KOLKATA: Even as winter is set to bow out, the city is heading for yet another spell of cloud and drizzles triggered by a western disturbance (WD) — the fourth since December to have an impact on the region — on February 3 and 4.
It is the La Nina effect that has led to frequent WDs this season and led to the chain of events that eventually led to a dip in temperature repeatedly during the last two months, said weathermen. Kolkata received 13 mm rain in January, making it the second wettest beginning to the year since at least 2017.
The WD will trigger light to moderate rain on Thursday and Friday and pull the mercury up to around 14-15 degrees. The minimum temperature is unlikely to slide below 14 degrees subsequently, said the Met office. Kolkata and south Bengal are set to receive light rain on Thursday and moderate rain on Friday.
“The La Nina effect has been at work this winter. It is triggered by a drop in surface temperature in a portion of the Pacific Ocean that leads to wind and generates weather systems across Europe and central Asia. Some of these sail into the Indian sub-continent and leads to clouds and rain. It sets off a chain of events that culminate in the clouds receding, paving the way for a return of the chilly northwesterly and a temperature drop,” explained Regional Meteorological Centre (RMC) director GK Das.
In winter months, La Nina causes a cold wave-like jet stream of wind that flows across Afghanistan, Iran and the Hindu Kush mountains. These strong and cold winds flow into India and help to pull the mercury down. An active La Nina also helps monsoon rain across India.
The frequent WDs this winter were interspersed by the chilly spells that often lasted nearly a week and saw minimum temperature tumble below 12 degrees thrice. On December 20, Kolkata recorded a minimum temperature of 11.2 degrees Celsius, the lowest of this season. On December 21, the minimum temperature had been 11.6 degrees while last Sunday, the minimum temperature clocked 11.8 degrees.
While in January 2021 Kolkata hadn’t received any rain, the city received 42.5 mm rain in January 2020. “On that occasion, too, it was the WDs and the La Nina effect that had been the reason. In south Bengal and Kolkata, the cool spells are punctuated by clouds and light rain, that, in fact, help to pull the mercury down. So, a slowing down of the La Nina effect will break the chain of events and prevent a temperature drop,” added Das.
A cyclonic circulation is expected to form across Bihar, Jharkhand and Bengal by Wednesday. This could be the final WD of the season and could cause moderate rain on February 4. Light drizzles are predicted on Wednesday. “Since we have reached the end of January, the mercury may not drop sharply again,” said Das.
