1405. Extreme Rainfall Caused by Global Warming Threatens Densely Populated Areas in Asia

On November 27, Cyclone Ditwah struck Sri Lanka with strong winds and extremely heavy rainfall, causing the worst flooding and landslides since the early 2000s. Meanwhile, Indonesia, Malaysia, and southern Thailand were hit by continuous heavy rains caused by Cyclone Senyar, which made landfall in Indonesia and Malaysia on November 26 and 27. This heavy rainfall resulted in significant loss of life, forced the evacuation of hundreds of thousands of families, and crippled transportation and energy infrastructure.

Sri Lanka's flood risk stems from its steep central highlands and low-lying coastal plains. Heavy rainfall rapidly transported runoff from the hills into the floodplains of densely populated areas. The Strait of Malacca region is formed by numerous volcanic islands, vast plains, and deltas. Many islands have limited natural drainage channels and deep valleys, making heavy rainfall prone to both flash floods and landslides.

The impact of climate change on tropical cyclones is complex. In addition to climate change, extreme rainfall in this region is known to be influenced by the El Niño (ENSO) and Indian Ocean Dipole (IOD) phenomena. In the Strait of Malacca, the current La Niña and negative IOD conditions were estimated to have contributed approximately 5% to 13% of the observed rainfall.

World Weather Attribution (WWA), which analyzes the causal relationship between extreme events and climate change, analyzed the five-day periods with the highest rainfall in two regions to assess the extent to which anthropogenic climate change has affected the probability and intensity of heavy rainfall events in this region.

The extreme rainfall associated with Cyclone Senyar over the Strait of Malacca was equivalent to a 1-in-70-year occurrence in today's climate, while the extreme rainfall associated with Cyclone Ditwa over Sri Lanka was equivalent to a 1-in-30-year occurrence in today's climate. Regarding the role of climate change, different observational datasets show a wide range of trends, but the trend direction is consistent, suggesting that extreme rainfall periods are becoming more intense in both study regions. In the Strait of Malacca region, the increase in extreme rainfall associated with rising global mean surface temperatures is estimated to be approximately 9% to 50%. This trend is even more pronounced in Sri Lanka, where five-day heavy rainfall events like those associated with Cyclone Ditwa are now approximately 28% to 160% more intense due to historical warming.

To determine whether the observed trends are attributable to anthropogenic climate change, we also evaluated high-resolution climate models, which are known to best capture rainfall patterns in the study region. However, these models are not well suited to simulating seasonal cycles in these small island regions, and most do not capture correlations with La Niña and the Indian Ocean Dipole. Therefore, without a more detailed assessment of the underlying processes and their representation in climate models, we cannot draw comprehensive conclusions quantifying the impact of climate change on these events.

Meanwhile, sea surface temperatures (SSTs) in the northern Indian Ocean were observed to be approximately 0.2°C higher than the 1991-2020 average, which is likely to have increased the energy available for tropical cyclone development and evaporation, leading to heavy rainfall. Without the trends associated with a 1.3°C increase in global warming, SSTs would have been approximately 1°C lower, below the 1991-2020 normal.

Rapid urbanization, the concentration of population and assets in low-lying floodplains and deltas, and infrastructure built in or near frequently flooded corridors have increased exposure to flood events. Cascading disruptions to transportation, energy, communications, and basic services have disproportionately affected low-income and socially vulnerable groups. While early warnings were issued in both Sri Lanka and Indonesia, ICT infrastructure failures prevented the warnings from reaching their intended recipients, and even those who received the warnings were often unable to predict the scale of the floods.

Contributor: IIYAMA Miyuki, Information Program