Seasonal march patterns of the summer rainy season in the Philippines and their long-term variability since the late twentieth century

Abstract

This study investigates the seasonal march patterns of rainfall in the Philippines from 1951 to 2012 and their long-term variability. In order to clarify the dominant patterns in the seasonal march of rainfall, an empirical orthogonal function (EOF) analysis was applied to pentad rainfall data of 30 stations. For the first EOF mode (EOF1), we obtained a pattern related to the summer rainy season. We then applied cluster analysis to the time coefficients of EOF1 in each year to classify the seasonal patterns of the summer rainy season. As a result, the patterns were classified into six clusters. We found a long-term change in the pattern appearances with three anomalous patterns frequently observed since the 1990s: (1) a pattern that has an indistinct dry season and a prolonged peak rainfall, (2) a pattern that has a distinct dry season and an earlier withdrawal of the summer rainy season, resulting in a shortened rainy season, and (3) a pattern with a distinct dry season as well as delayed onset and withdrawal of the summer rainy season. This study also shows the relations between these three patterns and the lower atmospheric circulation at the 850 hPa level around the Philippines. Consequently, large positive and negative anomalies in geopotential height were observed around the Philippines for the distinct and indistinct dry seasons, respectively. The duration and condition of the dry season were greatly affected by the strength and location of the subtropical high especially for February-March. It is also noteworthy that the timing of the onset (withdrawal) of the summer rainy season is clearly related to that of the onset of the westerly (northerly) wind in the zonal (meridional) component around the Philippines. Further, the duration and amount of peak rainfall were directly influenced by the strength of the westerly winds in the zonal component. These three anomalous patterns tended to appear in the years when the warm or cold event of the El Niño-Southern Oscillation (ENSO) occurred. This study suggests that the long-term variability in the seasonal march of rainfall is considerably influenced by the variability in ENSO.

Climatological seasonal changes of wind and rainfall in the Philippines

“…One pentad later in P28 (Figure 2e), the whole wind system in the north of 10 N abruptly changed from easterly to westerly, which clearly marks the onset of summer SW monsoon wind regime in the Philippines. The timing is almost the same as the first transition of the Asian monsoon (Matsumoto, 1997;Hsu et al, 1999;Ueda, 2005), or the climatological monsoon onset of the Philippines defined by Akasaka (2010), Akasaka et al (2007Akasaka et al ( , 2018, and Moron et al (2009). In the east of Luzon Island is the southwestern periphery of the WNPSH anticyclonic flow, and the convergence between the southwesterly and southeasterly is located in the east of the northern Philippines, while it is over Visayas.…”

“…As for the rainfall increase in the Indochina Peninsula, Kiguchi and Matsumoto (2005) and Kiguchi et al (2016) attributed to the influence of mid-latitude westerly trough to the occurrence of rainfall before the monsoon circulation onset. On the other hand, Akasaka (2010) and Akasaka et al (2007Akasaka et al ( , 2018, and Moron et al (2009) did not present such discrepancy between the rainy season onset and monsoonal wind changes. In order to investigate the reason for this discrepancy, we have plotted the similar time-latitude sections of rainfall and wind in each year, as in Figure 2, and found that in 9 years (1999,2000,2001,2004,2005,2007,2008,2009, and 2011) the increase of rainfall occurs earlier than the wind shift from easterly to westerly, while in other years, they change almost simultaneously.…”

“…They were able to identify the timing of the onset and withdrawal dates of summer rainy season in the country based on the sign change of the time coefficients of EOF1, which climatologically occur at P27 (May 11-15) and P62 (November 2-6) to P63 (November 7-11), respectively. Akasaka (2010) and Akasaka et al (2018) further investigated the interannual variations of the monsoonal seasonal march based on the same criteria used by Akasaka et al (2007). Moron et al (2009) utilized station-based rainfall and CMAP data from all over the Philippines and defined the onset of rainy season, for both datasets, as the first wet day of a 5-day period receiving at least 40 mm without any 15-day dry spell receiving less than 5 mm in the 30 days following the start of that period.…”

Interdecadal Shifts in the Winter Monsoon Rainfall of the Philippines

“…In particular, the rainy season over the western coast occurs from May to September (summer monsoon) and followed by a dry season, while the eastern coast experiences its rainy season from November to March (winter monsoon) [18][19][20][21]. Recently, a number of studies found long-term changes in the rainfall and temperature of the Philippines [19,[22][23][24][25][26][27][28]. For example, Cruz et al [25] found decreasing rainfall trends during the summer monsoon season (June to September) in stations located over the western coast of the country from 1961-2010.…”

“…(1) While there are many studies that have investigated the variability (i.e., long-term trends and interdecadal shift) of the summer monsoon rainfall of the Philippines [19,[22][23][24][25][26][27][28][29], the variability of the winter monsoon rainfall has received less attention. According to Assuncion and Jose [30], the winter monsoon accounts for 38% of the total annual rainfall in the Philippines, while the summer monsoon accounts for about 43%, suggesting that the rainfall contribution of the winter monsoon is equally important with that from the summer monsoon.…”