Interannual variations in seasonal march of rainfall in the Philippines

摘要

This study investigated interannual variations in seasonal march of rainfall in the Philippines by revealing onset and withdrawal pentads of rainy seasons from 1961 to 2000. For defining the onset and withdrawal of rainy season, the empirical orthogonal function (EOF) analysis was applied. As a result, the onset of summer rainy season, when started in mid-May on the average, was frequently delayed and fluctuated more greatly after the latter half of the 1970s. Such characteristics were not found in the onset of autumn rainy season, which corresponds to the increase in rainfall amount on the east coast. To clarify causes of the long-term change in the onset timing of the summer rainy season, we classified transition patterns of atmospheric circulation related to the onset of the summer rainy season by applying the EOF analysis to spatial anomalies of geopotential height at 850 hPa level. The first two dominant EOF modes showed three important triggers of the onset of the summer rainy season in atmospheric circulation: (1) the northeastwards shift in the subtropical high over the western North Pacific, (2) the evolution of the monsoon trough over the northern South China Sea and (3) the great approach of the easterly wave. Additionally, interannual variations in the time coefficients of EOF1 have a positive tendency on the boundary of the latter half of the 1970s and are significantly correlated with those in the onset of the summer rainy season. That is, it was suggested that the change of the onset timing in the summer rainy season after the latter half of the 1970s was related to a long-term change in transition patterns of atmospheric circulation connected with the onset of the summer rainy season.