Why has there been unusual rainfall this October?

If the seemingly non-stop rain for the past few days makes you wonder that this phenomenon might be a bit unusual, you would be right. 

The rainy season is nearing its end, yet some regions of the country are logging record rainfalls in over a decade.

The threats of floods amid large swathes of land going underwater have already become commonplace. 

But what is the reason behind this? And was this the last rain of the season?

The Bangladesh Meteorological Department notes three different factors are at work here: additional water vapour and air in the atmosphere; accumulation of large amounts of clouds; and the development of an Indian Ocean Dipole (IOD), also known as 'Indian Nino'.

"The rain that's been falling for the last few days can be defined as the last rain of the wet season," Md Bazlur Rashid, meteorologist at Storm Warning Center, Bangladesh Meteorological Department, told The Business Standard on Friday (6 October). 

"Such rainfall occurs every year in early October," he added.

However, this year, the rainfall has broken decade-long records. 

Rajshahi recorded 244mm of rainfall – the highest in over 10 years – in 24 hours till Thursday (5 October). Besides, Faridpur recorded 129.9mm of rain, which is the highest in the entire year for the district. 

On Friday, the met department recorded an alarming 476mm of rainfall in 24 hours at Nikli of Kishoreganj – the highest in the country. 

In addition, 340mm and 311mm of rainfalls were recorded in Mymensingh and Netrokona respectively. The capital, Dhaka, recorded 80mm of rainfall till 6pm Friday.

Meanwhile, tornadoes hit at least two districts in the last two days, killing at least one and destroying hundreds of homes. 

According to Bazrul, the conditions mentioned above are to blame for this bad weather. 

"Usually, these conditions do not happen at the same time," he said.

The meteorologist said, "Some additional rainfall occurs during the end days of monsoon. But this time, the condition has intensified due to the presence of two low pressures over land."

Bazrul explained that the low pressures combined and pulled a lot of water vapour and air into Bangladesh's atmosphere, which intensified the rainfall.

"Two depressions formed simultaneously in the Bay of Bengal and the Arabian Sea. The depressions quickly moved over land. On Thursday, the depressions passed through Sikkim in India and entered Rajshahi through the northern part of Bangladesh.

"And so there was a record amount of rain in Rajshahi on the day. It got stronger and proceeded towards Sylhet via Kishoreganj-Brahmanbaria via Mymensingh."

Besides, before bidding farewell, the monsoon gains strength and accumulates a large amount of clouds in the atmosphere.

On top of that, a positive Indian Ocean Dipole (IOD) or Indian Nino is impacting the Bay of Bengal.

The IOD is an ocean-atmosphere interaction very similar to the 'El Nino' fluctuations in the Pacific Ocean, playing out, as the name shows, in the Indian Ocean.