsoon season. The next of the reported decreasing summer monsoon rainfall section will describe the data and methodologies used trend. However, the rainfall contribution of TCs along to determine TC and monsoonal flow characteristics. the western Philippines is mainly due to the indirect The results discussed in Section 3 and Section 4 will effect of TCs, as their occurrence enhances the pre- summarize the present study. vailing southwest monsoonal flow (Bagtasa 2017). 2. Data and methodology This TC-monsoon interaction resulted in heavy precipitation events along the western region of the 2.1 TC and rainfall data Philippines. Cayanan et al. (2011) reported the in- In the present study, data from TCs that are formed fluence of TCs on monsoonal winds and interactions in the WNP basin from 1958 to 2017 were gathered with the Cordillera mountain range along western from the reanalyzed best track data via the website: Luzon. Heisterman et al. (2013) used the then newly Regional Specialized Meteorological Centre Tokyo acquired Tagaytay C-band radar to map the rainfall Typhoon Center of the Japan Meteorological Agency distribution of a heavy precipitation event across the (http://www.jma.go.jp/jma/jma-eng/jma-center/rsmc- Manila Bay area in August of 2012. Referred to as the hp-pub-eg/trackarchives.html). Rainfall data from Asian “2012 Habagat event”, this precipitation event was Precipitation - Highly Resolved­ Observational Data caused by the slow movement of ; Integration Toward Evaluation of the Water Resources located more than 1000 km away from Manila to the (APRHODITE) Monsoon Asia, version 1101R2 (Yata­ east of , Typhoon Haikui caused a continuous gai et al. 2012) from 1958 to 2007, and the National strong monsoonal flow resulting in over 1000 mm Climatic Data Center’s Global Summary of the Day of observed rainfall in Manila in a few days span. A (GSOD; http://www1.ncdc.noaa.gov/pub/data/GSOD) year later, the “2012 Habagat event” was followed from 2008 to 2017 were also used. Reanalysis data by a similar heavy precipitation that led to two heavy of mean sea level pressure, total column water vapor rainfall events with twenty-year return periods oc- flux, and 850 mb wind are from the Japanese 55-year curring on two consecutive years. During the “2013 Reanalysis (JRA55) project (Kobayashi et al. 2015), Habagat event”, Tropical Storm Trami (locally named which is carried out by the Japan Meteorological “Maring”) was similarly located to the east of Taiwan, Agency (JMA) at 1.25° grid resolution at 6-hour inter- strengthening the southwest monsoon flow which also vals (downloaded from https://rda.ucar.edu). led to widespread flooding, damaged properties, and loss of life in the Manila metropolis. Lagmay et al. 2.2 TC influence on rainfall (2015) discussed the influence of two volcanoes to the To determine if TCs influenced summer monsoonal west of Manila on the spatial and temporal distribution flow in the designated region, daily rainfall data at of heavy rainfall in both the 2012 and 2013 Habagat selected synoptic stations along the western coast of October 2019 G. BAGTASA 969

