Why do cyclones occur every Oct-Dec, right after the rainy season?

While the monsoon season ended around the 14th of October in Bangladesh this year, a low-pressure system in the Bay of Bengal developed into cyclonic storm Dana. Though it is expected to make landfall in India, its effects — including strong winds and higher-than-normal tides — were felt along the coastal areas of Bangladesh.

Cyclones during October-December are no new phenomenon. Bangladesh, alongside India, has faced such natural calamities numerous times, with nearly 300 cyclones recorded during the said period since 1891.

The question arises: What's so special about these months?

Abul Kalam Mallik, a meteorologist at Bangladesh Meteorological Department and a long-time researcher of cyclones, provided insights into this phenomenon. 

Analysing cyclone data from 1891 to 2023, he revealed that the Bay of Bengal experienced 274 cyclones during October-December. 

The highest number of Cyclones were recorded to happen during November. 

Mallik said the Bay of Bengal saw the creation of 133 Cyclones in the last 133 years while October recorded 94 and December 55.

A shift towards October

While November has seen more Cyclones in the long run, there has been a noticeable shift in cyclone formation in recent years. 

According to Mallik, October of 1967 witnessed the highest number of cyclones forming in the Bay - three. "Two of these made landfall in Bangladesh, one at Cox's Bazar and the other at Hatia of Sandwip, while the remaining one impacted Sri Lanka."

In recent years, Cyclone Hamoon made landfall in Bangladesh in 2023, impacting the country from 21-25 October. Similarly, Cyclone Sitrang affected the country from 22-25 October in 2022, making landfall at Sandwip. 

This year's Cyclone Dana also formed on 22 October and is expected to persist until 25 October. 

This indicates that cyclones have been forming during the same time period consistently over the past several years.

A post-monsoon effect

Meteorologist Mallik attributed this recurring pattern to the specific conditions prevailing after the monsoon season. 

"The equinox on 23 September marks a shift in sunlight angle, leading to rapid warming of seawater. This elevated sea temperature, combined with increased sunlight and evaporation, creates favourable conditions for cyclone formation in October and November," he said.

The meteorologist explained that for a cyclone to form, the sea water temperature must reach a minimum of 26.5 degrees Celsius, and the water depth should be at least 50 metres. 

He noted that these conditions are readily met in October, making the Bay of Bengal particularly susceptible to cyclone formation.

Mostofa Kamal Palash, a PhD researcher in meteorology and climate at the University of Saskatchewan, elaborated on the importance of sea surface temperature in cyclone intensification. 

"When a cyclone forms in the Bay of Bengal, meteorologists closely examine the region's surface water temperature map. Warmer sea water acts as fuel, accelerating the cyclone's growth and strengthening it as it moves over these areas," Palash told TBS.

He further mentioned that on 22 October, the sea surface temperature in the Bay of Bengal, particularly in the Odisha coast of India, was exceptionally high. 

"This warm water contributed significantly to the intensification of Cyclone Dana, which was projected to impact the coastal areas of Odisha," he added.