Impacts of ENSO on Philippine Tropical Cyclone Activity

Tropical cyclone (TC) activity in the Philippine region has strong seasonal variability. Investigating how the El Ni�o-Southern Oscillation (ENSO) phenomenon contributes to TC variability is the primary goal of this study. It provides better understanding of the characteristics of Philippine TCs during El Ni�o and La Ni�a phases of ENSO. This study produces a range of seasonal TC metrics, including: TC number, intensity, start and end dates, season length, TC days, accumulated cyclone energy (ACE), genesis locations, and tracks, during ENSO phases.

TCs in the Philippine region occur in all months and there are 2 distinct seasons: the less active (LAS) and more active (MAS) seasons. In neutral conditions, both seasons have above average numbers of TCs and landfalling TCs. During La Ni�a (El Ni�o) conditions, above (below) average TC numbers are observed in LAS while both La Ni�a and El Ni�o conditions observed below average in MAS. The landfalling TCs are above (below) average in La Ni�a (El Ni�o) conditions in both LAS and MAS.

Neutral conditions experience above average TC numbers in all three TC intensity categories in all months except in October-December where number of TCs with tropical depression (TD) intensity is below average. Above (below) average TDs are observed in January-March and October-December but below (above) average is observed in April-June in La Ni�a (El Ni�o) conditions. Both conditions have below average TD numbers in July-September. TCs in the tropical storm (TS) category are below average in January-June but above average in July-December in La Ni�a conditions, and below average all year round during El Ni�o conditions. Typhoon (TY) category TCs are below average in January-March and July-September, whereas they are above average in April-June and October-December in La Ni�a conditions. During El Ni�o conditions, there are below average TYs in most months, except in July-September.

Seasonal landfalling TD numbers are above average in neutral conditions except in April-June. Excluding January-March, La Ni�a conditions have below average landfalling TDs, especially in July-September. Below average landfalling TDs occur in January-March and October-December, while above average in April-September is observed during El Ni�o conditions. Above average landfalling TS numbers occur in neutral conditions in most seasons, except for October-December. The opposite is observed in La Ni�a conditions, where landfalling TS numbers are below average in January-September, whereas they are above average in October-December. During El Ni�o conditions, landfalling TS numbers are below average in all seasons. Landfalling TYs during neutral conditions are above average in January-March, but are below average for the rest of the year. Again, a reverse scenario is observed in La Ni�a conditions, except January-March, above average landfalling TYs are observed in April-December. In El Ni�o conditions, landfalling TYs are above average in January-March and July-September, but are below average in April-June and October-December.

During neutral conditions, the LAS mean start date is March 6 and the mean end date is May 12. The mean season length is 67 days. In La Ni�a (El Ni�o) conditions, the average start date of LAS is February 24 (March 16) whereas the average end date is May 9 (April 14) with 74 (32) days as the average season length. In MAS, neutral conditions have mean start date of June 18 and mean end date of December 16, with a mean season length of 181 days. La Ni�a (El Ni�o) conditions have an average start date of June 30 (June 14) while the average end date is December 8 (December 4). The mean season length for MAS is 162 (173) days in La Ni�a (El Ni�o) conditions.

The TC birthplaces in neutral conditions cover a broader longitude-latitude range. During LAS, birthplaces are within 110.9�-179.4�E longitudes and 2.6�-19.8�N latitudes, while MAS birthplaces are confined in 107.2�-177.8�E longitudes and 2.8�-26.5�N latitudes. In La Ni�a (El Ni�o) conditions, LAS birthplaces occur between 114.0�-163.3�E (mostly 136.0�-170.9�E) longitudes and 1.7�-18.3�N (3.2�-14.4�N) latitudes while in MAS, they lie within 111.0�-159.3�E (108.7�-177.2�E) longitudes and 3.0�-27.4�N (2.9�-25.5�N) latitudes.

In neutral conditions, the tracks of the TCs occupy 103.5�-179.7�E longitudes and 2.6�-37.7�N latitudes in LAS and in 84.6�-179.8�E longitudes and 2.5�-56.0�N latitudes in MAS. During La Ni�a (El Ni�o) conditions, tracks are within 104.8�-179.5�E (116.1�-170.9�E) longitudes and 1.7�-46.0�N (2.7�-30.0�N) latitudes in LAS and in MAS, TC tracks are within 97.1�-179.9�E (81.1�-179.8�E) longitudes and 3.0�-53.0�N (2.9�-53.0�N) latitudes.

Above average TC days and ACE are observed in neutral conditions in both LAS and MAS. The TC days and ACE in La Ni�a (El Ni�o) are above (below) average in LAS but below (above) in MAS.

This study highlights the influence of ENSO on Philippine TC activity. The impacts of ENSO on the number of TCs depend on the season, the ENSO phase, and on the intensity of the TCs. The average start of the season during LAS is earlier (later) relative to neutral conditions in La Ni�a (El Ni�o) conditions while the average end date is later (earlier) in La Ni�a (El Ni�o) conditions. The average season length of LAS is longer in La Ni�a conditions compared with neutral conditions; El Ni�o conditions have an even shorter season length. In MAS, relative to neutral and El Ni�o conditions, the average start date during La Ni�a conditions is late. El Ni�o conditions have the earliest average end date but La Ni�a conditions average end date is earlier than that of neutral conditions. MAS season length during La Ni�a conditions is the shortest, when compared with El Ni�o and neutral conditions.

In both LAS and MAS, the TC birthplaces are geographically closer to the Philippines in La Ni�a conditions and cover a broader latitude range, as opposed to El Ni�o conditions. TC birthplaces lie in a broader (narrower) longitude range in La Ni�a (El Ni�o) conditions in LAS but narrower (broader) in MAS. Most of the TC tracks during La Ni�a (El Ni�o) conditions occupy a broader (narrower) longitude range in LAS but La Ni�a (El Ni�o) conditions have a narrower (broader) longitude range in MAS.