Interannual variations in seasonal march of rainfall in the Philippines

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 3: () Published online 1 Jul 2 in Wiley InterScience ( DOI: 1./joc.175 Interannual variations in seasonal march of rainfall in the Philippines Ikumi Akasaka* Tokyo Metropolitan Research Institute for Environmental Protection, Sinsuna, Koto-ku, Tokyo 13-75, Japan ABSTRACT: This study investigated interannual variations in seasonal march of rainfall in the Philippines by revealing onset and withdrawal pentads of rainy seasons from 11 to 2. For defining the onset and withdrawal of rainy season, the empirical orthogonal function (EOF) analysis was applied. As a result, the onset of summer rainy season, when started in mid-may on the average, was frequently delayed and fluctuated more greatly after the latter half of the 17s. Such characteristics were not found in the onset of autumn rainy season, which corresponds to the increase in rainfall amount on the east coast. To clarify causes of the long-term change in the onset timing of the summer rainy season, we classified transition patterns of atmospheric circulation related to the onset of the summer rainy season by applying the EOF analysis to spatial anomalies of geopotential height at 85 hpa level. The first two dominant EOF modes showed three important triggers of the onset of the summer rainy season in atmospheric circulation: (1) the northeastwards shift in the subtropical high over the western North Pacific, (2) the evolution of the monsoon trough over the northern South China Sea and (3) the great approach of the easterly wave. Additionally, interannual variations in the time coefficients of EOF1 have a positive tendency on the boundary of the latter half of the 17s and are significantly correlated with those in the onset of the summer rainy season. That is, it was suggested that the change of the onset timing in the summer rainy season after the latter half of the 17s was related to a long-term change in transition patterns of atmospheric circulation connected with the onset of the summer rainy season. Copyright 2 Royal Meteorological Society KEY WORDS onset and withdrawal of rainy season; interannual variation; seasonal march; the Philippines Received September 28; Revised 27 May 2; Accepted 27 May 2 1. Introduction Annual and seasonal rainfall variations in the Philippines are primarily caused by seasonal shifts of the Asian monsoons and tropical cyclones (Flores and Balagot, 1). Rainfall is an important source of water for Philippine agriculture. Therefore, interannual variations in the seasonal march of rainfall, which are closely connected with the cultivation calendar, are of great interest to the Philippine economy. As the underlying research for this study, Akasaka et al. (27) investigated regional differences in the seasonal march of rainfall in the Philippines by using pentad rainfalls at 3 stations averaged over 4 years (11 2). The results showed that the seasonal marches of rainfall on the west and east coasts contrast sharply. The west coastal region has a distinct dry season during winter spring and a wet season during summer autumn. The summer rainy season starts simultaneously throughout the entire Philippines in mid- May and withdraws gradually from the northern part of the Philippines around November. In contrast, the east coastal region is wet throughout the year and experiences * Correspondence to: Ikumi Akasaka, Tokyo Metropolitan Research Institute for Environmental Protection, Sinsuna, Koto-ku, Tokyo 13-75, Japan. akasaka-i@tokyokankyo.jp maximum rainfall during autumn and winter (November February) rather than in summer (July August). On the east coast, a great increase in rainfall starts around late September. These regional differences are characterized both by the seasonal transitions in atmospheric circulation with abrupt changes over the Asian-Pacific monsoon region, as shown by Matsumoto (12b), and orographic effects. The relationship between topography and prevailing wind direction is an important factor characterizing the regional differences in seasonal march of rainfall over the Maritime Continent, including the Philippines (Chang et al., 25). While the climatological characteristics of the seasonal marches of rainfall in the Philippines have been shown by Akasaka et al. (27), the interannual variations of seasonal marches of Philippine rainfall have scarcely been investigated in comparison to those of other Southeast Asia and the South China Sea (SCS) areas (e.g. Zhang et al., 22; Huang et al., 24; Zhou and Chan, 27). The interannual variations in amount of rainfall in the vicinity of the Philippines are greatly influenced by El Niño/Southern Oscillation (ENSO). It is well known that a warm episode of ENSO leads to droughty conditions over the vicinity of the Philippines during May November (Ropelewski and Halpert, 7). Additionally, Jose () pointed out in a recent report Copyright 2 Royal Meteorological Society