Luzon during June to September (JJAS) were used. Table 1. SYNOP station ID and location. These stations are within the climate Type I of the Station ID Name Lon Lat modified Coronas climate classification (Philippine 98223 Laoag 120.53 18.18 Atmospheric, Geophysical and Astronomical Services 98325 Dagupan 120.35 16.08 Administration 2018). The climate Type I region, 98324 Iba 119.97 15.33 located along the western portion of the Philippines, 98426 Subic Bay 120.27 14.88 has a distinct wet season from June to September and 98425 Manila 120.98 14.58 dry season that occurs during the remainder of the 98429 NAIA* 121.00 14.52 year. Thus, this region is most affected by the summer 98430 Science Garden 121.02 14.63 southwest monsoon period. The rainfall values of the 98431 Calapan 121.18 13.42 selected stations are averaged as done in previous * NAIA-Ninoy Aquino International Airport studies (Cayanan et al. 2011; Cruz et al. 2013). Table 1 summarizes the selected synoptic station ID, name, and locations. Figure 1 shows the station locations used in this study and the topography of the Philip- pines. In the following analysis, HPE days were deter- mined using the daily rainfall values in the upper 85th, 95th, and 99th percentiles and averaged using data from over 8 synoptic stations. It is noteworthy that a HPE percentile case would encompass the more extreme HPEs. After which, the characteristics of TCs that coincide with HPE days, as well as the prevailing synoptic environment, were analyzed. 3. Results and discussion In this study, time periods of 1098, 366, and 73 days were determined as HPE time ranges, with daily mean rainfall in the upper 85th, 95th and 99th percentiles respectively: hereafter referred to as HPE85, HPE95, and HPE99, respectively. Almost all HPE days (92.8 % for HPE85, 97.3 % for HPE95, and 100 % for HPE99) coincide with at least one TC occurring in the WNP region. Most of these TCs did not make landfall over any Philippine landmass. Only 140 out of the 1,098 HPE85 days (12.8 %), 50 out of the 366 HPE95 days (13.7 %), and 11 out of the 73 HPE99 days (15.1 %) experienced TCs that made landfall over the Philippines. The result in the present study is consistent with the previous finding of Cayanan et al. (2011), which found that 11 out of 13 (or 84.6 %) heavy rainfall events in western Luzon Fig. 1. Philippine topographic map and SYNOP from 2002 – 2005 were caused by non-landfalling TCs station locations. over the ocean near the vicinity of Luzon, despite a different definition of heavy rainfall events. This suggests that while the presence of TCs in the WNP remote TC effect, HPE days that occur when any TC region is critical in inducing HPEs in the northwestern makes landfall over any Philippine landmass were Philippines, heavy rainfall does not necessarily come removed in the following analysis. from the immediate rainbands of a TC. In fact, since Out of the 1,588 TCs that formed in the WNP about 9 out of 10 HPEs coincide with non-landfalling from 1958 to 2017, 997 TCs occurred in the months TCs, the remote precipitation effect appears to be the of JJAS. Of which, 340 TCs (34.1 %) correlated to dominant cause of HPEs. To further investigate this HPE85, 148 TCs (14.8 %) correlated to HPE95, and 970 Journal of the Meteorological Society of Japan Vol. 97, No. 5

Fig. 2. Non-landfalling TC tracks during a) HPE85, b) HPE95 and c) HPE99 days. Lower row denotes normalized TC density count in 2.5° × 2.5° grids for d) HPE85, e) HPE95, and f) HPE99 days.

28 TCs (2.8 %) correlated to HPE99. Figures 2a, 2b, Japan. In addition, the grid cells in the northern SCS and 2c show the TC tracks for HPE85, HPE95, and also show considerable TC count. Most of the TCs in HPE99 days, respectively. The presented TC tracks the northern SCS region occurred simultaneously with are not full tracks from the best track data, but they another TC in the WNP. Previous studies (Cayanan are tracks that are coincident with HPE days. For et al. 2011; Bagtasa 2017) on TC-monsoon-HPE inter- simultaneously occurring TCs in the WNP region, it action in the Philippines confined their analysis to the is assumed that TCs closer to the Philippines exhibit oceanic WNP region northeast of Luzon. The resulting more effect on local precipitation. Here, we removed TC tracks presented here extend the region of TCs hy- TCs that occurred with a secondary TC when all of pothesized to cause heavy rainfall in the northwestern the location points more than 1,500 km from any part of the Philippines. Philippine landmass. This reduced the number of TCs In terms of intensity, Fig. 3 presents TCs classified in the analysis. However, the effect of TCs on the using 6-hour interval central pressure data from the environment, if any, is still included in the examina- JMA best track dataset. Here, the Koba table (Koba tion of reanalysis data presented below. The resulting et al. 1989) was used to relate TC central pressure tracks show that HPE-induced TCs occurred within a with maximum wind speed. The central pressure region extending longitudinally from 100°E to 145°E values of TCs were partitioned as follows: > 1000 hPa and latitudinally from 10°N to 40°N. Figures 2d, 2e, are TCs with less than gale force wind and classified and 2f show the corresponding normalized TC count as a Tropical Depression (TD). Pressures of 1000 hPa density on the 2.5° × 2.5° resolution grid cells of Figs. to 988 hPa are classified as a Tropical Storm (TS), 2a, 2b, and 2c, respectively. The density maps show 988 hPa to 973 hPa as a Severe Tropical Storm (STS), that most TCs passed through the region located to the 973 hPa to 910 hPa as a Typhoon (TY), and 910 hPa northeast of Luzon, or in the vicinity of a line segment or less as category 4 or 5 Severe TY (STY). Table 2 connecting northern Luzon and Naha in Okinawa, shows the TC intensity count distribution (in percent- October 2019 G. BAGTASA 971

