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Contribution of Tropical Cyclones to Rainfall in the

GERRY BAGTASA Institute of Environmental Science and Meteorology, University of the Philippines, Quezon City, Philippines

(Manuscript received 18 February 2016, in final form 10 January 2017)

ABSTRACT

Tropical cyclone (TC)-induced rainfall (TC rain) in the Philippines was investigated using a combination of ground and satellite observations to produce a blended 64-yr precipitation dataset. A total of 1673 TCs were examined using best track data from the Meteorological Agency. Rainfall from 100 (;1110 km) of the TC center was considered as TC-induced rainfall. TC rain contribution is highest in the northern Philippines, particularly along the western coast of (up to 54%), and lowest in the southern islands of Mindanao (6%). The high TC rain contribution is attributed to the enhancement of the Asian southwest monsoon by TCs located to the northeast of the Philippines. An unsupervised clustering method, k-means clustering, was used to divide the archipelago into four climate subtypes according to monthly rainfall variation. Interannual variability of rainfall from climate clusters with high TC rain contribution generally follows the variability of TC rain. On the other hand, the variability of low TC rain clusters is mainly influenced by El Niño–Southern 2 Oscillation (ENSO). All clusters show increasing trends of 16.9%–19.3% decade 1 in TC rain percentage contribution since 2000. This study hypothesizes that this increasing trend is due to changes in the charac- teristics of TC steering mechanisms and thermodynamic properties east of the Philippines in the past one and a half decades.

1. Introduction 58N–1158E, 158N–1158E, and 218N–1208E (dashed line in Fig. 1). Around half of these make landfall each year Changes in global rainfall patterns are of great con- (Cinco et al. 2016). cern in the midst of our warming climate. Studies of rain- Recently, several studies looked into the local trends fall patterns in global and regional scales have shown and variabilities of temperature and rainfall to eluci- variations in annual precipitation trends for different re- date climatic observations that can be vital for climate gions (Chadwick et al. 2013; Liu et al. 2013; Zhang and change adaptation measures. Cruz et al. (2013) Zhou 2011; Wang et al. 2012). In the Philippines, where the showed a decreasing rainfall trend in the western por- agricultural sector employs a third of its workforce (World tion of the Philippines during the boreal summer or Bank 2015), rain is the most important daily weather southwest monsoon season from 1960 to 2010. Their phenomenon. The Philippines is an archipelago consisting results also indicate a prolonged dry season for most of of more than 7100 islands situated between the western the western Philippines. Villafuerte et al. (2014) ana- rim of the Pacific Ocean and the South Sea (locally lyzed long-term trends and the variability of rainfall known as West Philippine Sea). It is dominated by com- extremes using seven extreme precipitation indices plex terrain, with long mountain ranges on the two largest (EPI). They found a drying trend from January to islands of Luzon and Mindanao to the north and south, March and an increasing trend during extreme rainfall respectively, and Visayas consists of smaller islands in the events from July to September. Longer trend analysis central Philippines. Its location subjects it to numerous from 1911 to 2010 by Villafuerte et al. (2014) indicates tropical cyclones (TC) that form in the northwestern Pa- the southwestward extension of the northwestern Pa- cific (NWP) basin (Weinkle et al. 2012). An average of 19.4 cific subtropical high (NWPSH) and the weakening of TCs enter the Philippine Area of Responsibility bounded westerly winds as causes of this drying trend. Cinco by coordinates 258N–1208E, 258N–1358E, 58N–1358E, et al. (2014) found mostly warming trends in surface temperature, as well as an increase in extreme tem- Corresponding author e-mail: Gerry Bagtasa, gbagtasa@iesm. perature and daily rainfall events, from 37 synoptic upd.edu.ph stations. Significant increases were distributed all over

DOI: 10.1175/JCLI-D-16-0150.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 09/29/21 09:23 AM UTC 3622 JOURNAL OF CLIMATE VOLUME 30

