KOLKATA: Even though monsoon is about set in across north Bengal by Saturday, conditions are yet to turn favourable for the arrival of rains in Gangetic Bengal and Kolkata. Unless a system forms over Bay of Bengal and moisture content in the air builds up further, monsoon currents were unlikely to move into south Bengal, said the Met office.
The normal date for the onset of monsoon in south Bengal is June 11. While monsoon arrived two days earlier than normal in Kerala this time, it won’t necessarily hasten the onset in other parts of the country, including Bengal, said weathermen.
“The onset of monsoon onset depends on several factors, including the existence of systems like a cyclonic circulation over Bay of Bengal, the volume of rain it triggers and the volume of moisture. Moisture depth needs to be substantial, extending up to a height of at least 3-4 km. Also, there should be enough clouds over the sea that travel to land,” said Regional Meteorological Centre (RMC) director GK Das. “These conditions are yet to form,” he added.
North Bengal receives monsoon earlier than the southern part of the state for two reasons: clouds moving in from Bangladesh and the presence of mountains. “Clouds from Bangladesh trigger rain reasonably quickly since the clouds hit the mountains leading to precipitation. In the south, this can’t happen so we need more clouds and favourable conditions like the presence of adequate moisture,” said Das.
Weather systems over Bay of Bengal, especially cyclones, help to hasten the movement of monsoon currents over Arabian Sea, triggering the arrival of monsoon in mainland India. A powerful cyclone over Bay of Bengal is always trailed by southwesterly wind which gets pulled by the system. Once the system passes away, the wind helps the movement of the monsoon currents over Arabian Sea.
“This leads to an early onset of monsoon in Kerala, which seems to have happened this time,” said a weat- her scientist.
Other factors like wind pattern, formation of local clouds and their movement and rain, too, play a role, said weather experts. This explains why even a powerful cyclone like Amphan that hit the Bengal coast on May 20, 2020, failed to hasten the onset of monsoon in Kerala, which received the rains on June 1, the scheduled date, experts said. Gangetic Bengal and Kolkata received monsoon on June 12 that year, a day after schedule.
According to IMD, monsoon currents could reach south Bengal on June 10. The currents, however, will reach Bangladesh and northeast India on June 5.
