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The excessive rainfall caused by Cyclone Senyar across Indonesia, Malaysia and Thailand and Cyclone Ditwah over Sri Lanka in the last week of November 2025 was made more intense by global warming along with natural climatic cycles such as El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), according to a study by the World Weather Attribution (WWA) consortium.
The observed rainfall trends from the two storms were not well represented by the available climate models for the region, hence the scientists could not quantify the exact role played by global warming and consequent climate change in making the cyclonic rainfall more intense.
The two storms killed 1,600 people across the four countries with hundreds still missing, millions affected, economic losses (not yet completely accounted and quantified) and unaccountable non-economic losses including erosion of mental health.
The devastation and suffering was also made more severe by the topography of the most affected regions, land use changes such as deforestation and urbanisation, inadequate health infrastructure, socio-economic vulnerabilities of the impacted population including lack of insurance and insufficient early warning, according to the study.
The primary weather factors in the origin of both cyclonic storms were a hyperactive Intertropical Convergence Zone (ITCZ) and accompanying atmospheric waves close to the equator. These helped organise “deep convection close to the equator, enabling slow-moving systems that produced prolonged, intense rainfall over both study regions”, according to the study. The ITCZ is a band of winds, clouds and rainfall along the equator whose movement defines rainfall seasons for tropical and sub-tropical regions.
The scientists found that the extreme rainfall from Cyclone Senyar in the Malacca Strait regions comprising of Indonesia’s Sumatra, parts of Malaysia and southern Thailand between November 23 and November 27 was a ‘once in 70 years’ event in today’s climate, which corresponds to a warming of 1.3°C since the pre-industrial period (1850-1900). Observational data of increase in rainfall from the region showed a wide variation across various datasets between 9 per cent and 50 per cent.
The extreme rainfall in Sri Lanka due to Cyclone Ditwah between November 24 and November 28 was a ‘once in 30 years’ event in a 1.3°C warmer world. The rainfall data from the country across different datasets showed that the increase in rainfall was much stronger and between 28 per cent and 160 per cent.
The increase in rainfall for both regions is expected under global warming based on the Clausius Clapeyron equation which says that for every 1°C rise in global average temperature, the moisture in the atmosphere will increase by 7 per cent. In addition La Niña, which is the cooler than normal phase of the ENSO and a negative IOD also added to the available moisture in the region which added to the intensity of the rainfall. “We estimate that the current La Niña and negative IOD conditions contributed about 5% to 13% to observed rainfall,” wrote the study authors.
For extracting a quantified attribution of the rainfall events over the two regions to global warming and consequent climate change, the researchers studied and analysed various climate models, none of which showed a clear trend.
“Models are not good at simulating the seasonal cycle over these small island regions, and the majority also do not capture the correlation with La Niña or the IOD,” according to the report. “Over both study regions, these models do not show consistent results regarding the direction of change, so there are probably aspects of the atmospheric circulation that are systematically misrepresented by the models,” the report added.
“This is surprising, given the substantial body of scientific literature reporting increases in heavy rainfall both within our study region and across the broader area,” the scientists cautioned.