Table 2. TC intensity count distribution and total number n of TC location points. TC HPE85th HPE95th HPE99th TD 6.9 % 3.0 % 0.0 % TS 40.2 % 47.3 % 43.9 % STS 22.1 % 23.3 % 25.4 % TY 30.3 % 25.9 % 29.6 % STY (cat 4 – 5) 0.5 % 0.6 % 1.1 % n 5364 1496 280

age) for the three HPE cases. TDs and STYs comprise only of a small percentage of all HPE cases. Most TCs over the WNP Ocean—particularly to the northeast of Luzon where most TCs crossed—are of the STS or TY classification. STS and TY collectively make up a majority of TC categories at 52.3 %, 49.2 %, and 55.0 % for HPE85, HPE95, and HPE99, respectively. However, simultaneously occurring TCs in the northern SCS are generally less intense at TS or STS categories. Other than the presence of intense TCs to the north- east of the Philippines, weaker TCs or low-pressure systems in the northern SCS are also essential in inducing remote HPEs. Using the JRA55 reanalysis data, Fig. 4 shows the mean JJAS and composite environment of the three HPE cases. Climatological summer southwest monsoon data shown in Fig. 4a are characterized by a westerly low-level wind flow from the Indian Ocean through the Indochina region. This monsoon’s winds shift from westerly to a southwest- erly direction in the central SCS as the winds meet the cross-equatorial jet at 110°E. The southwestward intrusion of the WNP subtropical high (not depicted in the figure) in the northern SCS in this season inhibits convection in most of the SCS. This intrusion also orients the monsoon trough in the southeastward direction from Indochina to Borneo, which places the area of active convection near the equator in the SCS (Chen et al. 2000). The summer monsoon is driven mainly by a variation in solar radiation and the differential heating of land and oceans. The area west of the Philippines is of particular interest to monsoon research as large variations in annual rain are found in this region; the onset of monsoons in this region signi- fy the start of the large-scale Asian summer monsoon (Wang 2002). Figures 4b, 4c, and 4d show the composite synoptic environment resulting from all HPE85, HPE95, and Fig. 3. Category of TCs for a) HPE85, b) HPE95, HPE99 days, respectively. The large environmental and c) HPE99 days. variability during HPE85 days did not show a signif- 972 Journal of the Meteorological Society of Japan Vol. 97, No. 5

Fig. 4. Composite synoptic environment of mean a) JJAS and b) HPE85, c) HPE95, and d) HPE99 days. Total col- umn water vapor flux magnitude (gray shading), 850 mb wind (arrow) and mean sea level pressure (dark gray line) are derived from the JRA55 reanalysis. Dashed lines show 95 % significance level for difference of zonal wind (blue) and column water vapor flux (red) from the JJAS mean.

icant (p < 0.05) difference from the JJAS mean zonal east coast of Taiwan. 2) A zonally-oriented eastward wind data and water vapor flux seen in the HPE95 and trough at the 1005 – 1007 hPa level extending to at HPE99 days. Nevertheless, the three HPE cases show least 135°E longitude along 20°N, which covers the common characteristics in the prevailing environment northern half of the Philippines. 3) The induced trough during HPE days regardless of heavy rainfall ranking; displaces the low-level southwesterly wind flow from these are as follows: 1) A strong cyclonic flow off the the central SCS to the Philippine Sea/western Pacific October 2019 G. BAGTASA 973