2 132 I. AKASAKA that the onset of the summer rainy season in the Philippines was delayed and its duration was shortened during the warm episodes of ENSO in 17. However, it is necessary to investigate whether or not these characteristics always appear during a warm episode of ENSO, by revealing the interannual variations in onset, withdrawal and duration of the rainy season. Thus, the purpose of this study is to reveal interannual variations in the seasonal march of rainfall in the Philippines by determining onset and withdrawal dates of the rainy season. This study also aims to clarify climatic factors in atmospheric circulation related to interannual variations in the seasonal march of rainfall, especially in the onset of the summer rainy season. 2. Data and methods The primary source of rainfall data used in this study is thesameasinakasakaet al. (27); it is recompiled from the daily rainfall data from 11 to 14 collected by Matsumoto (12a) and his related projects from the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), with data from the period between and 2 obtained directly from the PAGASA. Thirty-nine rainfall stations, which account for more than 8% of the data from 11 to 2, were used for this study (Figure 1). To exclude daily variations of rainfall, pentad rainfall data were calculated from 11 to 2, and were numbered as Julian pentad from the 1st pentad (for the first of January) to the 73rd pentad (for the end of December). To clarify the spatial and temporal structures of interannual variations in the seasonal march of rainfall, including regional differences due to orographic effect, the empirical orthogonal function (EOF) analysis was applied to 73 pentad rainfalls from 11 to 2 at 3 stations. To consider the statistical distribution, pentad rainfall data were normalized by a cubic root and then standardized for the EOF analysis. Next, the onset and withdrawal pentads of rainy seasons during 11 2 were defined by the result of the EOF analysis and the Mann Whitney U test. Details of this method are described in Section 3.2. The grid data of wind field and geopotential height at 85 hpa level, taken from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis (Kalnay et al., 1), were also used to analyse atmospheric circulation related to the seasonal march of rainfall. The pentad data were computed from daily data using the same method as for rainfall data. Then, to classify transition patterns of atmospheric circulation related to the onset of the summer rainy season, the EOF analysis was applied to spatial anomalies of geopotential height over the vicinity of the Philippines (8 17 E and 1 S 25 N) during the pentad previous to the onset of the rainy season and the following pentad (i.e. onset pentad). To remove secular changes, spatial anomalies of geopotential height were used for the EOF analysis. Figure 1. Distribution of observation stations used in this study. Circles and grey colours indicate observation stations and elevation, respectively. The Philippines is composed of three major islands: Luzon, Visayas and Mindanao Islands. 3. Results 3.1. Temporal and spatial structures of rainfall variation from 11 to 2 The results of the EOF analysis for rainfall show two dominant EOF modes. Although the signals in the factor loadings and the time coefficients of EOF2 are reversed compared with those shown by Akasaka et al. (27), mutual relationships shown from these two results have same characteristics on seasonal march of rainfall. The spatial pattern of the first EOF mode (EOF1), accounting for 32.8% of total variance, shows the same sign in the entire Philippines except the east coast of Mindanao Island, with especially high loadings in the west coastal region of 1 N (Figure 2). Over the 4 years studied, the mean time coefficients of EOF1 switch sharply from negative to positive in mid-may and gradually to negative around November (Figure 2). This means that the rainy season starts simultaneously in the entire Philippines in mid-may and gradually withdraws around November. These features appear markedly in the west coastal region, where the rainy season occurs in summer. The spatial pattern of the second EOF mode (EOF2), accounting for.% of total variance, reveals a different signal in the northwest part and the east coastal region Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

3 INTERANNUAL VARIATION IN SEASONAL MARCH OF PHILIPPINE RAINFALL EOF1(32.8%) contour interval:.1 EOF2(.%) contour interval: EOF pentad number EOF2 Figure 2. Spatial patterns and mean time coefficients of first two EOFs for pentad rainfall from 11 to 2, and indicate the spatial structures for EOF1 and EOF2, respectively, and shows the 4 years mean time coefficients of EOF1 and EOF2. EOF EOF year year pentad pentad Figure 3. Pentad numbers used for composite analysis in Figures 4 and 5. Black (white) squares denote pentad numbers when the time coefficients of EOF1 and EOF2 modes are above 1 (below 1). Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