FIG. 1. The Philippine Area of Responsibility (enclosed by the dashed line) and TC count density in the northwestern Pacific for (a) MAM, (b) JJA, (c) SON, and (d) DJF. The size of the gray rectangles extends from a count of 1 to 14 in increments of 1. the country. TC contribution on rainfall, however, was Chan 2002). As a result, TCs have longer lifetimes over not considered in these studies. the Pacific. This leads to more intensification and a ten- The climate of the Philippines is highly influenced by dency to track toward the northwest quadrant of the ENSO (Hilario et al. 2009; Ropelewski and Halpert NWP. Consequently, a northwesterly track leads to less 1996). Drier (wetter) conditions are associated with TC directly affecting the Philippines (Hilario et al. 2009). El Niño (La Niña) events. Lyon and Camargo (2009) Tropical cyclones are regarded as the most destructive observed a seasonal reversal in ENSO rainfall signal in hydrometeorological hazard in the Philippines. TCs in the Philippines. Above- (below) average rainfall is seen the last four decades have an estimated normalized cost of in El Niño (La Niña) years during boreal summer, and a more than $2 billion in damages (Cinco et al. 2016)and reverse rainfall anomaly is seen the following fall. TC thousands of lost lives. Historic accounts of TCs starting activity is also shown to be enhanced (reduced) during from the sixteenth century emphasize the destructive boreal summer of El Niño (La Niña) events due to nature of this hydrometeorological event (Ribera et al. changes in midlevel atmospheric moisture. A number of 2008). Aside from their destructive potential, TCs can also papers also reported on the relationship of TCs and have a significant contribution to the terrestrial hydrologic ENSO in the NWP basin (Atkinson 1977; Camargo and cycle (Coronas 1912; Flores and Balagot 1969). Moreover, Sobel 2005; Chan 2000; Corporal-Lodangco and Leslie understanding its impacts is vital to the proper manage- 2016; Wang and Chan 2002). In general, TC geneses ment of water resources (Dare et al. 2012; Ren et al. 2006). tend to occur farther to the southeast during El Niño Despite the importance of TCs in the Philippines, years (Corporal-Lodangco and Leslie 2016; Wang and there is limited literature on local TC-induced rainfall

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local orography generated active convection along the western, windward side of Luzon. Recently, Cinco et al. (2016) made an initial analysis of local TC-induced rain. The largest contribution of up to 50% was observed in the northern Philippines. However, their analyses focused on TC landfall frequency; no trends were found. The aim of this work is to quantify and characterize the amount of rainfall contributed by TCs using long-term gridded precipitation datasets over different seasons and to investigate the variability and trends in regions with distinct rainfall climate regimes. The next section will discuss the precipitation datasets used, TC track in- formation, and the method used to compute TC-induced rainfall. In section 3, the distribution of TC-contributed rainfall, its trends, and variabilities are presented. The influence of TCs on southwest monsoon rainfall is also considered. Section 4 summarizes the result of this study.

2. Data and method

FIG. 2. Observed vs gridded precipitation data (TRMM: red, This study analyzed TC-induced precipitation from APHRODITE: black). 1951 to 2014. Among the precipitation data used is the Asian Precipitation–Highly Resolved Observational (TC rain) contribution. Kubota and Wang (2009) stud- Data Integration Toward Evaluation of the Water ied the effects of TCs on rainfall in the NWP region using Resources (APHRODITE) Monsoon Asia, version 22 stations, including 7 stations from the Philippines. They 1101R2 (Yatagai et al. 2012). This dataset is composed showed that the interannual variability of rainfall in the of daily gridded precipitation covering Asia, including NWP is modulated by ENSO. Their results suggest that Southeast Asia, with a grid resolution of 0.25830.258. rainfall variability is controlled by changes in nonlocal The data cover a period of 57 yr from 1951 to 2007. circulations that modify TC genesis location and tracks. Rainfall from this dataset is derived from rain gauges Cayanan et al. (2011) reported cases of an indirect effect of that are up to 4.5 times denser than data available TCs located to the northeast of Luzon, northern Philip- through the Global Telecommunication System (GTS) pines. Heavy rainfall events were caused by the interaction of the World Weather Watch (Yasutomi et al. 2011). of TCs and southwesterly wind during the boreal summer To cover the years 2008–14, daily precipitation data monsoon period. Enhanced moisture flow coupled with from TRMM 3B42, version 7, for the period 1998–2014

FIG. 3. Mean total rainfall vs radius from the TC center.

