"Influence of Tropical Cyclones, Southwest Monsoon, and Tropical Cyclon" by Alwin Bathan, Lyndon Mark Olaguera et al.
 

Document Type

Article

Publication Date

6-11-2025

Abstract

This study investigates the processes driving interannual variability of rainfall over the western coast of the Philippines during the peak southwest monsoon (SWM) months (July–September) from 1961 to 2022. Specifically, it describes how tropical cyclones (TC), SWM, and TC-enhanced SWM influence SWM rainfall variability. Rainfall measurements from 11 stations along the western coast of the Philippines were classified into three types: direct TC rainfall (PD), indirect TC rainfall (PI), and SWM rainfall (PSM). The yearly contributions of PD, PI, and PSM to SWM rainfall were estimated to be 15.4 %, 33.1 %, and 51.5 %, respectively. Furthermore, four rainfall variability types were found depending on the characteristics of each year's rainfall anomalies: enhanced PD but suppressed PSM (D+S-), suppressed PD but enhanced PSM (D-S+), enhanced PI but suppressed PSM (I+S-), and suppressed PI but enhanced PSM (I-S+). Composite analysis using ERA5 data revealed the presence of anomalous winds and circulations unique to each type that causes variations in the TC tracks and the monsoon flow, which accounts for the rainfall variability. Notable ones are the anomalous anticyclone over Taiwan in the D+S- case, which pushes TCs closer to the Philippines, and the anomalous anticyclone east of the Philippines in the I+S- case, which enhances the trade winds.

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