4 134 I. AKASAKA Figure 4. Composite maps of rainfall, geopotential height and wind field at 85 hpa level for high (a and b) and low (c and d) time coefficients of EOF1. Circles and grey colours in and denote observation stations and elevation, respectively. Contour interval of geopotential height is 5 m. of the Philippines (Figure 2). Particularly, there is a distinct contrast between the west coast of Luzon Island and the eastern parts of Visayas and Mindanao Islands. Thus, EOF2 represents characteristics on both the west and the east coastal regions. Over the 4 years studied, the mean time coefficients of EOF2 indicate a steep change that corresponds to increases in rainfall amounts in the east coastal region in autumn (Figure 2). To reveal spatial patterns of rainfall, obtained from the two EOF modes, and their relation to atmospheric circulation, composite maps on rainfall, wind and geopotential height were made for high (above 1) and low (below 1) time coefficients. The pentads used for the composite analysis are shown in Figure 3. The high-time coefficients of EOF1 primarily appear in the summer rainy season during the end of May November (Figures 3 and 4), while the low-time coefficients appear in the dry season during January to late May (Figures 3 and 4). The wet and the dry seasons in the entire Philippines, especially the west coastal region, relate to the evolutions of the monsoon trough over the northern SCS and to the subtropical high over the western North Pacific (WNP), respectively (Figures 4 and ). In contrast, the high-time coefficients of EOF2 appear from the end of October through the following March (Figure 3). During this period, the eastern parts of Luzon, Visayas and Mindanao Islands experience maximum rainfall (Figure 5). The low-time coefficients of EOF2 appear during the end of May September when rainfall amounts increase in the northwestern part of the Philippines, especially in Luzon Island (Figures 3 and 5). Thus, EOF2 clearly reveals a contrast in seasonal marches of rainfall between the west and the east coastal regions. The contrast is mainly characterized by the prevailing east-northeasterly wind related to the evolution of high pressure over the southeast part of China in autumn winter (Figure 5) and by the Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

5 INTERANNUAL VARIATION IN SEASONAL MARCH OF PHILIPPINE RAINFALL 135 Figure 5. Same as in Figure 4 except for EOF2. EOF1 EOF year year pentad pentad Figure. Diagrams of the onset and the withdrawal pentads of rainy season from 11 to 2. Grey boxes denote pentads with positive value in the 5 pentad-running mean time coefficients of EOF1 and EOF2. Triangles and inverse triangles denote the onset and the withdrawal pentads of rainy seasons defined by the Mann Whitney U test, respectively. Dotted lines indicate the mean onset and withdrawal pentads of rainy season. west-southwesterly wind related to low pressure over the Eurasian Continent in summer (Figure 5). In summary, the results of the EOF and composite analyses showed that the spatial and temporal structures of rainfall variations from 11 to 2 have common characteristics with those of the climatological seasonal march of rainfall shown by Akasaka et al. (27). That is, the changes of sign in the time coefficients of the Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

