How Climate Change Cause Cyclones Unleash Disastrous Rainfall

As 2025 draws to a close, South Asian countries Sri Lanka and Indonesia have been devastated by Tropical Cyclones. Cyclone Senyar hit Indonesia’s Sumatra Island on 26 November, and Cyclone Ditwah struck Sri Lanka on 27 November.

While both cyclones have weak winds, the devastation they’ve caused is similar to that of cyclones with powerful winds, due to the sheer amount of moisture they dumped, which resulted in floods and landslides.

For instance, Ditwah has a peak wind speed of only 75km/h (47 mph) and is weaker than a strong cyclone, which has a wind speed of around 220 km/h. Cyclone Ditwah originated as a low-pressure system near the Sri Lankan coast on 26 November. It rapidly intensified into a cyclonic storm by 27 November.  

While meteorologists have accurately forecasted a heavy rainfall over 200mm, Ditwah’s translational speed, or how fast it is moving across the landscape, is slow, between three and eight kilometres per hour. Its slow speed turned Ditwah into a rain machine, pouring relentless rain that exceeded season averages. Some areas, like Gammaduwa (Matele), recorded 540.6 mm of rain in 24 hours, and Mullaithivu at 161.9mm within three hours.

Ditwah’s catastrophic toll included 1,739,923 affected individuals; this figure represents around 8% of the total population. Of those affected, 239,493 have been internally displaced, 640 confirmed deaths, and 211 people were missing (Sri Lanka, 2025).

The other tropical storm, Senyar, which formed in the Malacca Strait on November 25, has also brought devastation across three provinces on Sumatra Island: Aceh, North Sumatra, and West Sumatra.

According to NASA, it is unlikely that tropical storms would form within the Malacca Strait, a narrow waterway separating Malaysia and Indonesia’s Sumatra Island, because it is close to the equator, where the Coriolis effect is too weak to allow storms to rotate enough to form a cyclone.

However, in late November, it did just that. Not only did it form as a tropical cyclone, but it also sent a deluge and strong winds to Sumatra Island. Estimates from NASA satellite images show that the slow-moving tropical storm Senyar dropped close to 400 millimetres, or 16 inches, of rain, leading to flash floods and landslides.

Streams and rivers rapidly overflowed, carrying debris and sediment that washed over villages, towns, and cities. The disasters have killed nearly 1000, with more than 200 still missing, and displaced half a million people from their homes.   The estimated economic loss and damage from the Senyar was IDR 68.6 trillion, or around US$4.1 billion.

The Conversation “Even ‘weak’ cyclones are being turned into deadly rainmakers by fast-warming oceans” article explores how climate change is intensifying rainfall during cyclones. It notes that the fundamental driver of the heavy rain lies in the well-established principle of atmospheric physics, which states that for every degree of warming, the atmosphere holds an additional 7% of moisture.

So when tropical cyclones form, they tap into the atmosphere that essentially becomes a larger reservoir of moisture, and then release a deluge wherever the storm lands.

The article also notes that the Indian Ocean is rapidly warming. Oceans hold more than 90% of the excess heat trapped by GHGs, and long-term observations show a rising trend in ocean temperatures. During Ditwah and Senyar, the Indian Ocean was 1°C warmer than usual, which contributed to excessive rainfall during these cyclones.

The devastation caused by Ditwah and Senyar makes it clear how climate change can present new risks. Cyclones with high-speed winds are no longer the most dangerous; it’s those that could dump the most rain.

Advanced weather forecasting, early warning systems, and satellite data help predict the impacts of cyclones and storms, allowing people to better prepare and adapt.