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FIG. 4. Mean total column water vapor flux (arrows and gray shades) for (a) JJA, (b) cases where a TC is within 198–218N, 1258–1278E, and 2 2 (c) difference (b) 2 (a). The gray scale is white for a value of 0 and goes to black for a value of 1000 in increments of 100 kg m 1 s 1. were used. The dataset also uses a grid resolution of datasets per climate subtype. The scatterplot in Fig. 2 0.25830.258. Details are found on its website (http:// reveals that both datasets behave similarly and correlate mirador.gsfc.nasa.gov/collections/TRMM_3B42_daily__ highly (r 5 0.85) with observation. Observed precipitation 007.shtml). The TRMM Multisatellite Precipitation data provided by the National Climatic Data Center’s Analysis is widely used in climatological rainfall studies Global Summary of the Day (http://www1.ncdc.noaa.gov/ due to its long temporal and global spatial coverage (Liu pub/data/gsod) from 1990 to 2014 was used to assess the 2015; Breña-Naranjo et al. 2015; Wang et al. 2014). two datasets. A total of 31 synoptic stations distributed To address concerns over using two different datasets, across the Philippines were included in the assessment. both APHRODITE and TRMM data used the same grids TC reanalyzed best track data are from the website in the analysis. A comparison of annual rainfall of the two of the Regional Specialized Meteorological Centre datasets over a 10-yr overlap from 1998 to 2007 shows a (RSMC) Tokyo-Typhoon Center of the Japan Meteo- statistically significant correlation of r 5 0.96 at the 95% rological Agency (http://www.jma.go.jp/jma/jma-eng/ significance level (p , 0.05). However, the TRMM jma-center/rsmc-hp-pub-eg/trackarchives.html). A to- rainfall values have a mean wet bias of 12.1% compared tal of 1673 tropical cyclones over the same period as the to APHRODITE. Bias offset correction was applied precipitation datasets (1951–2014) in the NWP basin to the TRMM data for consistency. Figure 2 shows the were used in the analysis. Wind, geopotential height, comparison between observed mean annual rainfall and total column water vapor flux data at 1.25831.258 with the bias-corrected TRMM and APHRODITE resolution are from the Japanese 55-year Reanalysis

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FIG. 5. Daily mean (a) total rain, (b) TC rain, and (c) TC percentage contribution (TC/total) from 1951 to 2014. The color scales runs 2 2 (a) 0 (dark blue) to 10 (dark red) in increments of 2 mm day 1, (b) 0 (dark blue) to 5 (dark red) in increments of 1 mm day 1, and (c) 0% (dark blue) to 70% (dark red) in increments of 10%.

