Why in News?

• The southwest monsoon hit the Kerala coast on June 8 (May 29 in 2022). The normal date of the onset of the monsoon over the Kerala coast is June 1.
• However, monsoon is expected to remain weak for at least one more week, thanks to the development of cyclone Biparjoy in the Arabian Sea. 

What’s in Today’s Article?

• What do you mean by Monsoon?
• What is Meant by the “Onset of the Monsoon”?
• Conditions Determining the Onset of Monsoon
• Implications of Delay in/Early Onset of the Monsoon

What do you mean by Monsoon?

• A monsoon is a seasonal reversing wind accompanied by corresponding changes in precipitation and associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ).
  • ITCZ is the region that circles the Earth and where the trade winds of the Northern and Southern Hemispheres converge - creating a broad trough of low pressure.
  • Seasonal shifts (between equator and tropic of cancer) in the location of the ITCZ with the apparent movement of the sun results in the wet and dry seasons of the tropics.
• Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern.
• On the Indian subcontinent, as the rain bearing winds come from the southwest direction, it is known as the southwest monsoon.

What is Meant by the “Onset of the Monsoon”?

• The onset of the monsoon over Kerala signals the beginning of the four-month (June-September) southwest monsoon season, during which India gets more than 70% of its annual rainfall.
  • The monsoon covers the entire country by July 15.
• Contrary to what is sometimes assumed, the onset does not mean the first rain of the season.
• 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.
• 

Conditions Determining 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.
• 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.
• Wind field: 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 (passes through Kochi), and from longitude 55ºE to 80ºE.
• Heat: The 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 area between the 5ºN and 10ºN latitudes, and 70ºE and 75ºE longitudes.

Implications of Delay in/Early Onset of the Monsoon:

• Neither an early nor a late onset of the monsoon is unusual.
• A delay in the onset over Kerala does not automatically mean delays in the arrival of the monsoon over other parts of the country.
  • The northward progression of the monsoon is not uniform - it depends on local factors, including the creation of low-pressure areas.
  • Therefore, despite a late onset over Kerala, other parts of the country may start getting monsoon rain on time.
• A delay or an early arrival has no bearing on the quality or amount of rainfall, or its regional distribution across the country.