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What is the ‘onset’ of the monsoon?

GS Paper 1:

Topics Covered: Important Geophysical phenomena such as earthquakes, Tsunami, Volcanic activity, cyclone etc.

 

Context:

The southwest monsoon is likely to set in over Kerala on May 27, well ahead of its normal date of June 1.

  • If the forecast turns out to be accurate, this will be the earliest onset of the monsoon over Kerala since at least 2009.

 

The onset of southwest monsoon is announced by IMD Only after the following parameters are met:

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.

Wind field: The depth of westerlies should be upto 600 hectopascal (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: 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 watt per sq m (wm2) in the box confined by 5-10ºN latitude and 70-75ºE latitude.

 

Factors influencing the onset on Southwest Monsoon:

The onset needs a trigger in the form of a weather system in the proximity of the coastline. These are ocean born phenomena which accentuate the monsoon surge around the normal time of onset. These include:

  1. The low-pressure area or depression in the Bay of Bengal during the last days of May or the beginning of June.
  2. There are such systems in the Arabian Sea as well around the same time which results in onset over the mainland.
  3. ‘Cyclonic Vortex’ is another factor which appears in the Southeast Arabian Sea, off Kerala and Lakshadweep region. They also shift along the west coast to push the monsoon current.
  4. The formation of ‘trough’ off the west coast due to temperature differential between land and sea. This situation could be for a mild start and weak progress.
  5. Lastly the cross-equatorial flow, wherein the trade winds from the Southern Hemisphere crossover to the Northern Hemisphere can bring a strong monsoon surge towards the Indian mainland.

 

Monsoon in India- related key facts:

  1. Generally, across the world, the monsoons are experienced in the tropical area roughly between 20° N and 20° S.
  2. Out of a total of 4 seasonal divisions of India, monsoon occupies 2 divisions, namely- the southwest monsoon season and the retreating monsoon season.
  3. Southwest monsoon season over India, which brings more than 70 per cent of the country’s annual rainfall.

 

Insta Curious:

Curious about the factors influencing the formation (not the onset) of Southwest Monsoon? Read here: skymetweather.com

Did you know that Andaman and Nicobar Islands start receiving monsoon rainfall between May 15 and May 20 every year? This is earlier than the onset of monsoon in Kerala.

 

InstaLinks:

Prelims Link:

  1. What is El Nino?
  2. What is La Nina?
  3. What is ENSO?
  4. When do these events occur?
  5. Impact of ENSO on Asia, Africa and Australia.
  6. What is Indian Ocean Dipole?
  7. Differences between Southwest and Northeast monsoon.
  8. Factors influencing the onset on Southwest Monsoon.

Mains Link:

Discuss the impact of La Nina weather phenomenon on India.

 

[Q 1) Consider the following statements:

  1. Generally, across the world, the monsoons are experienced in the tropical area roughly between 20° N and 20° S.
  2. Southwest monsoon season over India brings more than 70 per cent of the country’s annual rainfall.

Which of the above statements is/are correct?

 

      1. 1 only.
      2. 2 only.
      3. Both.
      4. None of the above. ]

 

Sources: Indian Express.