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The Effect of Tropical Cyclones on Southwest Monsoon Rainfall in The Journal of the Meteorological Society of Japan, Vol. 89A, pp. 123--139, 2011. 123 DOI:10.2151/jmsj.2011-A08 The E¤ect of Tropical Cyclones on Southwest Monsoon Rainfall in the Philippines Esperanza O. CAYANAN Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), Quezon City, Philippines Tsing-Chang CHEN Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, U.S.A. Josefina C. ARGETE Institute of Environmental Science & Meteorology, College of Science, University of the Philippines, Quezon City, Philippines Ming-Cheng YEN Department of Atmospheric Science, National Central University, Chung-Li, Taiwan and Prisco D. NILO Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), Quezon City, Philippines (Manuscript received 30 May 2010, in final form 6 October 2010) Abstract Intense southwest monsoon (SWM) rainfall events causing massive landslides and flash floods along the west- ern sections of the Philippines were studied. These rainfall events, are not directly coming from the tropical cy- clones (TCs) for they are situated far north to northeast of Luzon Island. The heavy rainfall is hypothesized as caused by the interaction of strong westerlies with the mountain ranges along the west coast of Luzon that pro- duces strong vertical motion and consequently generates heavy rainfall. Four of heavy SWM rainfall cases were examined to determine how the presence and position of tropical cyclones in the Philippine vicinity a¤ect these SWM rainfall events; three cases with TC of varying positions within the Philippine area of responsibility (PAR) and the fourth case without TC. Using a spatial Fourier decomposition approach, the total streamfunction is de- composed into two flow regimes: monsoon basic flow (Waves 0–1) and tropical cyclone perturbation flow (Waves 2–23) over a domain of (20E–140W, 5S–35N). The purpose of this flow decomposition is to determine the latter’s e¤ect on or contribution to the monsoon activity. The analysis utilized the NCEP Final (FNL) data with 1 long. Â 1 lat. resolution. Results show that the tropical cyclones over the Pacific Ocean located northeast of Luzon generate strong southwesterly winds over the west coast of Luzon. These in addition to the southwesterlies from the basic flow strengthened the southwest winds that interact with the high Cordillera Mountain ranges Corresponding author: Esperanza O. Cayanan, Philip- pine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), Science Garden, Agham Road, Diliman, Quezon City 1100, Philippines. E-mail: [email protected] 6 2011, Meteorological Society of Japan 124 Journal of the Meteorological Society of Japan Vol. 89A along the west coast of Luzon. When the tropical cyclone is located north or north-northwest of Luzon, it gener- ates northwesterlies which converge with the southwesterlies from the basic flow. This results to enhancement of rising motion over western Luzon. The much stronger westerlies are then forced to rise above the mountains resulting to strong vertical motion that brings about heavy rainfall. 1. Introduction to northeast of Luzon. A good example of this is the big flash floods on the last week of August Lying in the vast Pacific Ocean and regarded as 2004 which inundated four western and central typhoon belt, the Philippines experiences an aver- provinces in Luzon putting them under state of age of 19 to 20 tropical cyclones per year and the calamity. The massive floodings were reported as peak occurs during the months of July and August. caused by heavy monsoon rains induced by Ty- From later part of May to September, southwest- phoon Marce which was over the Pacific Ocean erly winds or southwest monsoon (Habagat) prevail east to northeast of Luzon Island. A similar heavy over the country (Flores and Balagot 1969; Wil- rainfall event happened on the last week of August liams and Jung 1993), hence typhoon season coin- 1990 was caused by intense SWM due to the pas- cides with the southwest monsoon months. sage of three (3) consecutive TCs north of the Phil- Monsoon is traditionally defined in terms of ippines (Cayanan 1995). Another devastating event the annual reversal of the prevailing wind regime of this kind was the historical Philippine floods in (Ramage 1971) and a contrast between a rainy July 1972 where the Lingayen Gulf and the Manila summer and a dry winter. It is driven by the di¤er- ential heating between the huge land mass of Asia (with Tibetan Plateau as a raised platform of heat source) and the huge water mass of Indian Ocean. During northern summer or southwest monsoon, winds began to blow from the cooler sea towards the land (Das 1986) arriving over the western Phil- ippines as southwesterlies. On Philippine topography, mountain ranges with heights more than 500 m cover almost half of the Luzon Island. On the east coast of northern and central