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Rainfall Pattern in Assam

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June 27, 2022

What is the issue?

This monsoon, some areas received deficit rainfall while Assam received excess rainfall.

How about the arrival of monsoon this year?

  • In India, June 1 is regarded as the date of arrival of the monsoon, which accounts for about 80% of the rainfall in the country.
  • The monsoon landed three days ahead of the normal date in Kerala this year, but it turned sluggish on its western branch’s upward journey.
  • The central India suffered a deficit while the east and north-eastern parts battled excess rain leading to widespread floods in Assam and Meghalaya.
  • The key monsoon months are July and August and they bring nearly two-thirds of the monsoon rains.

How has the monsoon been so far this year?

  • The country received 2% less rain this year than it usually does between June 1 and June 23 every year.
  • The total rainfall was brought down by 34% over central India and 15% over peninsular India compared to the 32% more received by the east and northeast and 7% more by northwest India.
  • Meteorologists said the recent episode of heavy rainfall underlined the
    • Presence of the east-west trough in the lower levels of the atmosphere over the region
    • Incursion of large-scale moisture due to strong southerly and south-westerly winds from the Bay of Bengal
  • According to the IMD, Assam received 41% above normal rainfall during the pre-monsoon season (March to May), received 71% more than normal rainfall up to June 25.
  • A 2018 study by Indian Institute of Tropical Meteorology revealed that Assam had been witnessing a decreasing trend in the average monsoon rainfall since 1870 while experiencing sudden downpour days leading to frequent flooding.

What are the factors determining rainfall pattern?

  • Ecological and climate difference- Assam’s valleys experience both excessive and insufficient rainfall from time to time due to ecological and climate difference from one place to another.
  • Climate change- Climate change increases the water and surface temperature of Arabian Sea and Bay of Bengal by up to 2 degrees, forming low-pressure areas resulting in heavy rains.
  • Black carbon emissions- Aerosols, including black carbon, released by biomass burning, leads to a decrease in low-intensity rainfall while pushing up severe rain in the pre-monsoon season in northeast India.

What are the disturbances to monsoon in India?

  • Depressions- The important synoptic disturbances during the monsoons over India are lows, depressions, etc. that form mostly over the Bay of Bengal and produce a large volume of rainfall.
  • Position of offshore trough- The other synoptic disturbance which affects monsoon rainfall significantly is the position of offshore trough or vortex along the west coast of India.
  • Global phenomena- Monsoon rainfall in India is known to be affected by global phenomena such as El Nino or La Nina.
  • Other factors such as the Indian Ocean Dipole and Madden-Julian Oscillation also influence monsoon rainfall.

 

References

  1. https://www.thehindu.com/sci-tech/energy-and-environment/explained-what-accounts-for-deficit-in-some-areas-and-excess-rainfall-in-assam-this-monsoon/article65564490.ece
  2. http://www.bom.gov.au/climate/enso/history/ln-2010-12/IOD-what.shtml
  3. https://indianexpress.com/article/explained/madden-julian-oscillation-the-reason-behind-the-unexpectedly-good-june-rainfall/

 

Quick facts

  • El Niño- El Niño is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean.
    • In India, El Nino has been found to have strong links in suppressing the monsoon rainfall.
  • La Nina- La Nina refers to the periodic cooling of ocean surface temperatures in the central and east-central equatorial Pacific.
    • La Nina has been found to be helpful in bringing good rainfall to India.
  • Indian Ocean Dipole- IOD is defined by the difference in sea surface temperature between two areas (or poles) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia.
    • Positive event- warmer sea surface temperatures in the western Indian Ocean relative to the east bringing more rainfall to India
    • Negative event- cooler sea surface temperatures in the western Indian Ocean relative to the east bringing less rainfall to India
  • Madden-Julian Oscillation (MJO)- The MJO is a moving system of wind, cloud and pressure that brings rain as it circles around the equator.
    • MJO goes around the globe in 30-60 days on average. Sometimes, it can take 90 days.
    • In the active phase, MJO results in more than average rainfall for that time of the year, while in the suppressed phase, the area receives less than average rainfall.

 

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