What is the ‘onset’ of the monsoon, and why the delayed onset is not necessarily bad news

Contrary to what is sometimes assumed, the onset does not mean the first rain of the season. That can start happening in certain places even before the onset is declared.

For example, the Andaman and Nicobar Islands usually start receiving monsoon rainfall between May 15 and May 20, and it starts raining along the Kerala coast in the last week of May. However, ‘onset’ is a technical expression with a specific definition — and the IMD does not officially declare onset until certain prescribed conditions are met.

What are these conditions, which determine the onset of monsoon?

According to the IMD, the onset of the monsoon happens when there is a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region. IMD announces the onset of the monsoon only after certain precisely defined and measurable parameters are met.

The IMD essentially looks at the consistency of rainfall over a defined geography, the intensity of the rainfall, and the wind speed.

RAINFALL: The onset is declared if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep record at least 2.5 mm of rain for two consecutive days at any time after May 10. The onset over Kerala is declared on the second day, as long as specific wind and temperature criteria are also fulfilled.

The 14 enlisted stations are: (I) Minicoy, (ii) Amini, (iii) Thiruvananthapuram, (iv) Punalur, (v) Kollam, (vi) Alappuzha, (vii) Kottayam, (viii) Kochi, (ix) Thrissur, (x) Kozhikode, (xi) Thalassery, (xii) Kannur, (xiii) Kasaragod, and (xiv) Mangaluru.

WIND FIELD: The IMD says that the depth of westerlies should be up to 600 hectopascal (1 hPa is equal to 1 millibar of pressure) in the area that is bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE. The 10th parallel North passes through Kochi; and the area bound by the 55th and the 80th meridians East stretches from the middle of Iran to about Chennai.

The zonal wind speed over the area bound by 5-10ºN latitude (Maldives to Kochi) and 70-80ºE longitude (Arabian Sea to Chennai) should be of the order of 15-20 knots (28-37 kph) at 925 hPa.

HEAT: The INSAT-derived Outgoing Longwave Radiation (OLR) value — which is a measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere — should be below 200 watt per sq m (wm2) in the area between the 5ºN and 10ºN latitudes, and 70ºE and 75ºE longitudes.

Is a delay in the onset of the monsoon unusual?

The normal date of the onset of the monsoon over the Kerala coast is June 1, and this year’s delay is significant. However, neither an early nor a late onset of the monsoon is unusual.

So, in 2021, 2019, 2016, 2015, 2014, 2012, and 2011, the monsoon onset was realised after June 1. And in 2022, 2018, 2017, and 2010, the onset was realised before June 1. In 2020 and 2013, the monsoon was exactly on time, hitting the Kerala coast on June 1.

The IMD had predicted a delay of four days this year (onset on June 4), and a delay of six days in 2019 (onset on June 6); in both these years, the onset was realised on June 8. But there is always a margin of error in these predictions.

Does a delayed onset mean cascading delays across the country and for the rest of the season?

A delay in the onset over Kerala can potentially delay the arrival of the monsoon in other parts of the country, especially in the southern states, which normally start getting rain within days of the monsoon reaching the Kerala coast. The monsoon covers the entire country by July 15.

A delayed onset over Kerala does not automatically or invariably mean delays in the arrival of the monsoon over other parts of the country.

The northward progression of the monsoon after it has hit the Kerala coast is not uniform — it depends on local factors, including the creation of low-pressure areas. The monsoon may stall over certain places in certain years; or it may progress faster than usual.

It is possible, therefore, that despite a late onset over Kerala, other parts of the country will start getting monsoon rain on time.

The IMD said on Thursday that scattered showers are expected at many places along the Kerala coast over the next two days, but the widespread characteristic monsoon rainfall would have to wait at least until next week.

Does it therefore mean that an early onset would not necessarily mean a good monsoon?

Indeed, it would not — simply because the onset is just an event that happens during the progress of the monsoon over the Indian subcontinent.

A delay or an early arrival has no bearing on the quality or amount of rainfall, or its regional distribution across the country. In a recent year, the onset occurred two days in advance of the normal date and it rained heavily for about 10 days after that — however, the season as a whole ended with 14% less rain than normal.