The final week of November brought destruction across Sri Lanka, Indonesia and Thailand as Cyclones Ditwah and Senyar released continuous and intense rainfall. Millions of people were affected, more than 1,500 lives were lost, hundreds remain missing and damages reached massive financial levels. Sri Lanka’s president described the event as one of the most severe natural disasters the country has ever experienced.
Weak Winds but Extremely High Rainfall
Although both storms were devastating, neither of them were strong in terms of wind. Cyclone Ditwah recorded peak winds of around 75 kilometres per hour, a level that would be considered only a “gale” in the United Kingdom and far below the strength of historic cyclones. Yet Ditwah caused widespread flooding, landslides and destruction across South Asia.
Scientists believe this mismatch between weak winds and extreme rainfall is connected to climate change. A warmer atmosphere can store more moisture, nearly seven per cent more for every degree of warming. When storms form in this moisture-rich environment, they can release enormous amounts of rain even if wind speeds remain low.
Warmer Oceans Are Intensifying Rainfall
The world’s oceans have absorbed more than ninety per cent of the excess heat caused by greenhouse gas emissions. This long-term warming has raised sea temperatures significantly. Satellite data from the days leading up to Cyclones Ditwah and Senyar showed that parts of the eastern Indian Ocean were more than one degree Celsius warmer than usual.
Cyclones draw their energy from warm ocean water. The unusually warm sea surface increased evaporation and loaded the atmosphere with moisture. This moisture then fell as intense rainfall as the cyclones moved across the region.
Coastal Tracks Made the Storms Even More Destructive
Both cyclones formed close to land and travelled along coastlines, allowing them to stay above warm waters long enough to gather moisture while simultaneously dumping that moisture over nearby regions. Cyclone Ditwah also moved slowly as it neared Sri Lanka, causing repeated rainfall over the same areas and worsening the flooding.
This combination of warm oceans, slow movement and coastal proximity turned otherwise weak cyclones into powerful rainmakers capable of causing widespread devastation.
A Growing Climate Threat
These events indicate that the nature of cyclone risk is shifting. The most dangerous storms may no longer be defined by their wind speeds alone but by the amount of moisture they carry. Forecasting systems can accurately predict cyclone tracks and wind intensity, but rainfall-driven flooding remains far more difficult to forecast.
Preliminary scientific assessments suggest that the trend of weak cyclones producing extreme rainfall may become more common as ocean temperatures continue to rise. More research will follow, but the warning is clear: in a warming world, even weak storms can become deadly, and preparedness must adapt to this new reality.