The southwest monsoon typically sets in over Kerala on June 1 but the mainland onset has a ‘standard deviation’ of seven days, according to the Indian Meteorological Department (IMD). In the last 10 years, the earliest arrival was on May 29 in both 2018 and 2022 while the most delayed was June 8, 2019.

Standard deviation is the measure of dispersion. It means how far the data values are spread out from the mean value.

What are the IMD and Skymet projections on the expected date of onset? Can there be further delay?

Both weigh in a series of predictive atmospheric and oceanic factors during the summer and pre-monsoon months before estimating the onset date. They cannot afford to take chances since this date has a crucial bearing on how the monsoon would progress over the landmass and end the debilitating hot season. The date is eagerly looked to by the common man, the market, the government, and policymakers. It would also signal a day of deliverance for the Indian farming community and the animal population.

This year, the onset may be slightly delayed than normal, until June 4 with a model error of ± 4 days (which means the onset can happen on either side of June 4 but within a four-day window). In the last five years, the onset date fell within the window as predicted by the IMD.

Also read: Flirting with climate danger: UN forecasts 2 in 3 chance of briefly hitting key heat limit soon

A comparison of the predicted onset dates versus actual onset, in the last five years is instructive. Last year (2022), the actual onset date was May 29 against the prediction of May 27; in 2021, it was on June 3 (prediction - May 31); in 2020, June 1 (prediction - June 5); in 2019, June 8 (June 6); and in 2018, May 29 (May 29).

Skymet Weather has pushed the date back to June 7 (with an error margin of +/- 3 days). According to it, the onset will be delayed and the advancement slightly sluggish over Peninsular India after onset. Hot weather will continue deep into June this year over Central and Northern parts of the country.

What could be causing this delay?

Though the IMD has not expressly said so, the onset dynamics may already have been impacted by the extremely severe cyclone Mocha that ruled the roost over the Bay of Bengal for almost a week before making an eventful landfall as a Category-4 cyclone on the Myanmar coast.

The cyclone appears to have robbed the Bay, where monsoon flows from the Southern Hemisphere head first, of entire moisture build and kinetic energy. Global agencies have projected that the monsoon may take a while before organising itself again. The Andaman & Nicobar Islands and adjoining South Bay receive the monsoon first usually around May 20-22.

The IMD said on Wednesday it is very likely to advance into some parts of South Bay, the South Andaman Sea, and the Nicobar Islands ‘during the next 2-3 days.’ This would put the onset around the normal. Skymet Weather has not made a call but it sees twin cyclone Fabien just below the Equator disrupting the incoming monsoon flows.

These apart, the emerging El Niño tends to trigger ‘subsidence’ of airmass over the West Pacific and adjoining South-East and South Asia. Subsidence translates into higher air pressure at the surface level that militates against the formation of storms, clouding and precipitation.

Also read: ‘Developing’ El Nino may hit India’s maize, soya and rice output, says FAO

Does the delay in arrival matter to quantity of rainfall received in the monsoon season?

Several studies conducted in this respect have ruled out any significant correlation between monsoon onset and the rainfall received over the country as a whole except during the first week or so in the immediate neighbourhood of Kerala and Karnataka. It has been even found that the onset does not in any way portend its arrival over the northern parts of the country.

2018 (earliest onset in last five years), the year of Floods of the Century in Kerala as well as parts of Karnataka and Tamil Nadu, was when the monsoon delivered just around normal for the country as a whole (in fact, a deficit of nine per cent for the season falling within the ‘normal’ as categorised by the IMD).

Or even 2019 (the most delayed onset in the last five years) when a historically strong positive Indian Ocean Dipole (IOD), the equivalent of El Niño-La Nina in the Indian Ocean, transformed an otherwise weak monsoon into a super bumper season in the second half. A positive IOD is in the making this year, too. One can only wait and watch its impact, or the lack of it, on the larger monsoon.

Are there any crops which could be impacted if the arrival is pushed back?

The delay in monsoon, per se, is unlikely to affect any crop. However, the follow-up showers may be delayed as a result and it could delay sowing of oilseeds such as soyabean. A prolonged dry period could result in the Government asking farmers to switch over to crops that are not water guzzlers like paddy. It might recommend sowing nutri cereals, which consume less water.