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Contribution of Tropical Cyclones to Rainfall in the Philippines

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Contribution of Tropical Cyclones to Rainfall in the Philippines 15 MAY 2017 B A G T A S A 3621 Contribution of Tropical Cyclones to Rainfall in the Philippines 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 Japan 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 Luzon (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 China 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 Unauthenticated | Downloaded 09/29/21 09:23 AM UTC 15 MAY 2017 B A G T A S A 3623 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. Unauthenticated | Downloaded 09/29/21 09:23 AM UTC 3624 JOURNAL OF CLIMATE VOLUME 30 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.
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