Interdecadal Shifts in the Winter Monsoon Rainfall of the Philippines

This study investigates the interdecadal shifts in the winter monsoon (November to March) rainfall of the Philippines from 1961 to 2008. Monthly analysis of the winter monsoon rainfall shows that the shifts are most remarkable during December. In particular, two interdecadal shifts are identified in the December rainfall time series around 1976/1977 and 1992/1993. To facilitate the examination of the possible mechanisms leading to these shifts, the analysis period is divided into three epochs: 1961-1976 (E1), 1977-1992 (E2), and 1993-2008 (E3). The mean and interannual variability of rainfall during E2 are suppressed compared with the two adjoining epochs. The shift around 1976/1977 is related to the phase shift of the Pacific Decadal Oscillation (PDO) from a negative phase to a positive phase and features an El Nino-like sea surface temperature (SST) change over the Pacific basin, while that around 1992/1993 is related to a La Nina-like SST change. Further analysis of the largescale circulation features shows that the decrease in the mean rainfall during E2 can be attributed to the weakening of the low-level easterly winds, decrease in moisture transport, and decrease in tropical cyclone activity. In addition, the suppressed interannual variability of rainfall during E2 can be partly attributed to the El Nino-like SST change and the weakening of the East Asian winter monsoon.