(JRA-55) project, carried out by the Japan Meteoro- region of Mindanao, which experiences heavy annual logical Agency (JMA 2013). rainfall of about 3600 mm on average, receives only Rainfall within a 108 (;1100 km) radius from the TC 13.5% rain contribution from TCs. The lowest contribu- center was chosen in this study to be rainfall associated tion is seen in the south to southwest Mindanao at 6.2%. with TCs. Mean total rainfall within varying distance TC occurrence in the NWP basin varies seasonally. radii from the TC center was calculated for the whole The quarterly averaged number of TCs for December– study period. Similar to the results of Jiang et al. (2008) February (DJF) of the following year is 1.8 TCs, MAM at and Kubota and Wang (2009), Fig. 3 shows the mean 2.25 TCs, JJA at 11.28 TCs, and SON at 10.8 TCs. rainfall amount decreases with a larger TC influence Figure 1 shows the corresponding spatial TC count den- radius and becomes almost constant from around a 108 sity distribution in the NWP basin. High TC activity is radius onward. Hence, this was selected as the optimum seen in both JJA and SON. However, the months of SON range of TC-induced rainfall. show a more southwesterly track due to differences in environmental conditions (PAGASA Climate Data Sec- 3. Results and discussion tion, Climatology and Agrometeorology Branch 2011, unpublished data). Accordingly, the rainfall contribution a. TC–induced rainfall from TCs is also presented in the same quarterly partition. The distribution of total averaged daily rainfall, TC Figure 5 shows the quarterly total rain, TC rain, and rain, and TC percentage contribution from 1951 to 2014 in the TC percentage contribution. In the months of DJF the Philippines is shown in Fig. 4. The TC percentage (Figs. 5a,e,i), northeasterly winds or the northeast contribution is computed as the ratio of TC rain to total monsoon prevails and brings rain to the eastern coastal rainfall multiplied by 100%. The eastern Philippines re- regions (Akasaka et al. 2007). Also in the same season, gion is seen in Fig. 4a to have the most amount of rain at TC rain contributes up to 20% around southwestern 2 an annual average of 3700 mm or 10 mm day 1.Heavy Luzon; however, the value of total rainfall in this quarter rain in this region is due to orographic land–air interaction is only 3% of the annual rain or an average of 2 during the boreal winter monsoon period (Flores and 23 mm month 1. MAM (Figs. 5b,f,j) marks the transi- Balagot 1969). TC-induced rainfall (Fig. 4b) for all years is tion between the northeast and southwest monsoon re- apparent in the northern half of the country and most gimes. During this boreal spring transition, most pronounced along western Luzon. Consequently, the TC convection stays near and south of the equator (Chang percentage contribution (Fig. 4c)ismorethan40%in et al. 2005) with prevailing easterlies from the Pacific most of Luzon, and highest at 54.2% along the coastal Ocean. Similar to DJF, the TC contribution in MAM regions of the northwest. On the other hand, the northeast shows the same distribution but small total seasonal

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FIG. 6. Seasonal distribution of (a)–(d) total, (e)–(h) TC rain, and (i)–(l) TC percentage contribution. The color scales are as in Fig. 5.

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FIG. 7. TC Percentage contribution (TC/total rain) in JJA at the 2.58 TC influence radius. The color scale runs from 0% (dark blue) to 25% (dark red) in increments of 5%. rainfall (10% of annual rain). The southwest monsoon season usually starts in the second half of May and ends in September (Moron et al. 2009); monsoon winds bring in warm moist air from the to the western Philippines (Lagmay et al. 2015). A study by Asuncion and Jose (1980) showed that 43% of the average total annual precipitation in the Philippines falls during this season. The southwest monsoon season also coincides with peak TC occurrence in the months of July and August. A high precipitation amount during JJA is seen in the FIG. 8. (a) Climate clusters and (b) corresponding mean monthly western section of Luzon (Fig. 5c). The distribution of TC- rainfall variation from k-means clustering (cluster 1: blue, cluster 2: inducedrain(Fig. 5g) is consistent with the findings of red, cluster 3: green, and cluster 4: purple). Cayanan et al. (2011). A detailed discussion is found in the next section. Last, the southeast propagation of the in- on the southwesterly winds during the southwest mon- tertropical convergence zone (ITCZ) in SON results in the soon period in JJA. TCs located to the northeast of the shifting of monsoonal winds back to the northeast mon- Philippines enhance the moisture flux to the west that soon by the end of October (Moron et al. 2009). Figure 5l produces intense convection and orographic lifting shows that the TC rain percentage contribution along along the windward side of the Cordillera mountain northwest Luzon reaches 66.7% during this quarter, when range in northwestern Luzon. Figure 6a shows the mean TC tracks are more southwestward. In addition, a higher total column water vapor flux for JJA, and Fig. 6b shows TC rain contribution is seen in northern Mindanao. where a TC was inside an area northeast of Luzon bounded by 198–218N, 1258–1278E that occurred in the b. TC enhancement of southwest monsoon same quarter. The composite image of 44 TCs from 1958 Computed TC-induced rainfall is not only from the to 2014 with intensities stronger than a tropical storm in immediate rainbands of TCs but also from its influence Fig. 6b shows a large increase and a change in the