6 13 I. AKASAKA onset of rainy season (pentad) year Figure 7. Time series of the onset pentad of summer rainy season from 11 to 2 except for 1. Grey and dashed lines denote the 5-year running mean and the mean ± σ, respectively mm ( DPI). Figure 8. Duration of summer rainy season from 11 to. Dashed line indicates the mean duration for the 38 years. The durations of summer rainy season in 1 and 2 are not shown because the onset or the withdrawal pentad of summer rainy season in 1 and 2 could not be defined in this study. See in the text for the details. first two EOFs closely correspond to the transition period of rainfall distribution patterns in the seasonal march of rainfall. Therefore, in the following section, we define the onset and withdrawal pentads of the rainy season by using the first two EOF modes to discuss interannual variation in the seasonal march of rainfall Interannual variations in the onset and withdrawal of rainy seasons As mentioned above, the time coefficients of the first two EOF modes were used to determine the onset and withdrawal pentads of summer and autumn rainy seasons. Since the withdrawal of the autumn rainy season on the east coastal region is different in each latitudinal zone (Akasaka et al., 27), it has not been defined in this study. The detailed method is as follows. First, to consider the continuation of the rainy season, 5-pentad running mean values were calculated for the time coefficients, allowing the duration of the rainy season to be roughly determined. Then, to determine the onset (withdrawal) pentad of rainy season, the Mann Whitney U test was applied to a series of two groups, in which each group consists of six continuous pentads in the time coefficients, at the 5% significance level, for a two-tailed test. This test was conducted on every series shifted by 1 pentad for the period between 11 and 2. Finally, the first pentad in the second of the two groups with the smallest U statistics was selected as the turning point of the rainy or dry season, because the largest discontinuity was considered to occur during this period. The onset of the summer rainy season could be determined by using the time coefficients of EOF1 during 11 2, except for 1 when the time coefficients varied excessively and the positive sign, indicating an increase in rainfall, continued even in the dry season (Figure ). The anomalous continuation of positive time coefficients for EOF1 was shown during January March not only in 1 but also in 2. Although this means an increase in rainfall during the dry season, such a feature did not appear in other years. The withdrawals of the summer rainy season were defined, except for 2 when the positive sign of the time coefficients lasted until the end of the year. Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

7 INTERANNUAL VARIATION IN SEASONAL MARCH OF PHILIPPINE RAINFALL onset[-1] 2 [-1] onset[] 4 [] (e) onset[+1] (f) [+1] m/s 3 Figure. Composite maps of rainfall (b, d and f), geopotential height and wind at 85 hpa level (a, c and e) with centering the onset pentad of the summer rainy season during 11 2 except for 1. Here, denotes the onset pentad and and + refer to prior to and after onset pentad, respectively. Contour interval of geopotential height is 5 m. Contour interval of rainfall is 1 mm below 5 mm and is 25 mm above 5 mm. Contour lines above 5 mm are shown by thicker lines. The mean onset and withdrawal pentads of the summer rainy season occurred around the 2th pentad (5/ 25) and the 7th pentad (11/27 /1), and the standard deviation was 3.8 and.8 pentads, respectively. According to Wu and Wang (2), who examined interannual variability on the onset of summer monsoon over the SCS and the WNP using outgoing longwave radiation data for 175 (excluding months in 178), the mean onset of the rainy season and the standard deviation around the Philippines were the 27th 2th pentad (mid- May) and 3 4 pentads, respectively. Thus, the onset of the summer rainy season and its interannual variation in the Philippines might be closely connected to the onset of the WNP and the SCS summer monsoons. Viewed from the long-term variations, it can be noted that the onsets of summer rainy seasons were frequently delayed and varied more greatly after the latter half of the 17s (Figure 7). The duration of the summer rainy season was also calculated from the difference between the onset and the withdrawal pentad. The mean duration and the standard deviation were around 38 pentads and 8.57 pentads, respectively (Figure 8). The variations in the duration are significantly related to those in the onset and the withdrawal of summer rainy seasons at the % significance level (r =.4 and., respectively). Meanwhile, there is no significant correlation between the interannual variations in the onset and the withdrawal of rainy seasons. These results indicate that the duration of rainy seasons was particularly determined by the timing of the withdrawal of the rainy season and that the variations in the onset and the withdrawal were statistically independent. The onset of the autumn rainy season, which corresponds to the increase in rainfall on the east coast, Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

