A new study challenges the common belief that only tropical cyclones making direct landfall cause severe flooding in the Philippines during the annual “Habagat” or southwest monsoon season.
The study led by Ateneo de Manila University researchers indicates that cyclones hundreds of kilometers away are often more responsible for heavy rainfall than those that directly hit the country.
Researchers from Ateneo de Manila University, the Manila Observatory, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) and Japanese partner institutions analyzed 62 years of weather data from 1961 to 2022.
They found that the “indirect” effect of tropical cyclones accounts for an average of 33.1 percent of rainfall during the July-to-September Habagat season. This is more than double the 15.4 percent from cyclones making direct landfall. The remaining 51.5 percent comes from the monsoon itself, without tropical cyclone influence.
Tropical cyclones that do not make landfall can significantly enhance the Habagat by drawing in large amounts of moisture from surrounding seas, turning moderate monsoon rains into torrential downpours. The study found that cyclones forming farther from the Philippines, particularly northeast of Luzon, are more likely to intensify the monsoon.
A stark example occurred in July 2024 with Typhoon Gaemi (locally known as Super Typhoon Carina). Despite remaining far from the Philippine landmass, it enhanced the southwest monsoon, leading to Quezon City recording nearly a full month’s worth of rain in just 24 hours. The resulting floods across Luzon killed 48 people and caused over 8 billion pesos in damage.
In the four rainiest years on record (1962, 1972, 2012, and 2018), rainfall totals exceeded 2,000 millimeters during the monsoon season. The largest share of this rainfall, up to 41.5%, was attributed to the monsoon-enhancing effects of non-landfalling tropical cyclones.
Researchers hope these findings will improve extreme weather anticipation. Understanding these distinctions is crucial for local governments and disaster response agencies, especially as climate change increases the unpredictability of weather patterns, they said.
The study cites the need to monitor not just a tropical cyclone’s approach but also its formation and interaction with the monsoon system, vital information for flood-prone regions like Metro Manila, Zambales, Ilocos and Palawan.
