‘Moisture conveyor belt’ fuels extreme habagat rains, study finds
MANILA, Philippines — The Philippines’ most extreme southwest monsoon, or habagat, rains are more likely when a tropical cyclone northeast of the country combines with strengthened monsoon winds and a huge stream of moisture stretching thousands of kilometers from the Indian Ocean to the western Pacific, a new study found.
Researchers from the Ateneo de Manila University VORTEX Research Lab, Manila Observatory, Philippine Atmospheric, Geophysical and Astronomical Services Administration (Pagasa) and Tokyo Metropolitan University identified a recurring weather pattern behind some of the country’s worst habagat rainfall events.
The study, titled Synoptic conditions favoring tropical cyclone-enhanced southwest monsoon high precipitation events and published in Atmospheric Research, Volume 342, found that these extreme rainfall episodes are not caused by a tropical cyclone or the southwest monsoon alone.
Instead, several weather systems can work together across a vast area, allowing large amounts of water vapor to be carried toward the Philippines.
The study’s authors — Alwin Andriel L. Bathan, Lyndon Mark P. Olaguera, Faye Abigail T. Cruz, Jose Ramon T. Villarin, John A. Manalo and Jun Matsumoto — called this long, moisture-filled corridor a “moisture conveyor belt,” or MCB.
It can extend from the tropical northern Indian Ocean across Southeast Asia and the South China Sea toward a tropical cyclone in the western North Pacific.
“The MCB is a long band of enhanced moisture fluxes, traversing the country,” the researchers said.
A resident swims through deep floodwaters at the Archdiocesan Shrine of Our Lady of Lourdes in Cabetican, Bacolor, Pampanga, on Monday, August 10. (Grig C. Montegrande, Philippine Daily Inquirer)
The findings come from an analysis of 43 years of rainfall and atmospheric data covering July to September from 1981 to 2023. The researchers identified 382 days of unusually heavy rain linked to a tropical cyclone-enhanced southwest monsoon and grouped them into 53 separate extreme rainfall events, each lasting an average of 5.5 days.
Across those events, the researchers found a recurring sequence: Stronger monsoon winds develop over the northern Indian Ocean and carry more moisture eastward across Southeast Asia. As conditions become favorable for a tropical cyclone to develop, the tropical cyclone moves northeast of the Philippines, where its winds combine with the strengthened monsoon and help create the long moisture pathway toward the country.
When the moisture-laden air reaches Luzon, the island’s mountains force it upward, helping produce intense rainfall along the western side of the country.
Graphics by Ed Lustan/Inquirer.net
The researchers also found that other large-scale climate patterns and weather systems can strengthen this setup, including the Boreal Summer Intraseasonal Oscillation, or BSISO; El Niño-Southern Oscillation, or ENSO; and additional weather systems near the Vietnam-China border.
Together, these interacting conditions help explain why some habagat events can bring several days of exceptionally heavy rain even when the tropical cyclone itself remains far from the Philippines.
The southwest monsoon, locally called habagat, consists of moisture-laden winds blowing from the southwest toward the Philippines. It normally brings rain to the western side of the country, with August considered the peak month of the monsoon season.
But when a tropical cyclone forms northeast of the Philippines, its winds can strengthen the existing monsoon flow.
The study found that this interaction can set up the moisture conveyor belt, allowing moisture from much farther west to be transported toward the country.
The buildup, the scientists noted, does not happen overnight. They found that the first signs can appear about 12 days before the heaviest rainfall.
Around 12 days before an extreme rainfall event, stronger winds carrying additional moisture begin developing over the Arabian Sea. By about nine days before the event, these winds extend across the Bay of Bengal.
About six days before the peak rainfall, the stronger winds begin reaching the western North Pacific. Three days before the peak, a developing tropical cyclone can be seen east of the Philippines while the moisture flow from the Indian Ocean and South China Sea begins connecting with the tropical cyclone.