8 138 I. AKASAKA EOF1-onset[-1] EOF1-onset[](33.8%) EOF year EOF2-onset[-1] (e) EOF2-onset[](.3%) (f) EOF year Figure 1. Spatial patterns and the time coefficients of the first two EOF modes for spatial anomaly of geopotential height at 85 hpa level during onset[ 1]-onset[]. (a and b) and (d and e) denote the spatial structures for EOF1 and EOF2, respectively. and (f) show the time coefficients of EOF1 and EOF2, respectively could be determined except for 12, 14, and 1 (Figure ) because the time coefficients of EOF2 varied more excessively than those of EOF1. The mean autumn rainy season onset and the standard deviation were about the 5th pentad (1/3 7) and 4. pentads, respectively. The long-term change as shown in the onset of the summer rainy season is not found in the onset of the autumn rainy season. To clarify the cause of the long-term change in the onset of the summer rainy season, in the following section we focus on the interannual variation in transition patterns of atmospheric circulation related to the onset of the summer rainy season The transition patterns of atmospheric circulation related to the interannual variation in the onset of summer rainy season Climatological features To reveal the climatological transition pattern of atmospheric circulation before and after the onset of the summer rainy season, composite analyses were applied to rainfall, geopotential height and wind from 11 to 2, except for 1 (Figure ). In the pentad previous to onset of the summer rainy season (onset[ 1]), the easterly or southeasterly wind prevails over the vicinity of the Philippines, located under the southwestern edge of the subtropical high (Figure ). Since the ridge of the subtropical high extends towards Luzon Island, dry conditions are brought to the Philippines. In the onset pentad (onset[]), the southerly wind simultaneously starts to blow over the Philippines through a convergence of the southwesterly and southeasterly flows. Concurrently, rainfall amounts increase in the entire Philippines (Figure and ). After that (onset[+1]), the southwesterly wind prevails with a deepening of the monsoon trough over the northern part of the SCS (Figure (e)), and rainfall amounts continue to increase especially in the northwest part of the Philippines (Figure (f)). Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

9 INTERANNUAL VARIATION IN SEASONAL MARCH OF PHILIPPINE RAINFALL onset[-1] [-1] onset[] [] (e) onset[+1] (f) [+1] m/s Figure 11. The evolution of the composite rainfall (b, d and f), wind and spatial anomalies of geopotential height at 85 hpa level (a, c and e) with centering the onset pentad of summer rainy season in extreme years with high-time coefficients of EOF1. The composite years are shown in Table I. Here, denotes the onset pentad, and ( + ) refers to prior to (after) the onset pentad. Contour interval of spatial anomaly of geopotential height is 5 m. Solid and dashed lines indicate positive and negative values, respectively. Contour intervals of rainfall are 1 mm below 5 mm and 25 mm above 5 mm. Contour lines above 5 mm are shown by thicker lines Classification of the transition patterns of atmospheric circulation To clarify the cause of the long-term change in the onset timing of the summer rainy season, we first classified transition patterns of atmospheric circulation during onset[ 1] onset[] by applying the EOF analysis to spatial anomalies of geopotential height. This study focuses on the first two EOF modes because the third and higher modes will likely still be entangled with the rule of thumb by North et al. (2). The spatial pattern of EOF1, accounting for 33.8% of total variance, shows a strong contrast between north of N and around the Maritime Continent (Figure 1 and ). In onset[], significant negative signals concentrate, especially from the Bay of Bengal to the northern SCS (Figure 1). In extreme years with high-time coefficients of EOF1, these changes indicate that the western edge of the subtropical high is located further south than normal for onset[ 1]. In onset[], the monsoon trough develops with the prevailing southwesterly wind over the northern SCS (Figure 11 and ). Concurrently, rainfalls increase throughout the entire Philippines (Figure 11 and ). In onset[+1], increase in rainfall appears markedly on the west coast with the prevailing southwesterly wind over the SCS (Figure 11(e) and (f)). Conversely, in extreme years with low-time coefficients of EOF1, the western edge of the subtropical high stretches out over the Philippines, and no advance of the monsoon trough is shown around the northern SCS during onset[ 1] onset[+1] (Figure, and (e)). Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

