Climatology of Tropical Cyclone Activity in the Philippines: 1945–2011
The main aim of this study is to produce a comprehensive climatology of Philippine TCs extending existing work. TC activity in the Philippines runs year-round. The relatively quiet phase is from January to May, with less than one TC average per month, and is referred to as the less active season (LAS). The remainder of the year, June to December, is the more active season (MAS), with greater than one TC average per month. The two seasons differ in its characteristics in terms of TC count, landfall, season length, number of TC Days, TC birthplaces and tracks.
Large variations are present in the time series of annual number of Philippine TCs. The range is from a minimum of 10 in 1946 to a maximum of 28 in 1993. The annual average TC count for LAS is 2, and for MAS is 16. On average, the Philippines experiences 7 TC landfalls annually. The seasonal average landfall during LAS is 1 and is 6 for MAS. The season length for LAS has irregular fluctuations and is clearly seen. It depends on the start and end dates of the LAS. The season length ranges from a low of 0 days, for years when no TC exists in the domain, to a high of 155 days. However, the average season length is 61 days. TC activity in the Philippine region during LAS commences on March 6th and ends on May 5th on average. The MAS season length is much longer, averaging 174 days. The mean start and end dates of the season are June 20th and December 10th, respectively.
Of the 1,199 TCs that occurred in the Philippine region in 1945-2011, 453 made landfall in the Philippines. Seasonal variations in the frequency and landfall of the Philippine TCs are clear. July-September (JAS) has greater TC activity, as 49% of the total TC count is experienced during this period. The largest percentage of TC landfalls is observed in October-December (OND), accounting for 53% of the total landfalls. The Philippine TCs are classified as tropical depression (TD), tropical storm (TS) or typhoon (TY), based on the observed maximum sustained wind near the center. From 1945 to 2011, there were 192 TDs, 345 TSs, and 662 TYs. Most of the January-March (JFM) TCs are of TS intensity whereas TY intensities are predominant beginning from the second quarter up to the last quarter of the year, comprising more than 50% of the quarterly TC count. The greatest number of TY intensities is during the last quarter, accounting for 55% of the annual TC total. There are 103 TD landfalls, 132 TS landfalls, and 201 TY landfalls. Most of the TC landfalls in JFM are TDs. From April-June (AMJ) to OND, the majority of landfalls are of TY intensity.
The analysis of birthplaces and tracks of the Philippine TCs shows regular spatial and temporal progression. TC birthplaces can range from 2.5N° to 27.5°N, depending on the season. Relative to the geographical location of the Philippines, TC genesis positions extend east to 179.5°E and west to 107°E. As the months advance, the TC birthplaces and tracks also move northward until JAS, then regress in OND. In JFM, birthplaces and tracks are confined to the lower latitudes, and no TC develops in the South China Sea. The TC tracks generally are straight moving although some recurve. In AMJ, most of the TCs originate in the eastern side of the archipelago, but a small percentage start from the South China Sea. AMJ has higher cyclogenesis numbers than JFM, there are more recurving TCs, and the tracks extend farther north. JAS has the most cyclogenesis numbers, thus the birthplaces and tracks are the densest and largest in extent compared with other seasons. OND has less TC genesis and the latitudinal extent of birthplaces and tracks is not as large and dense compared to JAS, but has the greatest probability of landfall and has more straight-moving TCs. The seasonal spatial progression and retreat of birthplaces and tracks are divided into the LAS and MAS seasons. As LAS and MAS greatly differ in the number of TCs affecting the Philippines, this is reflected in the density of birthplaces and tracks. There are fewer TCs developing during LAS, and LAS birthplaces and tracks are confined to the lower latitude unlike the MAS where birthplaces and tracks reach farther north, and there is a significant increase in the number of TCs.
Wavelet analyses reveal ENSO as the dominant global mode affecting Philippine TC activity. In El Niño years, below average TC numbers are observed in both the LAS and MAS, with landfalling TCs below average except during July-September. In La Niña years, TC numbers and landfalling TCs are below (above) average in January-March and July-September (April-June and October-December).
A key finding is the identification of quiescent periods between the end of LAS and the start of MAS. The quiescent periods vary from 2 days to 5 months (average 1.5 months) for LAS/MAS transition and 8 days to 5.6 months (average 2.8 months) for MAS/LAS transition.