By the peak of the event, the moisture conveyor belt is fully established.
“By Lag 0, the TC is fully developed to the northeast of the Philippines. Large regions of the Philippines are covered in extremely high moisture flux anomalies, producing heavy rainfall over the country,” the study said.
In simpler terms, the researchers found a chain reaction: Stronger monsoon winds help move moisture eastward, a tropical cyclone develops and strengthens the circulation, and the combined winds create a long pathway that funnels moisture toward the Philippines.
Where tropical cyclones were during habagat-enhanced heavy rain events, 1981–2023. Blue lines show the paths of tropical cyclones during these events, while blue circles mark where the cyclone was on the day of heaviest rainfall. The yellow “×” shows the average cyclone location on these peak rainfall days. The black box marks the study area. (Courtesy of A.A.L. Bathan et al.)
The moisture carried by this pathway is concentrated mostly in the lower part of the atmosphere, below about 700 hectopascals (hPa), a measure of air pressure used by meteorologists to describe different levels of the atmosphere.
When these moisture-rich winds reach Luzon, the island’s mountains force the air upward. That upward movement helps clouds grow and intensify, producing heavy rain, particularly along the western coast.
“These low-level winds, which carry most of the moisture within the MCB, are responsible for the intense precipitation over the western coast of the Philippines when orographically lifted,” the researchers said.
“Orographically lifted” refers to a process described by the U.S. National Oceanic and Atmospheric Administration (NOAA) in which air is forced to rise as it moves over terrain such as hills or mountains.
As the air ascends, it cools, allowing water vapor to condense into clouds. With further cooling, this can result in rain or snow, often producing widespread cloud cover and enhanced precipitation over elevated areas.
A key finding is that the tropical cyclone associated with these extreme habagat events does not necessarily have to make landfall in the Philippines.
Most of the cyclones in the 53 events were located northeast of the country, and only five involved a tropical cyclone that eventually made landfall in the Philippines.
Graphics by Ed Lustan/Inquirer.net
The researchers deliberately excluded rainfall directly produced by a tropical cyclone’s own rainbands. Instead, they focused on cases in which the cyclone helped strengthen the southwest monsoon and, in turn, contributed to heavy rainfall over the country.
The study defined a tropical cyclone-enhanced habagat event based on the cyclone’s distance from the rainfall stations. The cyclone had to be at least 500 kilometers from any of the stations used in the analysis, meaning the event could occur either before the tropical cyclone approached the Philippines or after it had already moved away.
“[Tropical cyclone]-enhanced [southwest monsoon heavy rainfall events] do not necessarily imply that the associated [tropical cyclone] did not make landfall over the Philippines,” the scientists said.
“A [cyclone] may still produce a [tropical cyclone]-enhanced [southwest monsoon heavy rainfall events] as long as it occurred when the [cyclone] was at least 500 km away from any station, which could be either before approaching the country or after exiting it,” they added.
In other words, the study focused on the cyclone’s broader influence on the monsoon rather than rainfall falling directly from the cyclone itself.
GRAPHICS: Changes in moisture flow before and during the peak of heavy rainfall events. The maps show how much moisture was moving through the atmosphere, and in which direction, from 12 days before the peak rainfall day through the peak day. The numbers in parentheses show how many events were included in each map. Hatched areas indicate changes that are statistically significant at the 95% confidence level. (Courtesy of A.A.L. Bathan et al.)
The tropical cyclone’s distance from the country did not mean it had no effect. As it developed and moved northeast of the Philippines, its circulation could alter the direction and strength of the surrounding winds, helping draw more moisture toward the country.
The researchers also found that a large area of high pressure over the western North Pacific, known as the western North Pacific subtropical high, helped influence the tropical cyclone’s path.
The position of this high-pressure system favored a more northwestward track for the cyclone, keeping it northeast of the Philippines rather than sending it directly westward toward the country.
