How Bangladesh's familiar monsoon pattern is shifting over decades

Bangladesh's monsoon pattern is changing noticeably.

Meteorological data and experts point to heavier pre‑monsoon rainfall, a delayed arrival of the southwest monsoon, prolonged dry spells during the rainy season, and increasingly intense bursts of rainfall over shorter periods.

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The changing rainfall pattern is taking a growing toll on agriculture, particularly on farmers and marginalised communities.

Bangladesh Meteorological Department (BMD) data shows that rainfall in March this year was 31 percent above normal, while April recorded 76 percent more than average.

In contrast, June -- despite marking the start of the monsoon -- received nearly 30 percent less rainfall than normal.

By July 15, however, the country had already received around 85 percent of its normal rainfall.

The erratic weather has already caused widespread crop losses.

According to the Department of Agricultural Extension, torrential rain in March destroyed crops on about 2,131 hectares nationwide, including 916 hectares of potato fields in the northern region.

The Ministry of Agriculture said heavy rainfall between April 26 and May 4 caused crop losses worth about Tk 1,047 crore across seven haor districts.

More recently, floods and heavy rain between July 6 and 14 affected 27,011 hectares of cropland belonging to nearly 211,145 farmers in five districts of Chattogram and two districts of Sylhet, ministry data shows.

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The changing rainfall pattern is taking a growing toll on agriculture, particularly on farmers and marginalised communities. Photo: Azahar Uddin

“This year’s monsoon began in a highly unusual manner,” said Dr Bazlur Rashid, a BMD meteorologist.

“Winter rainfall was almost absent. Then the western disturbance remained active throughout March and April, producing much higher rainfall than normal,” he told The Daily Star.

“In June, when monsoon rainfall was expected, several heatwaves swept across the country. We experienced intense heat when it should have been raining, and excessive rainfall during periods that are usually much hotter,” he added.

A BMD report, Changing Climate of Bangladesh, published in April 2024, found significant changes in the country’s rainfall pattern after analysing weather observations from 1980 to 2023.

It says that although Bangladesh’s total seasonal rainfall has remained relatively stable, the distribution has changed markedly, with wet days in Dhaka and Rajshahi declining by about 2.7 days per decade while dry days increased by 2.5 to 2.7 days.

According to Dr Rashid, the southwest monsoon has been arriving two to five days later than its normal onset over the past decade.

“Instead of the traditional four to five days of continuous rainfall at the onset, the monsoon now enters weakly, followed by short‑duration extreme rainfall events,” he said.

He attributed the shift to changes in the movement of depressions over the Bay of Bengal, which increasingly steer moisture towards Chattogram and the hill districts before spreading to other parts of the country.

He added that the current influence of El Niño is contributing to reduced monsoon rainfall overall while increasing the likelihood of short, intense downpours concentrated in specific regions.