Climate change quadrupled rainfall in parts of Bangladesh, northeast India, new research shows

Extreme rainfall events (over 15 cm a day) have quadrupled in frequency over Bangladesh, India's Meghalaya plateau (Garo, Khasi and Jaintia hills) and surrounding areas (Assam) from 1950 to 2021 as a result of climate change, a new research paper has said.

Meghalaya has been a high rainfall region, with Cherrapunji and Mawsynram among the wettest places in India and globally, but the Indian Meteorological Department's (IMD) data indicates that monsoon rain over the northeastern region, including Meghalaya, is seeing a decreasing trend in the past 30 years.

"While the total rainfall in most of these regions is decreasing, our paper shows extreme rains are increasing," said Roxy Mathew Koll, co-author of the research paper.

Extreme rainfall events quadrupled over western Meghalaya (and northeast Bangladesh and India) and coastal southeast Bangladesh from May to October, from 1950 to 2021, compared with the first 30 years of baseline (1950 to 1980), according to the paper published in the Journal of Royal Meteorological Society last week.

Analysis by scientists from NASA Goddard Space Flight Center; Department of Marine, Earth and Atmospheric Science, North Carolina State University; Centre for Climate Change Research, Indian Institute of Tropical Meteorology, among others, found that warm Bay of Bengal sea-surface temperatures (SST) intensify the lower tropospheric moisture transport and flux through monsoon flow to inland areas where mountain-forced moisture converges and precipitates as rainfall during extreme events.

These events exacerbate flash floods and landslides.

"Bangladesh and Northeast India are geographically prone to flooding, and the dense population makes this region more vulnerable. Flood-causing extreme monsoon rains are on the rise over this region. The intensity of rainfall is rising, and weather models find it difficult to forecast the extreme rainfall amounts skillfully under a changing climate. These extreme rains are projected to increase further in these regions in the near future, which means that we should be prepared with a long-term vision. We should identify the hotspots and flood-proof each district that is prone to extreme rains and landslides," said Koll.

"To understand the role of climate change, we use high-resolution downscaled models from Coupled Model Intercomparison Project phase 6 [CMIP]. We find that the monsoon extreme event increase is ongoing, and the region of quadrupled events further extends over northeast Bangladesh and India and Southeast Bangladesh in future [2050–2079] compared with historical simulations [1950–1979]. A quadrupling of the intense daily moisture transport episodes due to increased monsoon flow instability, a northward shift of LLJ/monsoon flow, and increased moisture contribute to the increased future extreme events," the paper states.

The CMIP6 projection indicates that more devastating flood-causing extreme rainfall events will become more frequent in the future.

The team compared their findings with a couple of other datasets to find similar trends. On 17 June 2022, Cherapunji in Meghalaya Plateau received the third highest one-day rainfall (972 mm), whereas the highest ever recorded one-day rainfall was in June 1995 (1,563.3 mm).

In 2022, there were two consecutive extreme rainfall and associated flooding events over the NEBI [northeast region of Bangladesh (Sylhet division) and India (Meghalaya and Assam states)] from May to June.

The intense rainfall events caused the Surma–Kushiyara river basin to overflow, inundating most of the nine districts in the northeastern part of Bangladesh.

The most severe and damaged conditions were in Sylhet (84% submerged area) and Sunamganj (94% submerged area). During the May–June 2022 period, more than 80 people died and 244,060 ha of crops, 40,000 tube wells, and about $28.1 million worth of livestock were damaged due to flash floods, the paper states.

"Given that Bangladesh and India have the highest population densities in the world, and natural disasters like flash floods and landslides are very common due to frequent extreme rainfall events, it is crucial to improve the local extreme event warning systems. This article potentially serves the purpose of discussing what generally drives the extreme rainfall events over this region," it adds.

"Meghalaya gets rain due to strong monsoon southerlies hits the mountains, which is east-west oriented. An increase in heavy precipitation could be due to the increasing strength of monsoon winds and associated moisture flux convergence. Warming of the Bay of Bengal also may contribute. This result is obvious in view of other regions experiencing increasing trends in heavy precipitation. But I am worried about the quality of rainfall data available from the region," said M Rajeevan, former secretary of the Indian Ministry of Earth Sciences.

HT reported on 27 June 2022 on IMD's analysis of 'rainfall variability and changes over different states' for the period between 1989 and 2018 for Assam, which suggests that monthly rainfall for all monsoon months between June and September is recording a decreasing trend.

This pattern indicates that localised and episodic extreme rainfall events are increasing over these regions, which trigger disasters, but monsoon patterns have undergone a significant change in recent decades.