That track, the study noted, was important because it allowed the tropical cyclone to remain offshore while its winds helped strengthen the southwest monsoon.
The study described how the stronger monsoon winds extending eastward across the region eventually helped create conditions for a tropical cyclone to form and develop.
“These westerlies eventually reach the western North Pacific and, along with an increase in the mid-level moisture and low vertical wind shear, lead to the formation and development of a tropical cyclone,” the researchers said.
“Mid-level moisture” refers to water vapor higher in the atmosphere, while “vertical wind shear” describes a change in wind speed or direction at different heights. Low vertical wind shear generally makes it easier for a tropical cyclone to organize and strengthen.
The researchers added that the position of the western North Pacific subtropical high then helped determine the tropical cyclone’s path.
“The position of the western North Pacific subtropical high causes the tropical cyclone to move more northwestward, rather than west-northwestward, which allows the tropical cyclone to track northeast of the Philippines instead of landfalling directly,” the study said.
The study found that more than half of the 53 extreme rainfall events — 30 — involved more than one weather system. The additional system, however, was not always a full tropical cyclone. In some cases, it was a weaker tropical disturbance or the remnant of an earlier cyclone.
“It is worth noting that the additional systems in the multiple-system heavy precipitation events are not necessarily tropical cyclones since some additional systems have a maximum sustained wind speed of less than 63 km/h during the peak day of the heavy precipitation events,” the scientists wrote.
“Nevertheless, these systems are still of importance since Bagtasa (2023) showed that the remnant lows of the tropical cyclones before the main tropical cyclones during the 2012 and 2013 enhanced southwest monsoon events played a big role in the formation of the moisture conveyor belt and the presence of extreme rainfall over the western coast of the Philippines,” they added.
Graphics by Ed Lustan/Inquirer.net
In simpler terms, a weather system does not have to be strong enough to qualify as a tropical cyclone to affect the rainfall setup. Even the remnants of an earlier cyclone can help strengthen the flow of moisture toward the Philippines.
The researchers also found that the location of the additional system mattered.
“The location of the additional systems during the multiple-system heavy precipitation events is either to the east or to the west of the main tropical cyclones. The weaker systems to the east of the main tropical cyclones are spread apart across the Pacific Ocean; however, the weaker systems to the west of the main tropical cyclones feature a much more compact spatial distribution,” they said.
The researchers found that systems west of the main tropical cyclone were particularly important when they were near the Vietnam-China border.
There, an additional system could generate strong westerly winds across the Indochina Peninsula. Those winds helped carry moisture from the Indian Ocean toward the South China Sea, strengthening the moisture pathway that eventually fed into the main cyclone.
The study described the Indochina Peninsula as a “bridge” between the two bodies of water.
The stronger moisture flow was associated with more rainfall. Single-system events produced an average of 73.8 millimeters of rain on their peak day, compared with 82.8 mm for events involving multiple systems — about 12.2% higher.
Still, the scientists stressed that having an additional system was not required for an extreme habagat event.
“Although the presence of an additional system is not strictly a condition, since HPEs can still occur with a single system present, it generally increases both the likelihood of a heavy precipitation event and the rainfall associated with it, particularly when the additional system is located near the Vietnam-China border,” they said.
Graphics by Ed Lustan/Inquirer.net
The role of additional weather systems was also evident in two of the country’s most extreme enhanced-habagat rainfall events in recent decades.
During the August 2012 event, Science Garden in Quezon City recorded 1,007.4 mm of rain from Aug. 6 to 10. In August 2013, Sangley Point in Cavite recorded 1,067.4 mm from Aug. 18 to 22.
Previous research linked the extraordinary rainfall during both events to a moisture conveyor belt crossing the Philippines.
The new study examined these events alongside a much larger set of cases to identify the atmospheric conditions that repeatedly appeared before extreme rainfall, including how additional weather systems could strengthen the moisture flow toward the country. /dm