10 131 I. AKASAKA onset[-1] [-1] onset[] [] (e) onset[+1] (f) [+1] m/s Figure. Same as in Figure 11 except for extreme years with low-time coefficients of EOF1. It seems that the approach of cyclonic anomalies, propagating westwards (i.e. the easterly wave), are connected with an increase in rainfall throughout the entire Philippines (Figure, and ). The time coefficients of EOF1 have a positive tendency and start to change cyclically with large amplitude after the latter half of the 17s (Figure 1). We can conclude that there was a change in the transition patterns of atmospheric circulation shown by EOF1 during the period of The spatial pattern of EOF2 with.3% contribution shows a different signal between the eastern equatorial Indian Ocean and the WNP at the boundary around the Philippines in onset[ 1] (Figure 1). In onset[], the centre of the negative signal moves to the east of the Philippines, and the negative area spreads westwards throughout the Philippines (Figure 1(e)). In extreme years with high-time coefficients of EOF2, these changes correspond to the approach of cyclonic anomalies in atmospheric circulation during onset[ 1] onset[] (Figure 13 and ). In onset[+1], the southwesterly wind, which brings an increase of rainfall to the west coast, starts to blow over the southern SCS and the Philippines (Figure 13(e) and (f)). That is, a great approach of the easterly wave triggers an increase in rainfall throughout the entire Philippines just before the southwest monsoon onset (Figure 13, and (f)). On the other hand, in extreme years with low-time coefficients of EOF2, the southerly wind abruptly starts to blow over the Philippines with the northeastwards shift of the subtropical high and the southwest monsoon onset in onset[] (Figure 14). Concurrent with this, the summer rainy season starts throughout the entire Philippines (Figure 14 and (f)). This pattern is similar to the climatological pattern (Figure ). Unlike those of EOF1, the time coefficients of EOF2 show no trend (Figure 1(f)). Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

11 2 INTERANNUAL VARIATION IN SEASONAL MARCH OF PHILIPPINE RAINFALL onset[-1] 3 2 [-1] onset[] [] (e) onset[+1] (f) [+1] m/s Figure 13. Same as in Figure 11 except for extreme years with high-time coefficients of EOF2. In summary, the first two EOF modes show three important triggers in transition patterns of atmospheric circulation, which are connected with the onset of the summer rainy season: (1) the evolution of the monsoon trough over the northern SCS, (2) the northeastwards shift in the western edge of the subtropical high and (3) the great approach of the easterly wave. Additionally, high/low-time coefficients of EOF1 may be related to late/early onset of the summer rainy season (Table I). Extreme years with earlier and later onset of summer rainy seasons usually correspond with lowand high-time coefficients of EOF1, respectively. Furthermore, there is significant positive correlation between interannual variations in the time coefficients of EOF1 and in the onset of the summer rainy season at the % significance level (r =.4), suggesting that EOF1 may be associated with the onset timing. That is, the great approach of the easterly wave might trigger earlier onset of the summer rainy season by inducing a continuous increase in rainfall until immediately before the southwest monsoon onset. On the other hand, the later onset may be related to timing of the shift in the western edge of the subtropical high, located further southwards than normal, and of the evolution of the monsoon trough over the northern SCS in onset[]. 4. Conclusion This study investigated interannual variations in the seasonal march of rainfall in the Philippines by determining the onset and withdrawal of the rainy season. As a result of this study, some interesting characteristics were shown for the first time. The onset of the summer rainy season, which started in mid-may on average, was frequently Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

