Synoptic conditions and potential causes of the extreme heavy rainfall event of January 2009 over Mindanao Island, Philippines

Abstract

This study investigates the synoptic conditions that led to the heavy rainfall/flood (HRF) event in Mindanao Island, Philippines (122 −127°E; 5 −10°N), on January 2009 (JAN2009 HRF) that are less emphasized in previous works. Extensive flooding was reported over Cagayan de Oro City in the northern part of Mindanao, where the rainfall on January 10, 11, and 13, 2009, exceeded the 99th percentile of daily rainfall records of all January of the city from 1979 to 2017 by almost two times. A similar exceedance was also felt in Hinatuan station over the eastern coast of Mindanao Island on January 15, 2009. The interaction of a cold surge shearline over the northern Mindanao Island and the warm tropical easterlies led to enhanced moisture convergence. The warmer air mass is forced to ascend by the advancing colder air mass because it has lower density than the colder air mass. The enhanced moisture convergence and buoyancy difference by the air masses led to enhanced ascent and consequently rainfall along the cold surge shearline. Further analysis shows that enhanced anomalous easterly and northerly winds at 925 hPa are apparent over the Philippines. The anomalous easterly winds sustained the supply of warmer easterlies and collaboratively interacted with the northerly winds that supplied colder temperature air mass.The climatology of this HRF event was examined for all January from 1979 to 2017. The authors identified 15 other cases that are similar to the JAN2009 HRF event and performed lag composite analyses. The results show that the occurrence of these HRF events is facilitated by the southward expansion of the high-pressure system to the north of the Philippines, enhanced cold and warm temperature advections, and enhanced moisture convergence along the cold surge shearline. The results of this study have important implications for disaster mitigation during the northeast monsoon season when rainfall activities are, in general, less intensive over this region.

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

“…(2018), and Olaguera, Caballar et al . (2021) that usually causes heavy rainfall and flooding events in this region.…”

“…The lowest rainfall contribution was found over Luzon Island and to the west of the country, where they only contribute by about 40-50% to the mean daily rainfall. The high rainfall contribution to the northeast of Mindanao region may be related to the westward propagating cold surge vortices, which were found by Chen et al (2012aChen et al ( , 2012bChen et al ( , 2013Chen et al ( , 2015aChen et al ( , 2015b, Ogino et al (2018), andOlaguera, Caballar et al (2021) that usually causes heavy rainfall and flooding events in this region.…”

“…Yoshida et al (2014) showed that the BSISO can modulate the TC genesis frequency over the western North Pacific. During the boreal winter season (i.e., December-April), the Madden-Julian Oscillation (MJO) is the dominant eastward propagating ISO, which can interact with the cold surges and bring heavy rainfall to the east of the Philippines and the rest of Southeast Asia (e.g., Pullen et al, 2015;Lim et al, 2017, Olaguera, Caballar et al, 2021. For example, Pullen et al (2015) examined the unusual winter from November 2007 to March 2008 over the Philippines.…”

“…occur year‐round but are more frequent during the boreal winter season (i.e., November–March). These weaker TCs also cause heavy rainfall and catastrophic damages (Rodolfo et al ., 2018; Olaguera, Matsumoto et al ., 2021; Olaguera, Caballar et al ., 2021). However, they are not well examined compared to stronger TCs.…”

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

“…f. Cold-Surge-MJO (CS-MJO; when there is an enhanced northerly wind (i.e., days with averaged meridional winds exceeding one standard deviation of all NDJFM days from 1979 to 2019 along 15 N, between 122.5 and 130 E; Abdillah et al, 2021;Olaguera, Caballar, et al, 2021) and during strong and active MJO); g. CS-nonMJO (due to cold surge only); and h. Others (such as shearlines, which are formed at the trailing-end, along, or ahead of a cold front; .…”