Ocean, thereby resulting to 4) a stronger zonal compo- component of the total column water vapor flux and nent of wind velocity and water vapor flux along the 850 mb zonal wind are calculated along the north- northwestern coasts of the Philippines. Lastly, 5) the western Philippine coasts bounded by 12 – 18°N and deepening and stretching of the monsoon trough in the 119 – 120°E. Upon comparison to the mean JJAS northern SCS that spans from northern Indochina to value of 65.9 kg m−1 s−1, the HPE85, HPE95, and the Philippine Sea. TC presence in the northern SCS HPE99 cases show the increased values in water deepens the monsoon trough mentioned in (5). This vapor flux of 241.5 kg −1m s−1, 415.3 kg m−1 s−1, and region of low pressure enhances the convective activ- 521.6 kg m−1 s−1, respectively. From the JJAS mean, ity in the area and confines the vertically-integrated water vapor flux increased by 266.7 % for the HPE85 westerly water vapor flux from the Indian Ocean case, 530.5 % for the HPE95 case, and 691.9 % for along a narrow path within 10°N to 15°N in the SCS. the HPE99 case. Along the same region in the north- This process significantly increased the zonal wind western Philippines, the 850 mb mean zonal wind strength and the amount of eastward moisture trans- increased from 1.9 m s−1 to 5.7 m s−1 for the HPE85 port along the SCS to the Philippine Sea over western case (194.3 % increase), 8.9 m s−1 for the HPE95 case Luzon. Kudo et al. (2014) referred to this water vapor (365.1 % increase), and 10.9 m s−1 for the HPE99 case transport as the “moisture conveyor belt” (MCB). (468.8 % increase). The increase in zonal wind and The MCB forms when TC-induced westerlies overlap water vapor flux, coupled with the orographic forcing, with monsoonal westerlies in the SCS. They serve as is thought to produce precipitation along the western a pathway for WNP TCs to accumulate moisture from Luzon region (Cayanan et al. 2011; Racoma et al. remote oceans. Other than the MCB, variations in 2016). summer monsoon westerlies can also produce short- Figures 5a and 5b show the mean annual HPE days term and sub-seasonal strong phasing due to diabatic per decade (for HPE85, HPE95, and HPE99) and the warming in the confluent zone east of the Philippines total annual HPE precipitation per decade, respective- (Holland 1995). This may explain why some HPE ly. Statistically significant (p < 0.05) increasing trends days did not coincide with a TC in the WNP. Howev- are seen in the number of annual HPE days and total er, as the composite maps show the apparent presence annual HPE precipitation amount per decade in the of a TC and MCB in the HPE days, this leads to the upper 85th percentile rain with r = 0.97 and r = 0.84, conclusion that TCs to the northeast of the Philippines respectively. Annual HPE days per decade for the indeed caused most HPEs in western Luzon. The flux years 1960 – 1969 was at 20.7 days per year, while of moisture via the MCB is transported to the inner HPE days per decade from 2010 to 2017 (normalized core of TCs and induces a release of latent heat near to 10 years) was 26.9 days per year. This results in an the eyewall. This typhoon-induced convective heating increase of 29.8 % in the last 5 decades or an average intensifies TCs further and induces equatorial Rossby of 1.3 days per year per decade. An even larger in- waves (Fujiwara et al. 2017). In turn, the westward crease in the total annual HPE rainfall amount result- phase propagation of equatorial waves produces low- ing from precipitation in the upper 85th percentile was level westerlies in the SCS, which further intensifies observed. Total annual HPE85 rainfall increased by the westerlies along the MCB. This TC-MCB feed­ 63.5 % from 442 mm in the 1960’s to 722 mm in the back effect can sustain vapor flux into TCs. This Vapor current decade. No significant trends were observed flux typically peaks at the start of the decay stage for the HPE95 and HPE99 cases for both annual when TCs are located in the region southwest of HPE days and total annual HPE rainfall amount. Japan (Takakura et al. 2018). This explains why most The increase in heavy local precipitation due to TC- HPE-inducing TCs are located in the region northeast monsoon interaction in the upper 85th percentile is of the Philippines. likely due to an increase in TC count to the east of The main difference between more extreme HPE Taiwan in the recent two decades (Tu et al. 2009). cases is the eastward extent of the induced eastward This study showed that the presence of TCs in the trough (1005 – 1007 hPa) and the size of the closed WNP region east of Taiwan exhibits the most effect isobar over the monsoon trough that extends from on remote precipitation in western Luzon. Changes the northwest SCS to the northern Philippines. The in WNP sea surface temperature, low-level relative synoptic environment during HPE99 days shows a vorticity, precipitable water, and vertical wind shear larger extent of the 1002 hPa isobar that leads to a (Chu et al. 2007) were shown to be strongly related to larger pressure gradient force, resulting in stronger the northward shift of TC tracks (Wu and Wang 2004; monsoonal flow and a stronger MCB. The mean zonal Tu et al. 2009) toward the mid-latitudes of East Asia 974 Journal of the Meteorological Society of Japan Vol. 97, No. 5

