Why is it in the News?

The southwest monsoon is progressing normally, and conditions are suitable for its onset on the Kerala coast in the next five days, the India Meteorological Department (IMD) said on Monday (May 27).

What does the ‘onset of monsoon’ Mean?

• The onset of the monsoon over Kerala marks the beginning of the four-month, June-September southwest monsoon season over India, which brings more than 70% of the country’s annual rainfall.
• The onset of the monsoon is a significant day in India’s economic calendar.
• According to the IMD, the onset of the monsoon marks a crucial transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region, and the Department announces it only after certain defined and measurable parameters, adopted in 2016, are met.

Onset & Advance of Monsoon:

• Broadly, the IMD checks for the consistency of rainfall over a defined geography, its intensity, and wind speed.
• Rainfall: The IMD declares the onset of the monsoon 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. In such a situation, the onset over Kerala is declared on the second day, provided specific wind and temperature criteria are also fulfilled.
  • The 14 enlisted stations are Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kasaragod, and Mangaluru.
• Wind field: The depth of westerlies, prevailing winds that blow from the west at midlatitudes — should be up to 600 hectopascals (1 hPa is equal to 1 millibar of pressure) in the area bound by the equator to 10ºN latitude and from longitude 55ºE to 80ºE.
  • The zonal wind speed over the area bound by 5-10ºN latitude and 70-80ºE longitude should be of the order of 15-20 knots (28-37 kph) at 925 hPa.
• Heat: According to IMD, the INSAT-derived Outgoing Longwave Radiation (OLR) value (a measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere) should be below 200 watts per sq m (wm2) in the box confined by 5-10ºN latitude and 70-75ºE latitude.
• Northern Limit of Monsoon (NLM): Southwest monsoon normally sets in over Kerala around 1st June.
  • It advances northwards, usually in surges, and covers the entire country around the 15th of July.
  • The NLM is the northernmost limit of monsoon up to which it has advanced on any given day.

In general, the Andaman and Nicobar Islands start receiving monsoon rainfall between May 15 and May 20 every year, and it usually starts raining along the Kerala coast in the last week of May.