What is the ‘onset’ of the monsoon, and why the delayed onset is not necessarily bad news
- The southwest monsoon has set in over the Kerala coast, the India Meteorological Department (IMD) said recently.
- This year, along with 2016 and 2019, is the most that the monsoon’s onset has been delayed in the last couple of decades.
Onset of the monsoon
- It signals the beginning of the four-month (June-September) southwest monsoon season.
- During this period India gets more than 70% of its annual rainfall.
- Does not mean the first rain of the season.
- It can start happening in certain places even before the onset is declared.
- Onset is a technical expression with a specific definition and the IMD does not officially declare onset until certain prescribed conditions are met.
Conditions which determine the onset of monsoon
- Significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region.
- Parameters:
- 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.
- Wind Field:
- The IMD says that the depth of westerlies should be up to 600 hectopascal 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;
- The area bound by the 55th and the 80th meridians East stretches from the middle of Iran to about Chennai.
- 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.