Unauthenticated | Downloaded 09/29/21 09:23 AM UTC 3628 JOURNAL OF CLIMATE VOLUME 30 direction of moisture flow. A mean increase of 411% TABLE 1. Summary of mean and std dev of total rain, TC rain, and 21 zonal water vapor flux to the west of the Philippines from TC percentage contribution (3100%) for all clusters (mm day ). 21 21 90.2 to 371 kg m s is derived from JRA-55. Figure 6c Total rain TC rain TC/total 3 100% shows the moisture flux anomaly of the composite 44-TC Cluster Mean (std dev) Mean (std dev) Mean (std dev) case. This interaction with the southwest winds increases rainfall in the northwest region (Cayanan et al. 2011; 1 6.79 (0.95) 3.05 (0.90) 44.5 (10.0) 2 5.68 (0.72) 2.30 (0.70) 39.2 (8.9) Lagmay et al. 2015). 3 5.18 (0.80) 1.41 (0.48) 26.7 (7.0) The calculated TC rain percentage contribution dur- 4 7.60 (1.43) 1.64 (0.64) 21.1 (6.6) ing JJA in Fig. 7 using a TC influence radius of 2.58 (;277 km) to simulate precipitation only from the rainbands shows a different spatial distribution. Here, 2 the northeast region of Luzon is most affected (up to trend of 20.2 mm decade 1 ( p , 0.05) from 1960 to 23%), whereas a comparably lower TC rain contribution 2010, consistent with the findings of Cruz et al. (2013). is found along western Luzon. This further supports the However, all clusters as shown in Fig. 9 show an in- selection of a 108 TC influence radius used in this study. creasing trend from year 2000 onward, with linear trend lines shown as the dashed line and sharp peaks are seen c. Interannual variability of TC rainfall in the years 2008 and 2011. These peaks are more pro- In this work, k-means clustering was utilized to group nounced for cluster 4 along the eastern Philippines. regions of distinct climate subtypes according to monthly Anomalous heavy rainfall events that occurred during rainfall variation based on the current 64-yr precipitation the months of JFM in those 2 yr were likely the result of dataset. This type of unsupervised clustering technique very strong trade winds in those years as indicated by a looks for the least variation within a cluster and the high western Pacific trade wind index (not shown). maximum difference between clusters (Everitt et al. 2001). Maximum precipitation is observed along the eastern However, it does not determine the optimum number of region, as it is on the windward side of the prevailing clusters or the value of k. Here, k 5 4 was chosen. monsoon (Chang et al. 2005; Akasaka et al. 2007; Moron The resulting climate clusters and their corresponding et al. 2009). An increasing trend in the west Pacific trade monthly rainfall variability are shown in Fig. 8. Figure 8a winds was reported by England et al. (2014) to account shows clusters 1 and 2 covering the western coast and the for the warming hiatus since 2000. Also, this possibly remainder of western Luzon and Visayas, respectively. explains the increasing annual rainfall trend and an These regions have two pronounced seasons: dry from unprecedented rainfall amount to the east of the Phil- November to April and wet the rest of the year (Fig. 8b). ippines in the past one and a half decades. Cluster 3 covers the central (longitudinal) Philippines, Seasonal and interannual variabilities of rainfall in the where rainfall is evenly distributed throughout the year, Philippines are influenced by the extreme phases of and cluster 4 describes the eastern regions, where there ENSO (Hilario et al. 2009; Lyon et al. 2006; Ropelewski is no dry season but more rain during the winter mon- and Halpert 1996). This is seen in the positive peaks in soon period. Table 1 summarizes the total rain, TC rain, total annual rainfall (Fig. 9) in 1971, 1999, 2008, and and the TC rain percentage contribution and their cor- 2011—all La Niña years—and negative peaks in 1983, responding standard deviation (std dev) for each of the 1987, 1997, and 2010, which were El Niño years. A four clusters. The interannual variability of TC-induced comparison of the ENSO-3.4 index and total rain rainfall for each climate cluster is also examined. yielded a high correlation for clusters where the TC Figures 9a–d show the annual total rain and TC rain contribution is small. For cluster 1, the ENSO-3.4 index for each climate cluster. The variability of TC rain in the and total rain has a low correlation of r 520.3, cluster 2 northwestern Philippines correlates well with the vari- has a moderate correlation of r 520.58, while cluster 3 ability of total rain. A correlation coefficient of r 5 0.69 has r 520.74 and cluster 4 has r 520.69—all at the for cluster 1 and r 5 0.72 for cluster 2, both at the 99% 95% significance level. Since clusters 1 and 2 are af- significance level, were found. This implies that TC rain fected by the TC-enhanced southwest monsoon, there is has a significant influence on the overall rainfall vari- no distinctive reduction in TC rain in El Niño years due ability in this region. However, this relationship was not to the tendency of TCs to track toward the northwest observed for clusters 3 (r 5 0.53) and 4 (r 5 0.35). Be- quadrant of the NWP. This increases TC occurrence to cause TC rain makes a low contribution in those clusters, the northeast of Luzon, leading to enhanced monsoon it was not a significant determinant of total rainfall. winds. In addition, the low correlation can also be ex- The mean annual rainfall of clusters 1 and 2 (Fig. 9a,c, plained by the late seasonal reversal toward the final respectively) follows a small but significant decreasing quarter of an ENSO year (Lyon et al. 2006); therefore,

