Intensive observation PALAU2013 and data rescue study for Philippine summer monsoon onset

Philippines are located in the western side of tropical western Pacific. There are distinct summer monsoon in the western side and winter monsoon in the eastern side of the country. This study focuses on the onset of summer monsoon in the western Philippines during May to July. Intensive observation of Pacific Area Long-term Atmospheric observation for Understanding of climate change (PALAU2013) was conducted by launching additional upper-air observation in Cebu, Laoag, and Puerto Princesa during May to August 2013. We captured the onset of summer monsoon in June 10, 2013. Low level strong southwesterly wind associated with moistening air was penetrated in the central Philippines during the onset. However, this structure became vague in JRA25 reanalysis in 2008, when there was no continuous upper-air observation. Upper-air observation is necessary to represent the structure of monsoon onset. The recovery of historical station back to 1901 called “Data rescue” was conducted by Monthly Bulletins of Philippine Weather Bureau from 1901 to 1940. We created rainfall dataset in the Philippines from 1901 to 2012 by connecting recovered data and PAGASA station data. Summer monsoon onset was defined by using 8 station rainfall data in the western Philippines. The onset date becomes earlier after 1990s and tends to occur in middle May. Early onsets were also seen in early 20th century.