The Role of the Moisture Conveyor Belt in High Precipitation Events Along the Western Coast of the Philippines During the Southwest Monsoon Season

This study examines the role of the moisture conveyor belt (MCB) in high precipitation events (HPE) during the southwest monsoon (SWM) season in the Philippines. Using rainfall data from 11 weather stations along the country’s western coast, all high precipitation days (HPDs) from July to September between 1961 and 2022 were identified and categorized into direct tropical cyclone (TC), indirect TC, and SWM HPDs. A relationship between TC distance and intensity with HPD rainfall amount was not found, implying that there are other factors that modulate rainfall. HPEs were identified by grouping together HPDs that were less than 5 days apart. Lag composites of water vapor and surface latent heat fluxes revealed the presence of an MCB over the tropical northern Indian Ocean and the western North Pacific (WNP) during HPEs, but not during non-HPEs. This means that not all TCs to the northeast of the Philippines are able to produce HPEs. The presence of an MCB, which transports moisture from the Indian Ocean towards the Philippines, is integral to the occurrence of HPEs. Also, the Boreal Summer Intraseasonal Oscillation (BSISO) influences MCB formation and HPD occurrence. HPDs usually occur during Phases 5–7 of the BSISO since enhanced convection over the WNP leads to the formation of TCs, while anomalous westerlies over the tropical northern Indian Ocean create the strong monsoon westerlies necessary for MCB formation. Understanding the role of the MCB in the occurrence of HPEs may help improve the forecasting of such events in the Philippines.