The contribution of non‐tropical cyclone vortices to the rainfall of the Philippines

Abstract

Weaker disturbances than tropical cyclones (TCs) such as tropical depressions and cold surges can significantly induce heavy rainfall and flooding events over the Philippines. However, the analysis of these disturbances including their rainfall contributions are often neglected in previous studies. As the first attempt to address this research gap, this study investigates the rainfall contribution of non‐TC vortices over the Philippines from 1979 to 2020. Only those rainfall‐producing non‐TC vortices that formed and appeared within a 500‐km radius from the Philippine coastline were examined in this study. A total of 7,686 non‐TC vortex days (50% of the total days during the analysis period) were identified. The mean rainfall contribution of these non‐TC vortices was found to be highest over the northeastern Mindanao Island (80–90% of the mean daily rainfall) and lowest over the central and western regions of Luzon Island (50–60%). Seasonal analysis of the occurrence frequency of these vortices shows that they are most frequent during the December–February (DJF) season. In this season, the rainfall contribution may increase to 50–80% of the mean daily rainfall over the whole country, while in the other seasons, the rainfall contribution may only increase to as much as 60%. Higher frequency of extreme rainfall days associated with these non‐TC vortices were also found during the DJF season. The frequency of occurrence and percentage rainfall contribution of these non‐TC vortices in relation to the different phases of the Boreal Summer Intraseasonal Oscillation (BSISO) during boreal summer (June–October) and the Madden–Julian Oscillation (MJO) during boreal winter (December–April) were also examined. Higher frequency and percentage rainfall contribution over the country were found during Phases 4–6 of both the BSISO and MJO, during which their respective active convections transition from the Maritime Continent to the western North Pacific.

Quantifying the influence of the Madden–Julian oscillation on rainfall extremes during the northeast monsoon season of the Philippines

“…Surigao and Hinatuan City stations both have 4% percentage contributions from TC-nonMJO. Such low percentage contributions from TCs alone over Mindanao Island were also noted by Cinco et al (2016), Bagtasa (2017), and Olaguera et al (2023). The contribution from TC-MJO ranges from 0% to 6%, with the largest in Virac station (6%).…”

“…What systems are included in the "other disturbances"? To answer this question, we teased out further the extreme events caused by other disturbances and found that 656 days (16%) out of the 4011 extreme days due to other disturbances matched with the non-TC vortices in Olaguera et al (2023). Hence, the category "other disturbances" used in this study also include disturbances that were not classified as LPS.…”

“…Previous studies that examined the impact of LPS on rainfall used a 500 km distance from the station location (e.g., Dang‐Quang et al, 2016; Olaguera et al, 2023). To estimate the optimum distance by which an LPS can induce rainfall over the Philippines, we followed the same methodologies of Kubota and Wang (2009) and Bagtasa (2017) in determining the optimum distance for TC influence.…”

“…Extreme rainfall events, which are associated with devastating floods, are one of the hazards that affect the Philippines. Various factors that contribute to these extreme rainfall events have been explored in recent studies and include, but not limited to, tropical cyclones (TCs), cold surges (CS), shearlines, MJO, BSISO, and non‐TC vortices (Abdillah et al, 2021; Bagtasa, 2017; Cayanan et al, 2011; Chen et al, 2015a, 2015b; Cruz & Narisma, 2016; Olaguera, Matsumoto, et al, 2021; Olaguera, Caballar, et al, 2021; Olaguera, Cruz, et al, 2022; Olaguera, Manalo, et al, 2022; Olaguera, et al, 2023; Olaguera & Matsumoto, 2020; Pullen et al, 2015; Racoma et al, 2022). Aside from the copious amounts of rainfall that each of the above factors may bring, these factors may also interact with the background circulation or with one another, which makes the extreme rainfall events in the Philippines more complex and severe (Olaguera, Cruz, et al, 2022).…”

Climatological Analysis of Rainfall over Hinatuan City, Surigao del Sur in eastern Mindanao—the wettest location in the Philippines

“…We checked whether this vortex has been detected as an LPS, but surprisingly it is not (not shown). Another index for non-TC vortices was used by Olaguera et al (2023) based on Vort and surface winds. We found that this vortex was detected by their index.…”