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FIG. 9. Total rain (black and red) and TC rain (green and purple) from APHRODITE (solid) and TRMM (dashed) 2 for (a)–(d) clusters 1–4 (mm day 1) and trends (2000–14) of TC rain (black dashed lines).

ENSO has a relatively small influence on the annual total (Agaton) and Severe Tropical Storm (STS) Noguri and TC rainfall. In contrast, clusters 3 and 4 have a small (Espada)—to come within 100 km of Philippine landmass. TC rain contribution. Most of the rainfall in these clusters In the southern Philippines, 2011–13 show the highest TC- is produced from non-TC mechanisms (i.e., the interaction induced rainfall due to several southerly extreme TC between land and trade wind during the northeast mon- events: Typhoon Washi (Sendong) in 2011; Typhoon soon) that are well correlated with ENSO events (Webster Bopha (Pablo) in 2012; and STS Sonamu (Auring), TS and Yang 1992). Shanshan (Crising), and TS Podul (Zoraida) in 2013. TC-induced rainfall variability is highest for clusters Increasing trends in TC rainfall are seen in all clusters located in the northwestern half of the country (clusters 1 from 2000, with a higher trend along the eastern region. and 2). A single or a few TC-induced heavy precipitation In clusters 1 and 2, increasing TC rain resulted in the events can affect the total annual rainfall of certain years. reversal of the previously reported (Cruz et al. 2013; In 1986, Typhoon Wayne (local name: Miding) made a Villafuerte et al. 2014) negative southwest monsoon substantial contribution to TC rain over Luzon and is rainfall trend. The TC rain and TC percentage contri- considered one of the longest-lived tropical cyclones in bution trends from 2000 to 2014 are summarized in the NWP basin (Fatjo and Wells 1986). Recently, Ty- Table 2. Cluster 4 shows the highest increasing trend of 2 2 phoon Parma (Pepeng) and Tropical Storm (TS) Ketsana 1.95 mm day 1 decade 1 in TC rain, while cluster 2 (Ondoy) increased the total rainfall in 2009 (Yumul et al. shows the highest increase in TC percentage contribu- 2013). In 2012 and 2013, two weeklong heavy pre- tion. Significant increases ( p , 0.05) are observed in all cipitation events due to the enhancement of southwest- clusters. erly winds by several persistent TCs affected the west of This study hypothesizes that changes in TC steering central Luzon (Lagmay et al. 2015). The TCs described mechanisms—namely, a southerly subtropical steering have resulted in a total accumulated rainfall volume in ridge and stronger environmental flow during SON and the upper 95th percentile. In 2002 when TC rain was December (SOND) in the last two decades—affected lowest, most TCs that formed in the NWP basin had TC movement. This leads to TCs shifting to more northeastward recurvature due to a weak NWPSH westward, southerly tracks. Figure 10 shows the decadal steering. This resulted in only two TCs—TS Tapah southwestward migration of the NWPSH represented by

