Cool summer can impact monsoon onset: Experts

PUNE: A cool summer or pre-monsoon season this year can impact the onset and progress of the southwest rains in India, top weather experts are of the view.
If the landmass of India tends to cool down during summer, as happened this year, the temperature gradient between the land and sea decreases. This can lead to a delay in the onset of the southwest monsoon over the Indian subcontinent, which normally sets over Kerala around June 1.
Dr M Rajeevan, former secretary of ministry of earth sciences, on Friday told TOI, “The pre-monsoon period in India, from March to May, is crucial in setting the stage for the southwest monsoon. The onset and progress of the southwest monsoon are affected by a variety of factors, including the sea surface temperature, land surface temperature and atmospheric conditions. These are a few of the parameters for forecasting monsoon onset and progress over India.”
Dr Rajeevan said a rainy pre-monsoon period in India and associated land cooling during March to May so far this year could delay the onset and progress of the southwest monsoon.
Dr Rajeevan said a delay in the onset and progress of the southwest monsoon might not affect the seasonal monsoon. “Warmer-than normal pre-monsoon temperatures tend to lead to an earlier on set and stronger progress," he said. India experienced an unusual drop in temperature this April, with the country seeing its coolest nights since the last five years, also, coolest and wettest days after 2020. The experts said conditions close to the onset (May) would have a stronger impact on the onset than March-April.
DS Pai, head, Environment Monitoring an d Research Center of India Meteorological Department (New Delhi), told TOI, “The onset and strength of the monsoon in India are determined by a variety of factors, including the temperature contrast between the land and ocean. Typically, the land warms up during the pre-monsoon or summer season, which helps to pull the monsoon towards the land regions.”
He s aid, “If the warming over land is insufficient during this period, it could hinder the process of pulling the monsoon towards the land region. This summer, there is significant moisture over the land. It will take time for the moisture to evaporate off the top soil. The top soil needs to be warm enough to pull the monsoon towards the land regions.”
Pai, however, added that the evolution of the thermal pattern over land surface and Indian Ocean leading to setting of monsoon flow was only one of the factors determining the monsoon onset. “The evolution of wind and convective patterns in the Asia-Pacific region are also important. A cyclone is also soon likely to form over the Bay of Bengal. This can play an important role in the evolution of thermal, dynamic and convective patterns, leading to monsoon onset over the region. The next few days would be crucial to determine the timing of the monsoon onset this year . IMD’s forecast on monsoon onset will be released soon, based on these parameters,” he said.
Dr Roxy Mathew Koll, senior climate expert from Indian Institute of Tropical Meteorology (IITM), Pune, told TOI, “Pre-monsoon temperatures can be one of the factors that can influence the monsoon onset. An El Nino can enhance the tropospheric temperature gradient by increasing the temperatures over Eurasia and thereby weaken the monsoon winds.”