No tropical cyclones formed in the Bay of Bengal during the pre-monsoon season (March-May) in 2015, 2018 and 2025, which was an unusual phenomenon indicating the increasing impacts of climate change on this region.

However, this year, the southwest monsoon reached the Teknaf coast on 24 May, which was earlier than the usual deadline. The arrival of the monsoon this early has happened after 16 years. Previously, in 2009, the monsoon had entered the coastal areas of Bangladesh at almost the same time.

In 2024, the southwest monsoon arrived at the area stretching from Cox’s Bazar to Sylhet on 30 May.  

During the period from March to May, the Bay of Bengal typically sees one or two tropical cyclones, but this year it experienced an unusual calm with no cyclones forming, which is a rare deviation from the region’s usual pattern.

The warm waters of the pre-monsoon season act as fertile ground for storm development, but, this year no cyclone occurred amid the early onset of southwest monsoon, said Dr Mohan Kumar Das, executive director of the National Oceanographic and Maritime Institute (NOAMI), and the joint secretary of the South Asian Meteorological Association (SAMA).

“The southwest monsoon began on 8 June in 2023, on 32 May in 2022, on 6 June in 2021 and on 8 June in 2020, according to the report “Changing Climate of Bangladesh: Trends” and changes detected in weather observations from 1980 to 2023 in Bangladesh.

On 26 July 2015, Cyclonic Storm Komen developed late during the monsoon-forming period and intensified by 29 July – making it a rare intra-monsoon cyclone, said Dr Mohan.

He told the Daily Sun that tropical cyclones do not originate during the southwest monsoon season in the Bay due to the vertical wind shear or a change in wind speed and/or direction with altitude. High vertical wind shear disrupts the formation of cyclones by tilting the vortex and preventing the cyclone from organizing and intensifying.

Moreover, the southwest monsoon creates a stable atmospheric condition which is not conducive for the formation of cyclones.

Cyclones require unstable conditions where warm moist air can rise and condense to form clouds and precipitation, said the meteorologist.

Besides, the Coriolis force, which is essential for the formation of cyclones, is minimal at the equator and increases with latitude. During the southwest monsoon, the winds blow from the southwest towards the equator where the Coriolis force is minimal, thus not favouring the formation of cyclones, he added.

However, in the last decade and a half, cyclones have often been observed in the month of May.

Cyclone ‘Remal’ hit the country on 26 May 2024, ‘Mocha’ on 14 May 2023, ‘Asani’ on 11 May 2020 (formed in the Bay of Bengal but hit the Indian State of ‘Andrha’ Pradesh), ‘Yaas’ on 26 May 2021, Cyclone ‘Amphan’ on 20 May 2020, Cyclone ‘Fani’ on 4 May 2019, Cyclone ‘Mora’ on 30 May 2017 and Cyclone ‘Roanu’ on 21 May 2016.