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TABLE 2. Trends in TC rain and TC percentage contribution correlation (r 5 0.7) between TC maximum wind speed 21 (3100%) for all clusters since 2000 (decade ). in the NWP and the warm pool SST. Moreover, the 21 frequency of TCs with maximum wind speed greater Cluster TC rain (mm day ) TC % contribution 2 than 150 km h 1 has also been increasing with 29.6% of 1 11.48 116.9 2 11.45 119.3 169 TCs in 1984–93, 30.5% of 141 TCs in 1994–2003, and 3 11.13 116.5 41.9% of 160 TCs in 2004–13. 4 11.95 118.9 Are we seeing wetter TC conditions in the Philippines due to the warming of global surface temperature? The aforementioned studies on the changes of some dynamic the 5870-m geopotential height at the 500-mb isosurface and thermodynamic factors seem to suggest so. The during SOND for the periods 1975–84 (gray solid line), long-term southwest progression (Villafuerte et al. 1985–94 (gray dashed–dotted line), 1995–2004 (dotted 2014) and a more recent westward extension of the line), 2005–14 (dark dashed line), and 2011–14 (dark solid NWPSH (Sui et al. 2007; Zhou et al. 2009) are partly line). The NWPSH is an important steering mechanism of explained by warming SST trends. Warming SST in the TCs in the NWP basin (Xiang et al. 2013). TCs move warm pool region produces more frequent intense TCs along the periphery of the NWPSH, and its westward in the tropical western Pacific, and consequently more extent determines whether a TC recurves northward or TCs with intense rain field. Surface warming also leads moves toward a westerly direction. Also, stronger east- to higher moisture content in the atmosphere (Durre erly steering flow (850 mb) is seen from a wind anomaly et al. 2009; McCarthy et al. 2009). Last, a larger TC (vectors) for 2011–14 as compared to the 1980–2010 cli- rainfall area in the NWP basin compared to Atlantic matological mean. This may also explain the higher TC TCs is a manifestation of high relative SST in the NWP landfall occurrences along the southern regions of the region (Bretherton et al. 2004; Lin et al. 2015). But while Philippines found in the analysis by David et al. (2013).In studies had shown an increase in the destructiveness of addition, thermodynamic factors such as an increase in tropical cyclones (Emanuel 2005; Webster et al. 2005; tropospheric humidity (Dai 2006; Chan 2008; McCarthy Wu et al. 2008) since the 1970s, interdecadal variations et al. 2009) and warmer sea surface temperature to the of dynamic factors conducive to TC formation and in- east of the Philippines (Comiso et al. 2015) likely con- tensification that are coupled to oceanographic cycles tributed to the precipitation amounts of TCs affecting the should also be considered, as Chan (2008) emphasized. Philippines. Furthermore, the 16–32-yr variation in TC intensity that Most studies about the effects of global warming on Chan (2008) reported started its upward cycle around TCs focus on TC frequency and intensity. There are a the year 2000, which coincides with the increase in TC few studies, however, that deal with changes in TC rain contribution. Further investigation is warranted to rainfall area. Since the TC rainfall area or the TC rain sufficiently answer whether TC rain is indeed increasing field is easily observable in satellite images, the TC rain due to climate change. It is a question vital to the miti- field is often used as a measure of TC size. It can be gation and adaptation efforts geared toward anthropo- influenced by a TC’s humidity, vorticity, vertical wind genic climate change in the Philippines. shear, and latitude. The TC rain field is also closely re- lated to the TC wind field (Matyas 2010). 4. Summary and conclusions Using TRMM satellite observation and a global cir- culation model, Lin et al. (2015) showed that the TC rain Tropical cyclones are of great socioeconomic impor- field is controlled primarily by relative sea surface tance to the Philippines. In this study, the contribution of temperature (SST), or SST over a region compared to TC-induced rainfall was characterized using combined the tropical mean SST, whereas the TC rainfall rate in- ground and satellite data to produce a blended 64-yr creases with absolute SST. Both of these phenomena are precipitation dataset. Out of the 1673 TCs analyzed, observed in the tropical western Pacific Ocean to have 1273 TCs or 76% had rainfall contribution on Philippine 2 significant increasing trends (Dai 2006; Durre et al. 2009; landmass. The highest TC rain (as much as 1400 mm yr 1) McCarthy et al. 2009). In relation to this, various studies and percentage contribution (54.2%) averaged over the have shown that TC strength is well correlated with SST entire study period is found along the northwest coast of (Emanuel 2005; Webster et al. 2005; Mei et al. 2015). Luzon in the northern Philippines. TC rain contribution Comiso et al. (2015) observed that the regional SST in this region was mainly due to the influence of TCs lo- to the east of the Philippines, referred to as the warm cated to the northeast of the Philippines on the prevailing pool (108S–108N, 1508–1708E), has been warming southwesterly winds during the summer monsoon season 2 by 0.28 decade 1. Their results also showed a high and not directly from the immediate TC rainbands. JJA

