Development of Local Southwest Monsoon Index in the Philippines

Abstract

Based on previously reported Asian-Australian Monsoon indices, this study characterizes the intensity of the southwest (SW) monsoon associated rainfall and its variability in different subregions of the western Philippines. Reanalysis and satellite-based datasets are utilized to derive these monsoon indices, which include the southerly and westerly wind shear indices, and outgoing longwave radiation-, and mean sea level pressure-based indices, spanning from 1991 to 2020. Subsequently, these indices were integrated to develop a local SW monsoon index (LSWMI) in the Philippines, which was compared and assessed with climatological gridded-and ground-based rainfall datasets to quantitatively describe the spatiotemporal dynamics of the SW monsoon season over the 30-year period. Results show that the proposed LSWMI can sufficiently capture the occurrences of heavy rainfall events in western Philippines. Moreover, the LSWMI is also capable in describing the evolution (onset, peak and decay) and distinct spatiotemporal characteristics of the SW monsoon, as it propagates from northern to southern Philippines. Overall findings demonstrate the significance of utilizing the LSWMI in characterizing and quantifying the SW monsoon, which ultimately provides new insights on advancing the monsoon monitoring and forecasting capabilities in the country.