12 13 I. AKASAKA delayed and fluctuated more greatly after the latter half of the 17s. Such characteristics were not found in the interannual variation in the onset of the autumn rainy season, which corresponds to the increase in rainfall amount on the east coast. To clarify causes of changes in the interannual variation of the onset of the summer rainy season, we classified transition patterns of atmospheric circulation during onset[ 1] onset[]. The results showed three important triggers of the onset of the summer rainy season in atmospheric circulation: (1) the northeastwards shift in the subtropical high over the WNP, (2) the evolution of the monsoon trough over the northern SCS and (3) the great approach of the easterly wave. The interannual variation in the onset of the summer rainy season was characterized by abrupt changes in these factors of atmospheric circulation patterns during onset[ 1] onset[]. Thus, these factors greatly influence the onset timing and process of the summer rainy season. Numerous earlier studies have investigated climatological characteristics of atmospheric circulation associated with the onset of the southwest monsoon or the rainy season over the vicinity of the SCS and the WNP, including the Philippines (e.g. Wu and Wang, ). They showed the northeastwards shift of the subtropical high over the WNP and the evolution of the monsoon trough over the SCS as climatological triggers of the southwest monsoon or rainy season onset. They bring changes in the prevailing wind direction from the easterly to the westerly wind to the vicinity of the Philippines. In addition to these two triggers, this study presents the great approach of the easterly wave as an important trigger related to the summer rainy season onset in the Philippines from the viewpoint of interannual variations. Also, the great approach of the easterly wave may induce early onset of the summer rainy season in the Philippines because it leads to an increase in rainfall amount just before the onset of the southwest monsoon. On the other hand, a delayed onset of the summer rainy season may be connected with timing of the shift in the western edge of the subtropical high, located further southwards than normal, and the deepening of the monsoon trough. Furthermore, the transition pattern, related to delayed onset, has an increasing trend after the latter half of the 17s. This turning point corresponds to the period when the delayed onset of the summer rainy season started to clearly appear. Taken in a wider perspective, the long-term change in the onset timing of the summer rainy season is likely related to the relationship between ENSO and the WNP summer monsoon, as pointed out by Tanaka (17), or the rise of sea surface temperature in the Tropics after the latter half of 17s, showed by Nitta and Yamada (). Further studies are needed to clarify the relationship between the long-term changes in onset timing of the summer rainy season and the transition pattern of atmospheric circulation during onset[ 1] onset[], Table I. Relationship between early/late onset of the summer rainy season and low-/high-time coefficients of the first two EOF modes. Year Onset pentad Onset - mean EOF1 EOF : Above +1 ž :Below 1 The second column indicates the onset pentad of the summer rainy season, which written down in order of the earlier onset. In the third column, each year s anomalous onset pentad is given with + and denoting early/late pentads compared with the mean onset pentad (i.e. 2 pentad). Open (closed) circles in the fourth and the fifth columns denote the extreme years with the standardized time coefficients more than 1 (less than 1) in each EOF mode. specifically there is a need to analyse them with variations of sea surface temperature that drives atmospheric variation in the Tropics. Acknowledgements The author wishes to express gratitude Prof. Jun Matsumoto, Tokyo Metropolitan University, late Dr Jose. Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

13 2 INTERANNUAL VARIATION IN SEASONAL MARCH OF PHILIPPINE RAINFALL onset[-1] [-1] onset[] [] (e) onset[+1] (f) [+1] m/s Figure 14. Same as in Figure 11 except for extreme years with low-time coefficients of EOF2. D. Rondal, Department of Agriculture, the Philippines, and Philippine Atmospheric, Geophysical and Astronomical Services Administration for kindly preparing rainfall data and giving helpful advices on this study. The author would like to sincerely thank prof. Wataru Morishima, Nihon Universtiy and prof. Takehiko Mikami, Teikyo University, for their helpful comments and support. The author is also grateful to prof. Hideo Takahashi, Tokyo Metropolitan University, Dr. Yoshiyuki Kajikawa, University of Hawaii, Dr. Hisayuki Kubota, Japan Agency for Marine-Earth Science and Technology, and two anonymous reviewers for their thoughtful suggestions. This research was supported by the Global Environmental Research Fund (B-1) of the Ministry of the Environment, Japan, and by the Data National Key Technology, Ministry of Education, Culture, Sports, Science and Technology. References Akasaka I, Morishima W, Mikami T. 27. Seasonal march and spatial difference of rainfall in the Philippines. International Journal of Climatology 27: 7 725, DOI: 1./joc Chang CP, Wang Z, Mcbride J, Liu CH. 25. Annual cycle of southeast Asia-Maritime Continent rainfall and the asymmetric monsoon transition. Journal of Climate : Flores JF, Balagot VF. 1. Climate of the Philippines. Climates of northern and eastern Asia. In World Survey of Climatology, Vol. 8, Arakawa H (ed). Elsevier: Amsterdam; 3. Huang R, Huang G, Wei Z. 24. Climate variations of the summer monsoon over China. In East Asian Monsoon, Chang C-P (ed). World Scientific: Singapore; Jose M.. Documentation and Analysis on Impacts of and Responses to Extreme Climate Events-CLIMATE SECTOR. PAGASA. Kalnay E, Kanamitsu M, Kistler R, Collins W, Deaven D, Gandin L, Iredell M, Saha S, White G, Woolen J, Zhu Y, Chelliah M, Ebisuzaki W, Higgins W, Janowiak J, Mo KC, Ropelowski C, Wang J, Leetmaa A, Reynolds R, Jenne R, Joseph D. 1. The NCEP/NCAR 4-year reanalysis project. Bulletin of the American Meteorological Society 77: Copyright 2 Royal Meteorological Society Int. J. Climatol. 3: ()

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