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FIG. 10. NWP SOND mean wind anomaly (850 mb) for 2010–14 relative to 1980–2010 (vectors) and 5870-m geopotential height (500 mb) for 1975–84 (gray solid line), 1985–94 (gray dashed–dotted line), 1995–2004 (dotted line), 2005–14 (dark dashed line), and 2010– 14 (dark solid line). and SON show the highest TC-induced precipitation, as potential of TCs’ intense winds was most likely also af- these coincide with the highest TC occurrence months fected by the said changes. A separate paper to describe and a climatological southerly–westward TC track, re- their impacts is underway (G. Bagtasa 2017, unpublished spectively, that influence precipitation distribution. The manuscript). The Philippines is an agricultural country southern islands of Mindanao showed the least amount of with many rain-fed farmlands and together with a rap- rain from TCs. Nevertheless, TC rain contribution still idly increasing population, it is important to understand reached ;20%inthatregionduringSON. the spatiotemporal characteristics of TC-induced pre- To examine the interannual variability of TC rain cipitation to assess its implication on water resources contribution, the Philippines was grouped into separate management and food security of the country. climate subtypes according to monthly rainfall vari- Acknowledgments. The author would like to ac- ability using the unsupervised k-means clustering knowledge the generosity of the Oscar M. Lopez Center method. A k-means for k 5 4 was used in the cluster for Climate Change Adaptation and Disaster Risk analysis. Clusters 1 and 2, which cover western Luzon Management Foundation, Inc. (OML Center) for sup- and northwest Visayas, have the highest TC-induced porting the WHATSUP project (NP2013-07UP1). The rain and percentage contribution. On the other hand, author would also like to thank the three reviewers for cluster 4, which covers the eastern coast, has the lowest. their valuable comments, which helped to improve the Positive (negative) peaks in total annual rainfall co- quality of the paper. incide with La Niña (El Niño) years. However, regions with less TC rain contribution have a higher correlation to the ENSO-3.4 index. This leads to the conclusion that REFERENCES ENSO has more influence on non-TC rain. Akasaka, I., W. Morishima, and T. Mikami, 2007: Seasonal march Increasing trends in both TC rain and TC percentage and its spatial difference of rainfall in the Philippines. Int. contribution were seen for the whole Philippines after the J. Climatol., 27, 715–7252, doi:10.1002/joc.1428. 1997–99 ENSO event. These trends are likely due to Asuncion, J., and A. Jose, 1980: A study of the characteristics of the more westward TC tracks brought about by changes in northeast and southwest monsoons in the Philippines. Na- tional Research Council of the Philippines Assisted Project, 49 dynamic and thermodynamic factors. These factors af- pp. [Available from Philippine Atmospheric Geophysical and fected TC movement and intensity in the recent one and a Astronomical Services Administration, Agham Road, Dill- half decades. Aside from precipitation, the destructive man, Quezon City, Metro Manila 1100, Philippines.]

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