Luzon, lies the Sierra Madre Range (from Cagayan to Quezon Province). Along the western coast of northern Luzon, the Cordillera Mountain Range runs down to central Luzon (the longest on Fig. 1) and linked by the Cordillera Range with Sierra Madre Range on the east. Statistics show that 43% of the average annual rainfall in the Philippines is derived from the south- west monsoon (SWM; Asuncion and Jose 1980). The average SWM rainfall distribution has a maxi- mum rainfall of about 2,000 mm (or greater) con- centrated over mountain areas (Iba, 3,011 mm and Baguio, 2,645 mm) and heavy rains in the range 1,500–1,999 mm are found along the western por- tion of Luzon (Asuncion et al. 1981). During the SWM season, the heavy rainfall events in the west- ern sections of Luzon cause flash floods over low lying areas and landslides along mountain slopes. In these disastrous events, it is noted that the heavy Fig. 1. Location of western Luzon stations rainfall are not directly coming from the tropical plotted on topographical map of the Phil- cyclones (TCs) but due to the SWM surge enhanced ippines. (Topographical map from Aka- by the presence of tropical cyclones situated north saka, 2008 PMS presentation). February 2011 E. O. CAYANAN et al. 125 Bay, due to flood waters joined to become an in- land sea (Gordon 1973). For the same flood event, Asuncion and Jose (1980) noted that this unusual condition was brought about by the presence of tropical cyclones that enhanced the monsoon ac- tivity at the time when the tropical disturbance was north to northeast of the region. This kind of weather condition was frequently observed, how- ever, no scientific study has yet been done to ex- plain this phenomenon. It is hypothesized that the heavy rainfall in the western section of Luzon is caused by the monsoon southwesterly flow enhanced by the TC and its in- teraction with the mountains along the west coast. The strong southwesterlies are said to be a result of the combination of the monsoon southwesterlies and the westerlies generated by TCs present in the vicinity of Luzon Island. During the rainy season, Fig. 2. Coverage of the Philippine area of rainfall amount on the west coasts of the Philip- responsibility (PAR), bounded by 25N– pines peaks when southwesterlies are strongest 120E, 25N–135E, 5N–135E, 5N– (Akasaka et al. 2007). The strength of these wester- 115E, 15N–115E and 21N–120E). lies is modulated by the position of the northward propagating 30–60 day monsoon trough and west- ern north Pacific anticyclone (Chen et al. 2000; by 119E–122E, 14N–19N) represented by ar- Chen and Murakami 1988). Four TCs moved rows drawn to scale. The heavy rainfall events iden- through the vicinity of the Philippines were ana- tified (shaded) in the series and the corresponding lyzed to substantiate the hypothesis introduced tracks of tropical cyclones during the events are above. also included in the figures. In the 5-year time series, 19 heavy rainfall events 2. Data and methodology were identified and investigated. Out of the 19 A five-year (2002–2006) daily rainfall data at events, 17 cases or 89.5% involve a tropical cyclone 11 western Luzon stations in the Philippines during in the PAR. This proves that tropical cyclones con- the southwest monsoon months, June–September, tribute much to the enhancement of rainfall during was analyzed. The rainfall stations used are shown SWM season. From the 17 events with tropical cy- in Fig. 1. The tracks of tropical cyclones within clones, 11 cases involved TCs over the ocean in the the Philippine area of responsibility (PAR) during vicinity of Luzon Island and only 6 events with TC the period were also considered in the study. The crossing the Philippine Islands. On the wind direc- PAR, as illustrated Fig. 2, is bounded by the lines tion, 14 events have SW winds up to 850 hPa level. joining the following points: 25N–120E, 25N– The remaining 5 events with variable winds (N, 135E, 5N–135E, 5N–115E, 15N–115E and NW, NE) involve TCs crossing the Philippine Is- 21N–120E. In the rainfall analysis, the 5-day lands. This shows that heavy rainfall events occur rainfall running mean was plotted in a time series when the winds over the western sections of Luzon shown in Fig. 3. In this study, heavy rainfall event is southwesterly. is defined or described as those with total rainfall of Four cases of heavy rainfall events are selected at least 200 mm dayÀ1 at 11 stations observed for for presentation and discussion in this study. The 5 consecutive days in the time series. The presence heavy rainfall events are selected to represent di¤er- of tropical cyclones (represented by small circles ent conditions/scenarios of heavy rainfall events. in yellow-depression, blue-storm or red-typhoon) The presence and positions of tropical cyclones are within the PAR on day to day basis is also indi- considered in the selection.
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