An analysis of tropical cyclone (TC) data from 1951 to 2013 in the Philippines revealed that an average of 19.4 TCs enter the Philippine Area of Responsibility (PAR) every year and nine TCs cross the country. Time series analysis of the TC datasets shows no significant trends in the annual number of TCs in PAR but a slightly decreasing trend in the number of landfalling TCs in the Philippines, particularly in the last two decades. However, while the analysis shows fewer typhoons (above 118 kph), more extreme TCs (above 150 kph) have affected the Philippines. The study also confirms that the Northern island of Luzon is most frequently hit by TCs, and that TC-associated rainfall is greatest in this region compared to the southernmost part of the country. The impact of TCs shows a consistently increasing trend in economic losses and damages. Further understanding of past and future trends of TC activity in the Western North Pacific Basin, and the PAR, including the impacts associated with them, will provide valuable insights for climate change adaptation and disaster risk management.

Seasonal rainfall in the Philippines is known to be modulated by the El Niño Southern Oscillation (ENSO) phenomenon, with ENSO warm (cold) events frequently contributing to drought (excessive rainfall) in many areas. Here, using rainfall data from 40 observing stations across the Philippines, it is shown that the seasonal rainfall response to ENSO reverses sign between boreal summer (July–September) and fall (October–December) for both El Niño and La Niña. Above‐median, boreal summer rainfall is observed at several stations in the north‐central Philippines during El Niño before anomalously dry conditions become manifest the subsequent fall. Conversely, during La Niña below‐median summer rainfall occurs at several stations before the onset of anomalously wet conditions in fall. The results are shown to be not simply a consequence of ENSO transitions during boreal spring, but are associated with changes in the large scale monsoon system during the life cycle of ENSO events.

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The spatio-temporal variability of boreal summer monsoon onset over the Philippines is studied through the analysis of daily rainfall data across a network of 76 gauges for the period 1977 to 2004 and the pentad Merged Analysis of Precipitation from the US Climate Prediction Center from 1979 to 2006. The onset date is defined using a local agronomic definition, namely 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. The onset is found to occur rather abruptly across the western Philippines around mid-May on average and is associated with the setup of a "classical" monsoonal circulation with low-level easterlies subsequently veering to southerly, and then southwesterly. The onset manifests itself merely as a seasonal increase of rainfall over the eastern Philippines, where rainfall occurs throughout most of the year. Interannual variability of the onset date is shown to consist of a spatially coherent large-scale component, rather similar over the western and eastern Philippines, with a moderate to high amount of local-scale (i.e. station scale) noise. In consequence, the large-scale signal can be easily retrieved from any sample of at least 5-6 stations across the network although the localscale coherence and fingerprint of the large-scale signal of the onset date are found to be stronger over the central Philippines, roughly from Southern Luzon to Northern Mindanao. The seasonal predictability of local onset is analyzed through a cross-validated canonical correlation analysis using tropical Pacific and Indian Ocean sea surface temperature in March and the 850 hPa May wind field from dynamical forecast models as predictors. The regional-scale onset, defined as the average of standardized local-scale anomalies in onset date, shows good predictive skill (r ≈ 0.8). Moreover, most of the stations show weak to moderate skill (median skill = 0.28-0.43 depending on the scheme) with spatial averaging across stations typically increasing skill to > 0.6.

The long-term variability of Philippine summer monsoon onset from 1903 to 2013 was investigated. The onset date is defined by daily rainfall data at eight stations in the northwestern Philippines. Summer monsoons tended to start earlier in May after the mid-1990s. Other early onset periods were found during the 1900s, 1920s, and 1930s, and an interdecadal variability of summer monsoon onset was identified. Independent surface wind data observed by ships in the South China Sea (SCS) revealed prevailing westerly wind in May during the early monsoon onset period. To identify atmospheric structures that trigger Philippine summer monsoon onset, we focused on the year 2013, conducting intensive upper-air observations. Tropical cyclone (TC) Yagi traveled northward in the Philippine Sea (PS) in 2013 and triggered the Philippine monsoon onset by intensifying moist low-level southwesterly wind in the southwestern Philippines and intensifying low-level southerly wind after the monsoon onset in the northwestern Philippines. The influence of TC was analyzed by the probability of the existence of TC in the PS and the SCS since 1951, which was found to be significantly correlated with the Philippine summer monsoon onset date. After the mid-1990s, early monsoon onset was influenced by active TC formation in the PS and the SCS. However, the role of TC activity decreased during the late summer monsoon periods. In general, it was found that TC activity in the PS and the SCS plays a key role in initiating Philippine summer monsoon onset.