Stating that there exists a possibility that the 'La Nina' effect will peak in the winter, the India Meteorological Department (IMD) said that there can be some "below normal temperatures" but that too depends upon a lot of other factors.
La Nina is a phenomenon, wherein the sea surface temperatures along the equatorial Pacific Ocean are mostly below normal. It is the exact opposite of El Nino, when trade winds weaken, and warm water is pushed back east.
The World Meteorological Organisation (WMO) says that the El Nino/La Nina Southern Oscillation (ENSO) has a major influence on climate patterns in various parts of the world and impacts the year-to-year variability of the monsoon over South Asia.
This naturally-occurring phenomenon involves fluctuating ocean temperatures in the central and eastern equatorial Pacific, coupled with changes in the atmosphere. The current La Nina conditions had originally been around since 2020 and weakened earlier this year.
The WMO, which monitors the developments closely, said sea surface temperatures in the eastern-central Pacific are predicted to be below-average to average during September-November 2021, in the range of minus 1 to 0 degrees Celsius. For October-December 2021, they are predicted to range from minus 1.1 to 0.1 degrees Celsius deviation from the average.
"Currently, La Nina has just reached the threshold, at minus 0.8, this has to sustain. When SST is minus 0.5 and lower, then only we can call that La Nina conditions," IMD's climate research and services centre, Pune, chief D.S. Pai said.
"As per the model forecasts, there exists a possibility of La Nina that will peak in the winter and will continue up to the spring season of next year. However, the La Nina intensity is not expected to be strong as it has started its journey only now," he said.
"We can expect some below normal temperatures. But that is not the only factor. We also need to factor in extra-tropical mid-latitude activity. All depends on what the models tell."
Earlier in the year, while issuing its 'Long Range Forecast for South-West Monsoon 2021' on June 1, the IMD had said: "La Nina conditions, which peaked in November last year, started weakening in the early part of 2021 and turned into neutral ENSO conditions by end of April 2021."
The South Asian Climate Outlook Forum session had on June 10 updated its 'Consensus Statement' on the seasonal forecast over South Asia for the 2021 Southwest Monsoon Season (June-September) wherein commenting on the ENSO Conditions over the Pacific Ocean, it had echoed IMD's announcement: "La Nina conditions (colder than normal SSTs over the equatorial Pacific) were developed during the second part of the previous year, which peaked in November. However, La Nina conditions over the equatorial Pacific started weakening in the early part of 2021 and currently ENSO neutral conditions are prevailing over the equatorial Pacific Ocean."
For the IMD, winter is only in December, January and February months, not November, except for some northernmost parts, where there would be snowfall and cold air will start.
"Northeast monsoon will start moving southwards when the mid-latitude regime will come down. As that happens, cold air will come southwards. As soon as the monsoon leaves, the mid-latitude regime will take over. When that happens, only then winter sets in, which is generally in December," Pai said.
Just as during monsoon, low pressure yields more rainfall; in winters, if there is Western Disturbance, it will bring more cooler air from the north. And then, there are Arctic Oscillations that can lead to colder temperatures in some areas that can, in turn, have an influence on north India.