Philippine scientists have long-ignored alarms of the indirect impacts of typhoons during the southwest monsoon season. Their research shows how much these systems impact rainfall patterns – even if they’re not making direct landfall. This issue is especially serious for flood-prone regions, such as Metro Manila, Zambales, Ilocos and Palawan.
Tropical cyclones that remain offshore strengthen the Habagat, or our southwest monsoon. They may create their own weather by drawing in a huge amount of moisture from the adjacent oceans. That much moisture can easily turn a beneficial monsoon rain into a dangerous torrential downpour. Studies have indicated that rainfall associated with tropical cyclones accounts for 33.1% of the total rainfall in the southwest monsoon season. Surprisingly, this contribution happens even when these storms never make landfall.
Tropical cyclones that develop well over the open ocean usually track north or northeast of Luzon. This movement greatly increases the likelihood of improving the monsoon. These downstream effects add up to over two-and-half times the negative direct impacts on rainfall. And tropical cyclones serve to supercharge the monsoon season. They even account for as much as 41.5% of all rainfall in this short time span.
In the past, tropical cyclones hundreds of kilometers away from the Philippines were more often associated with heavy rainfall. Hurricanes that make landfalls have the opposite effect on precipitation. In July 2024, Typhoon Gaemi tracked deep offshore. Though it was short-lived, it massively increased the strength of the southwest monsoon. In Quezon City, over 900mm (almost a month’s worth) fell in just 24 hours. These heavy downpours increase the chances of destructive flooding.
The four wettest years on the record books 1962, 1972, 2012 and 2018. Each of these years saw more than 2,000 mm of rainfall during the monsoon season. Adding those ingredients together shows the indirect effects of tropical cyclones to make up the majority of the rainfall’s share. The rest, 51.5%, is owed to the monsoon itself, without any doing of the cyclone.
The dangers posed by these deluges should not be treated lightly. Just this past spring in 2024, disastrous flooding due to cyclone-boosted monsoon rains resulted in catastrophic losses. The disaster claimed 48 lives and caused damages of more than 8 billion pesos. Therefore, the knowledge of these indirect impacts is critical to the people living in flood-affected areas.
