From CK Nayak
NEW DELHI, Feb 13: The frequency of extreme rainfall events over Meghalaya, Assam, and Bangladesh has quadrupled from 1950 to 2021 as a result of climate change but the India Meteorological Department (IMD) data indicate a decreasing trend in monsoon rain over the past 30 years.
“While the total rainfall in most of these regions is decreasing, our paper shows extreme rains are increasing,” Roxy Mathew Koll, the co-author of a new research paper said.
Extreme rainfall events quadrupled over western Meghalaya (and northeast Bangladesh and India) and coastal southeast Bangladesh from 1950 to 2021, compared with the first 30 years of baseline (1950 to 1980), the paper published in the Journal of Royal Meteorological Society last week said.
The intensity of rainfall is rising, and weather models find it difficult to forecast the extreme rainfall amounts skilfully under a changing climate. These extreme rains are projected to increase further in these regions in the near future, which means that one should be prepared with a long-term vision, the study suggested.
“We should identify the hotspots and flood-proof each district that is prone to extreme rains and landslides,” Koll said.
The CMIP projection indicates that devastating flood-causing extreme rainfall events will become more frequent in the future.
“To understand the role of climate change, we use high-resolution downscaled models from the Coupled Model Intercomparison Project (CMIP). We find that the monsoon extreme event increase is ongoing, and the region of quadrupled events further extends over the Northeast and Bangladesh,” Koll said.
“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 said. Extreme rainfall means more than 15 cm a day.
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).
“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 said.
“Meghalaya gets rain due to strong monsoon southerlies hitting the mountains, which are east-west oriented. An increase in heavy precipitation could be due to the increasing strength of monsoon winds and associated moisture flux convergence,” the study said.
“Warming of the Bay of Bengal also may contribute. This result is obvious given 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.
IMD’s analysis of ‘rainfall variability and changes over different states’ for the period between 1989 and 2018 for Assam, 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.
The analysis was done 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 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.