In an age of new normals, the Indian Meteorological Department (IMD) has changed the dates of normal onset of monsoon across several parts of India, mainly in central and northwest India.
While announcing that the southwest monsoon will be "normal" (within 96 to 104 per cent of the Long Period Average or LPA) this year, M. Rajeevan, earth sciences secretary, said a detailed study of the monsoon behaviour between 1961 and 2019 was carried out by the IMD, specially its Pune division, based on which the department has announced new normals as regards the onset date across the country.
While the date of onset over Kerala remains June 1, its normal advance dates over Maharashtra, Gujarat, Madhya Pradesh, Telangana, Andhra Pradesh, Chhattisgarh, Odisha, Jharkhand, Bihar and parts of Uttar Pradesh have got delayed by three to seven days. On the other hand, the monsoon will now arrive almost a week earlier on July 8 over extreme northwest India, as against the old normal of July 15.
Similarly, new withdrawal dates have been announced, after studying data between 1971 and 2019. The changes are rather more appreciable, especially over northwest and central India, with a week or fortnight later than the old normals. The withdrawal of the southwest monsoon from south India remains the same--October 15.
In some cities, these normals are going to be rather more significant. Agra, for instance, has just got its monsoon shortened with a delayed onset at June 30 (previously June 23) and an earlier withdrawal on September 14 (previously September 22). The Mumbai monsoon, on the other hand, has got a slight extension, starting a day later than previously on June 11, by stretching nine days longer, with a withdrawal on October 8, as against the previous September 29.
Delhi's pattern is not very different from the new normal, with a four-day late onset on June 27 as against the earlier June 23 and a three-day late withdrawal on September 25 than the previous years. Ahmedabad's entire monsoon diary has shifted by a week with onset on June 21 (previous June 14 and withdrawal on September 30 (previously September 22).
In northeast India, the monsoon season has shrunk marginally, with a two- to five-day delayed start, though not much change in withdrawal dates.
Monsoon studies in India is a serious business, not just for its science, but also for its economic impact on the country, that largely depends on monsoon precipitation to quench its thirst and nurture its crops. Monsoon forecast by the IMD these days is based on two models--its in-house statistical ensemble one, and the dynamical model launched under the National Monsoon Mission.
Monsoon forecasting is based on studying several global phenomena, most important being the sea surface temperature (SST) gradient between the North Atlantic and North Pacific between December and January, the SST in the equatorial south Indian Ocean in February, East Asia mean sea-level pressure during February and March, northern Europe Land Surface Air Temperature during January and Equatorial PAcific warm water volume during February and March.
Rajeevan said that both the El Nino Southern Oscillation and Indian Ocean Dipole were neutral; this is good news as a positive El Nino is not good for the monsoon. Some models predict La Nina being active during the second half of the season. La Nina brings more rain.
The chances of a normal monsoon is high at 41 per cent, an above normal monsoon has a 21 per cent chance, and there is a nine per cent chance of excess rain. On the other hand, there is a 20 per cent chance of below normal, and a nine per cent chance of deficient rain.