The onset of the southwest monsoon over Kerala, which happened on June 8, marks the beginning of the four-month monsoon season over India. It came a week after its normal date of June 1.
The southwest monsoon brings over 70% of India’s annual rainfall. Though an important marker for the season, the timing of the onset does not have any bearing on the quality or amount of rainfall during the season. It is just an event during the progress of the monsoon.
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. The onset of the monsoon is announced when certain criteria are fulfilled after May 10. If at least 60% of the 14 designated meteorological stations in Kerala and Lakshadweep report at least 2.5 mm 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; if not, it is not declared. The IMD announced that all the conditions were fulfilled on the morning of June 8.
A delayed onset has the potential to delay the arrival of the monsoon in 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 monsoon over the entire country would be delayed. The northward progression of the monsoon from the Kerala coast depends on a lot of local factors, including the creation of low pressure areas. It is possible, therefore, that despite the onset happening late, other parts of the country start getting rains on time.
In pictures | Colours of rain: Southwest monsoon arrives in Kerala
The IMD has maintained that most of the northern and eastern parts of the country, which start receiving rainfall only towards the end of June and early July, would have monsoon rain 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 the Kerala coast also means that the June rainfall is likely to be deficient. But the IMD has predicted that some of this deficiency would be made up for in July and August, and that the overall seasonal rainfall would be around the 96% of the normal that it has predicted. India as a whole receives about 89 cm of rainfall in a normal monsoon season.