The start of monsoon over Kerala finally happened today, a week after its normal date of June 1. The Indian Meteorological Department (IMD) had last month predicted that the commencement would be delayed this year and would happen within four days of June 6.
The onset marks the beginning of the four-month, June-September monsoon season over India, which brings over 70 per cent of India annual rainfall.
Though, an important marker for the monsoon season, the timing of onset does not have any bearing on the quality or amount of rainfall during the season. It is just an event that happens during the progress of monsoon over the Indian subcontinent.
The onset is announced when certain criteria is fulfilled anytime after May 10. If at least 60 per cent of the 14 designated meteorological stations in Kerala and Lakshadweep report at least 2.5 mm of rain for two consecutive days after May 10 — and a few other conditions relating to wind and temperature are also fulfilled — the onset of monsoon is said to have taken place.
The Andaman and Nicobar Islands generally start getting monsoon rainfall between May 15 and 20, and the Kerala coast generally begins to have monsoon rainfall sometime in the last week of May. However, if the prescribed conditions are not met, the onset is not declared.
The IMD declared that all those conditions were fulfilled on Saturday morning.
A delayed onset has the potential to delay the arrival of monsoon to other parts of the country as well, especially in south India, most of which starts getting rainfall within days of it reaching the Kerala coast. However, it does not mean that the arrival of monsoon over the entire country would be delayed.
The northward progression of monsoon, after reaching the Kerala coast, depends on a lot of local factors, including the creation of low pressure areas. It is possible, therefore, that despite the start happening late, other parts of the country start getting rains on time.
The IMD has maintained that the most of northern and eastern parts of the country, which start receiving rainfall only towards the end of June and early July, would have monsoon rains on time. But southern and central India, which have normal monsoon arrival dates in the first, second and third weeks of June, would obviously get delayed rains.
The delay in the onset over Kerala coast also means that the June rainfall was likely to be deficient. But the IMD has predicted that some of this deficiency would be made up in July and August, and that the overall seasonal rainfall would be around the 96 per cent of normal that it has predicted. India as a whole receives about 89 cm of rainfall in a normal monsoon season.
The overall amount of rainfall that India gets during the four month monsoon season, and the regional distribution of rainfall, is not influenced by the date of the onset of monsoon.