Fig. 5. a) Number of annual HPE days (HPE99 × 10 and HPE95 × 3) and b) total precipitation per decade from 1960 to the 2010’s for the upper HPE85 (blue), HPE95 (red) and HPE99 (green) percentile rainfall conditions and linear trend line of HPE85. Decade 2010 – 2017 was normalized to 10 years.

rather than a westward track to the SCS. This is par- along the northwestern Philippines. More than nine ticularly true for the decade after 2000. In the current out of ten HPE days coincide with TC occurrences in decade, a higher TC count was also observed in the the WNP basin, indicating that TCs are essential in Philippine Sea, which likely contributed to an even producing HPEs. Landfalling TCs only account for higher number of annual HPE days and total HPE 12.8 %, 13.7 %, and 15.1 % of HPE85, HPE95, and precipitation. The La Niña-like decadal cooling of HPE99 days, respectively. Thus, this study focuses on the tropical Pacific (Kosaka and Xie 2013) may have the non-landfalling TCs that led to most heavy precipi- reduced the frequency of anomalous anticyclone ac- tation events in the northwestern Philippines. TC track tivity in the Philippine Sea (Lyon and Camargo 2009) density plots for all cases show that most TCs were that led to a larger TC count in the eastern Philippine in the vicinity of a line segment connecting northern region. In the future, as the thermal contrast between Luzon and Okinawa in Japan. Composite synoptic the eastern and western Pacific further intensifies, the conditions are shown to exhibit similar features that northern hemisphere summer monsoon (Wang et al. indicate TC enhancement via prevailing monsoons. 2012) coupled with the northward shift of TC tracks These are summarized as follows: 1) cyclonic flow in (Wu and Wang 2004) will likely cause more HPE days the WNP east of Taiwan, 2) a zonally-oriented east- in a warmer globe. ward trough at the 1005 – 1007 hPa isobar extending to at least 135°E longitude along 20°N, 3) displacement 4. Summary and conclusions of southwesterly flow from the SCS to the Philippine Tropical cyclones exhibit a significant influence on Sea, 4) a 194.3 – 468.8 % increase in zonal wind speed weather in the Philippines. The country’s geographic and 266.7 – 691.9 % increase in total column water position at the western rim of the WNP basin makes vapor flux along the northwestern Philippines, and 5) it vulnerable to TC trajectories. Even though the wind the deepening of the monsoon trough in the northern speeds of TCs can be destructive, the rainfall provided SCS, mostly due to simultaneously occurring TCs in by TCs provides a significant contribution to the coun- that region. When the westerly wind produced by TCs try’s freshwater resources. According to a previous to the northeast of the Philippines overlaps with the study, up to 54 % of annual rainfall in the northwest background monsoon westerlies in the SCS, strong region of the Philippines is TC-induced. Moreover, moisture transport to the Philippine Sea is induced. researchers have shown that most of TC-induced pre- Referred to as the “MCB”, this type of moisture intru- cipitation comes from TCs’ interaction with prevailing sion has more influence on HPEs than climatological monsoons in the months of JJA (Bagtasa 2017). This monsoon variability and is the main mechanism that study characterizes this TC-monsoon interaction. produces nearly all HPEs in the northwestern Philip- Three heavy rainfall cases were analyzed, namely pre- pines. cipitation in the upper 85th, 95th, and 99th percentiles An increase of 29.8 % in the mean annual HPE85 of daily rainfall averaged from eight synoptic stations days-per-decade and a 63.5 % increase in total annual October 2019 G. BAGTASA 975

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