Many Filipinos think that only typhoons directly hitting the country cause floods. However, new research shows a different story. Tropical cyclones hundreds of kilometers away can be more dangerous during the “Habagat” or southwest monsoon season.
The annual Habagat season spans from July to September. During this time, distant tropical cyclones can enhance the monsoon. This enhancement pulls in moisture and causes heavy rainfall.
Indirect Effects Pack a Punch
The study reveals that the “indirect” effects of these cyclones contribute more rainfall than their “direct” hits. On average, direct hits account for only 15.4% of the Habagat rainfall. But the indirect effects contribute over twice that amount. The monsoon itself, without cyclone influence, accounts for the remaining 51.5%.
The research indicates that an average of 33.1% of the rainfall during the southwest monsoon season comes from tropical cyclones that do not make landfall.
These cyclones enhance the Habagat, drawing in moisture from the seas. This turns moderate rains into torrential downpours.
Location Matters
The location where a tropical cyclone forms, its “genesis point,” also plays a role. Cyclones forming farther from the Philippines tend to move northeast of Luzon. These are more likely to enhance the monsoon.
In contrast, those forming closer to the country often take shorter paths westward. Thus, they have a weaker effect on the southwest monsoon.
A Case Study: Typhoon Gaemi (Carina)
A stark example occurred in July 2024. Typhoon Gaemi (locally known as Super Typhoon Carina) stayed away from the Philippines. Yet, it enhanced the southwest monsoon significantly.
Quezon City recorded almost a month’s worth of rain in just 24 hours. Floods across Luzon resulted in 48 deaths and over 8 billion pesos in damage. This happened even though the typhoon did not make landfall.
Decades of Data Analyzed
Researchers from various institutions analyzed 62 years of weather data, from 1961 to 2022. They focused on rainfall patterns along the western coast of the Philippines during the peak monsoon season.
In the four rainiest years on record (1962, 1972, 2012, and 2018), rainfall exceeded 2,000 millimeters. The indirect effects of tropical cyclones contributed the largest share. Up to 41.5% of the total rainfall was attributed to their monsoon-enhancing effects.
Call for Better Monitoring
Researchers emphasize the need to improve monitoring of tropical cyclones and the monsoon. Understanding the distinctions between rainfall types is crucial.
Local governments and disaster response agencies need this information. Climate change increases the unpredictability of both cyclones and seasonal rainfall.
The study highlights the need to monitor cyclone formation and interaction with the monsoon. This knowledge is critical for flood-prone regions like Metro Manila, Zambales, Ilocos, and Palawan.
