Climatology of Philippine tropical cyclone activity: 1945-2011

The Philippine region occupies the southwestern western North Pacific (WNP) Ocean, between 5 degrees-25 degrees N and 115 degrees-135 degrees E. About 70% of WNP tropical cyclones (TCs) formed in or entered the Philippine region during 1945-2011. Here, a climatology of Philippine TC metrics is developed, including mean annual frequencies, landfalls, TC days, season lengths, season earliest and latest start and end dates, genesis locations, and tracks. Two distinct TC seasons, the less active season (LAS; 1 January-31 May) and more active season (MAS; 1 June-31 December), are evident. Philippine TC annual median LAS frequency is 2 [interquartile range (IQR) is 2], and median landfalling frequency is 1. The annual median MAS frequency is 15 (IQR is 4.5), and median landfalling frequency is 6. About 55% of Philippine TCs reach typhoon intensity. A quiescent (TC-free) period occurs between LAS and MAS, ranging from 2 days to 5 months (median 1.2 months) for LAS to MAS transitions, and 6 days to 7 months (median 2.85 months) for MAS to LAS transitions. The interannual variability of the annual average lifetime maximum intensity (LMI) for all TCs and landfalling TCs decreased slightly during the satellite era (the years since 1980). The TC annual average latitude of LMI in the satellite era exhibits a poleward migration; however, for landfalling TCs it is equatorward. Wavelet analysis shows El Nino Southern Oscillation as the dominant mode affecting Philippine TCs, consistent with other studies. The wavelet analysis also indicates possible decadal and multi-decadal modes. In El Nino years, TCs frequently recurve or decay before reaching the Philippine region, producing below normal numbers and landfalls in LAS and MAS. In La Nina years, TC numbers and landfalls are below normal in January-March and July-September, but above normal in April-June and October-December. The climatology developed here has social and economic relevance: allowing planning, providing early risk assessment, and mitigating impacts through timely preparation and management.