Long-term Trends and Variability of Rainfall Extremes in the Philippines

The Philippines, like many other countries, has experienced recent extreme rainfall events that caused damage to properties and loss of human lives. These lead to a question whether those events are related to human-induced climate change or part of natural climate variability. To address that question, this study investigates trends and variability in rainfall extremes in the Philippines during boreal summer (JAS) and fall (OND) from 1951 to 2010. Observed daily rainfall data from 35 meteorological observing stations are utilized; then, rainfall extremes are described using seven precipitation indices. Trend analyses show that the country's precipitation tends toward wetter (drier) condition as indicated by significant increasing (decreasing) trend in the maximum consecutive 5-day rainfall totals and significant decreasing (increasing) trend in the longest dry spell duration during JAS (OND). Inter-annual variability in the rainfall extremes during OND (JAS) is strongly (weakly) controlled by the El Niño Southern Oscillation (ENSO) as suggested by high (low) correlations between the Niño 3.4 index and precipitation indices at most stations. However, it is important to note that spatial incoherency exists in the observed trends; thus, factors affecting precipitation extremes, such as the strong influence of ENSO during OND but a non-dominant effect during JAS, may differ from one station to another in the country. Furthermore, care should be taken when interpreting trends because aside from the pronounced year-to-year variation, high inter-decadal variability is present in the indices of extreme precipitation as shown at the stations with earlier pre-1940s rainfall data. Although trends in the rainfall extremes can be partly attributed to high inter-annual and inter-decadal variability as suggested by observations, the potential impact of global warming could also not be discounted since; it may affect ENSO, tropical cyclones and other factors that cause rainfall extremes in the country.