Long-term trends and variability of rainfall extremes in the Philippines
Introduction
Trends and changes in precipitation and temperature extremes have been a focus of research over the past decade. Consistent with this focus, a suite of extreme precipitation and temperature indices was defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) to enable uniformity in analysis of climate extremes in different regions around the world (e.g., Klein Tank et al., 2006, New et al., 2006, Zhang et al., 2005). Consequently, Alexander et al. (2006) were able to incorporate studies across different regions to obtain a global picture of trends in extreme precipitation and temperature. Unlike those of extreme temperature, trends in extreme precipitation were commonly found to be spatially incoherent. In this regard, more recent studies have maximized the spatial availability of observations to examine trends in precipitation extremes. For instance, Endo et al. (2009) investigated trends in extreme precipitation using almost the entire network of stations in Southeast Asia. They were able to provide a clearer picture of trends across the neighboring countries in the region. However, Endo et al. (2009) analyzed trends using annually computed extreme precipitation indices (EPI), whereas, Klein Tank et al. (2006) emphasized that trends obtained from annually analyzed EPI, specifically in monsoon regions, may be dominated by wet or dry seasons. Thus, further analysis of trends in EPI in the Southeast Asian region, including the Philippines, where rainfall is influenced by monsoons, is needed.
The prevailing surface winds brought by the monsoons and the topographic effect are the main causes of rainfall seasonality over Southeast Asia (Chang et al., 2005). In the Philippines, mountain ranges are mostly stretched with a north–south orientation along the east and west coasts and reach elevations higher than 1000 m (see Fig. 1). Thus, rainfall in the Philippines shows seasonally and spatially contrasting characteristics. The rainy season in the country generally begins around mid-May, when the western North Pacific subtropical high (WNPSH) moves northeastward, enabling the southwesterly wind brought by the Asian summer monsoon to propagate over the Philippines (Akasaka, 2010). Subsequently, the East Asian winter monsoon is established around November (Ding, 1994) and brings northeasterly surface winds that cause the wetness (dryness) of the windward (leeward) eastern (western) coasts of the Philippines. Hence, agricultural activities throughout the country are patterned according to this rainfall seasonality. However, the eventualities of heavy rainfall events leading to floods, on the one hand, and rainfall deficits causing droughts, on the other hand, are embedded during these seasons. Such eventualities of heavy rain and drought were experienced during a recent five-year period (2004–2008), which directly affected the agricultural and energy sectors of the country as well as the Philippine economy (Yumul et al., 2011).
Droughts in the Philippines generally coincide with strong El Niño (EN) events (Jaranilla-Sanchez et al., 2011, Jose and Cruz, 1999); whereas, excessive rains in the country often occur during La Niña (LN) conditions (Hilario et al., 2009, Yumul et al., 2008). Because LN events are generally associated with excessive rains in the Philippines, the country was expecting a wet condition in 2007, a strong LN year, but drought was instead experienced from June to July of that year (Yumul et al., 2011). However, this seasonally opposite rainfall response with El Niño–Southern Oscillation (ENSO) in the country had been noted initially by Lyon et al. (2006), who showed that the seasonal total precipitation in the Philippines during July–September tends toward a wetter (drier) condition during EN (LN), while an exactly opposite behavior occurs during October–December. Nevertheless, these studies did not directly investigate ENSO-extreme rainfall relationships on a seasonal perspective over the entire region of the Philippines; therefore, this study focuses on this aspect.
On a multidecadal timescale, the Pacific Decadal Oscillation (PDO) has been shown to influence global precipitation anomalies that are well-pronounced over extratropical regions (Mantua and Hare, 2002). As the influence of PDO is only secondary in tropical regions, less attention has been given to its impact on rainfall over the Tropics. Studies that have examined the influence of PDO include Sen Roy et al. (2003) and, just recently, Krishnamurthy and Krishnamurthy (2013), both of which showed that the positive (negative) phases of the PDO are associated with rainfall shortages (excesses) over India. In the Philippines, decadal variability in rainfall needs to be further investigated. For instance, Jose et al. (1996) showed an increasing trend in both seasonal and annual total rainfall during 1951–1992 in the northwestern section of the Philippines, whereas Cruz et al. (2013), who used rainfall data from 1961–2010, showed a drying trend over the same region. Moreover, remarkable floods were experienced in the Philippines in the 1960s, 1970s, and 2000s, whereas several droughts were recorded in the 1980s and 1990s (Hilario et al., 2009). To determine whether these extreme rainfall events in the country are associated with PDO remain to be confirmed.
While there are increasing concerns, whether the extreme rainfall events that cause disastrous impacts in the Philippines are due to a changed climate, this present study aims to investigate trends and variability of rainfall extremes in the country. Specifically, this paper (1) examines long-term trends in seasonally computed EPI, thereby considering the known rainfall seasonality; (2) includes recently digitized rainfall data for a subset of stations to assess the obtained trends in a century-long temporal perspective; (3) utilizes long-term atmospheric reanalysis to suggest possible causes of the obtained trends; and, finally, (4) investigates how ENSO affects the interannual variability of the EPI and how the PDO affects their multidecadal variability to minimize the impacts of extreme rainfall occurrences, which is especially important in a disaster-vulnerable country such as the Philippines.
The datasets that we used and methods that we employed are described in the next section. The trends in EPI, their possible causes, and their decadal and interannual variability associated with PDO and ENSO are presented in Section 3. Finally, we summarize our most important findings and provide conclusions in Section 4.
Section snippets
Datasets
Daily rainfall data from 35 meteorological stations of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) are utilized in this study (Fig. 1). These selected stations have minimal missing data (less than 10%) during the entire 60 years (1951–2010) of observation. Additionally, 4 of these 35 stations, which have near-complete rainfall records from as early as 1911, are included to understand variations of precipitation extremes in a longer temporal context.
Trends in EPI (1951–2010)
The trends in seasonal wet days total rainfall (PCPTOT), maximum 5-day rainfall (RX5day), and maximum length of dry spell (LDS) during 1951–2010 are shown in Fig. 4.
Four (three) stations showed significant (10% level) decreasing (increasing) trends in PCPTOT during JFM. The trends in RX5day at some stations contributed to the trends obtained in PCPTOT, particularly in the eastern and southeastern sections of the country during JFM. On the other hand, the decreasing trends in PCPTOT during JFM,
Conclusions
Motivated by recent occurrences of extreme rainfall events in the Philippines and the increasing concerns whether these eventualities are a consequence of climate change, as an initial step, this present study investigated the long-term trend and variability of rainfall extremes in the country. Given the known seasonal and spatial variation of rainfall in the Philippines, rainfall extremes were described using seven EPI, which were computed seasonally, by utilizing 60-year (1951–2010) daily
Acknowledgments
We thank Dr. Esperanza Cayanan of PAGASA for her assistance in the rainfall data acquisition. We deeply appreciate the help of Drs. Xiaolan Wang and Yang Feng in using the RHtests for daily precipitation. The Green Network of Excellence (GRENE) program and a Grant-in-Aid for Scientific Research (no. 23240122) from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) supported the recovery and digitization of the pre-1940s rainfall data in the Philippines. This work
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Present affiliation: Department of Geography, Senshu University, Tokyo, Japan.