La Nina behind floods in Bangladesh

La Nina is moderately active in the Pacific Ocean this year. This would cause more rains across the Ganges and Brahmaputra basins ultimately leading to more floods, said Dr Rashed Chowdhury in an interview with The Daily Star.

Adjunct faculty at the School of Sustainable Engineering and the Built Environment in Arizona State University, he said there was a direct link between La Nina being active and floods in Bangladesh.  

Bangladeshi-born Rashed, who had also worked as a principal scientific officer at Pacific Climate centre, said La Nina was active in the Pacific even during earlier floods in Bangladesh like in 1954, 1955 1968, 1974, 1987, 1988, 1998, 2004, 2007, and recent ones in 2017 in 2019.

"It means, if there is active La Nina, there is high probability of flood in Bangladesh."

La Nina is a climate pattern that describes the cooling of surface ocean waters along the tropical west coast of South America.

Saying that predictions are never 100 percent accurate, he said, "The connection between La Nina and floods in the country, however, is very clear.

"The severity is not clear. It could be catastrophic or could be normal. But those who work on atmosphere and oceanic dynamics have been discussing the possibility of floods this year.

"I have written about it for a newspaper in April and have been writing about it on social media ... It is not sudden and I am not at all surprised by it."

About is the La Nina phenomenon, Dr Rashed said it is very hard to explain briefly. 

"We can say it is an interaction between the ocean and the atmosphere. Sometimes, the sea surface temperature of a certain place – what we call Nino 3 or 4 -- in the eastern Pacific Ocean either increases or decreases.

"Now the temperature is colder than normal." 

When it is colder, the monsoon wind becomes very strong and brings in moisture. That is what is causing so much rainfall in the greater Ganges and in the Brahmaputra-Jamuna basin, he said.

Rashed added this intense rainfall will take place in coming months too and all the water will go down to the Bay of Bengal through Bangladesh.

Asked about the current forecasting system in Bangladesh, and whether it is possible to forecast specific weather events earlier, he said countries like the USA and many South American countries have been using seasonal forecasting based on La Nina using oceanic atmosphere dynamics.          

He further said Bangladesh currently just mentions level of water but not the area of inundation.  

"If they want to do it, they have to find out the correlation between depth of water and the area of inundation. Then they would able to say how much area would be inundated if the water level increases to a certain level.  

With seasonal forecasting, which is the modern system, it is possible to forecast an event three months ahead. 

"If we could forecast the flood three months ago, the government could have been far better prepared," he said.