A climatological analysis of the monsoon break following the summer monsoon onset over Luzon Island, Philippines

Abstract

This study investigates the climatology of the monsoon break following the onset of the summer rainy season over Luzon Island (120−122.5° E, 13−22° N) in the Philippines from 1979−2017. The first post-onset monsoon break is remarkable in stations located over the north and central Luzon Island and occurs climatologically in early June. Composite analysis of the large-scale circulation features during the monsoon break period shows that this break is associated with the westward extension of the western North Pacific Subtropical High (WNPSH), which weakened the monsoon southwesterlies and induced enhanced low-level divergence over Luzon Island. The westward extension of the WNPSH may be facilitated by the phase change of the boreal summer intraseasonal oscillation (BSISO). About 59% (23/39) of the monsoon break cases occurred when suppressed convection, associated with the dry phases of the BSISO, is apparent over the western North Pacific. This suppressed convection favors the westward expansion of the WNPSH. With the occurrence of the monsoon break in early summer, the seasonal march of the early summer monsoon over the Philippines can be divided into three phases: (1) the monsoon onset phase, which occurs between mid to late May under the influence of the westerly/southwesterly low-level winds, (2) the monsoon break phase, when rainfall decreases over Luzon Island in early June, and (3) the monsoon revival phase, when rainfall increases again due to the intrusion of monsoon southwesterlies over the Philippines. This study highlights the complex features of the summer monsoon onset and the impact of the WNPSH on the local climate of the Philippines in early summer.

Influence of boreal summer intraseasonal oscillation on rainfall extremes in the Philippines

“…We also examined the June-July 2007 dry spell over Luzon region and found that the days with low rainfall amounts coincided with the suppressed BSISO phases (Phases 1-4) over the western North Pacific. This result corroborates the results found by Olaguera et al (2021) who demonstrated that the majority of the post onset monsoon break cases over the Luzon region occur during the suppressed phases of the BSISO over the western North Pacific.…”

“…Figure 7a shows the BSISO phase-space diagram from June 1, 2007 to August 31, 2007, while Figure 7b shows the average daily rainfall from the eight PAGASA stations (Laoag, Vigan, Dagupan, Iba, Science Garden Ambulong, Coron, and Baguio stations) located on the western coast of the Luzon region. These stations are similar to those used by Cruz et al (2013) and Olaguera et al (2021) in characterizing the southwest monsoon season of the Philippines. From June 1-24, the active convection of the BSISO is located over the Bay of Bengal and convection is suppressed over the western North Pacific.…”

“…They suggested that this is due to the westward extension of the WNPSH that suppressed convective activities and TCs over the Luzon region and non-northward migration of the Inter Tropical Convergence Zone. Olaguera et al (2021) examined the monsoon breaks following the southwest monsoon onset over Luzon region from 1979 to 2017 and found that most of their identified breaks coincided with the suppressed phase of the BSISO (Phases 1-4). Motivated by these findings we therefore examined the possible role of the BSISO in inducing the dry weather condition during the southwest monsoon season of 2007.…”

“…Only few studies have examined the impact of the BSISO on the rainfall of the Philippines. Olaguera et al (2021) examined the impact of the BSISO on the monsoon break following the southwest monsoon onset over Luzon Island. They found that about 59% of the total monsoon break cases in Luzon from 1979 to 2018 occurred concurrently with the suppressed phases (Phases 1-4) of the BSISO.…”

Diurnal to Intraseasonal Precipitation Variation Around the Northwestern Coast of Luzon Island: Results From YMC‐BSM 2018 Field Campaign

“…The western part of the Philippines and coastal waters to its west are a region in the Asian monsoon domain with heavy precipitation in boreal summer (Matsumoto et al., 2020; Shige et al., 2017; Xie et al., 2006), primarily caused by horizontal convergence and uplift of lower‐tropospheric southwesterly monsoon winds due to topography. Precipitation variability in this region is caused by various atmospheric disturbances, such as typhoons (Bagtasa, 2017, 2019; Cayanan et al., 2011), quasi‐biweekly oscillations (QBWOs; Natoli & Maloney, 2021), and boreal summer intraseasonal oscillations (BSISOs; Bagtasa, 2020; Ho et al., 2008; Natoli & Maloney, 2019; Olaguera et al., 2021). Furthermore, the diurnal cycle is conspicuous (Hilario et al., 2021; Natoli & Maloney, 2019), as in other tropical coastal areas (Kikuchi & Wang, 2008; Yang & Slingo, 2001).…”