IMD predictions of Monsoon

In News

• India Meteorological Department (IMD) predicted a borderline-level normal summer monsoon rainfall this year. 

About IMD 

• It is an agency of the Ministry of Earth Sciences.
• It is the principal agency responsible for meteorological observations, weather forecasting and seismology.
• It is also one of the six Regional Specialised Meteorological Centres of the World Meteorological Organisation.
• IMD releases the long range forecast in two stages in April and June.

Predictions by IMD

• Normal to below normal rainfall:
  • The rainfall during the June-September period is likely to be 96% of the long-term average Period.
  • Normal to below normal rainfall in some areas of north west India and some parts of west central India and some pockets of north east India.
• El Nino impact:
  • The El Nino is the key factor responsible for the relatively less rainfall this year.
  • Since 2019, India has been under the influence of the converse ‘La Nina’ and therefore is getting substantial monsoon rains.
  • The El Nino event, triggered by warm water in the equatorial Pacific, can affect weather patterns worldwide and in India, El Nino linked to drier conditions and reduced rainfall.
• Current La Nina:
  • The current La Lina conditions, which is usually favourable to monsoon, has changed to neutral over the equatorial Pacific region.

IMD Categorisation Method of Monsoon

• Long Period Average (LPA): 
  • LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) averaged over a long period like 30 years, 50-years etc. 
  • It acts as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season.
• Categories of Rainfall:
  • Large Excess: ≥60% of  long period average (LPA).
  • Excess: 20% to 59% of  long period average (LPA).
  • Normal:  -19% to +19% of  long period average (LPA).
  • Deficient: -59% to -20% of  long period average (LPA).
  • Large Deficient:  -99% to -60% of  long period average (LPA).
• Predicting the Monsoon: Monsoon season is from June to September in India as a whole, the long period average (LPA) is 88 cm and standard deviation is 9cm (about 10% of mean value).
  • Therefore, when the rainfall averaged over the country as a whole is within ±10% from its LPA or 90% to 110% of LPA, the rainfall is said to be “normal” and when the rainfall is <90% of LPA it is considered to be below normal and when it is >110% of LPA, the rainfall is said to be “above normal”.

Monsoon & its onset

• A monsoon is a seasonal change in the direction of the prevailing, or strongest, winds of a region. 
• The monsoon that starts around the first week of June, making the first landfall in Kerala.
• From October to November, the retreating monsoon or the Northeast monsoon sets in, and brings rain to the eastern coast of India, especially Tamil Nadu. 
• Various factors such as the availability of energy in the atmosphere, the intertropical convergence zone, the Coriolis effect and jet streams, play a role in facilitating the southwest monsoons.

Effects of Monsoon on Indian Subcontinent

• Agriculture & Economy: Adequate and timely monsoon showers are vital for India’s agriculture sector, the main source of livelihood for some 60% of its population and which accounts for about 18% of the economy. 
  • Nearly half of India’s farmland, which has no irrigation cover, depends on annual June-September rains to grow crops such as rice, corn, cane, cotton and soybeans.
  • Monsoon uncertainty can pose potential risk to the economy.
• Rivers: The monsoon brings water and sediment not only to Indian rivers but also to rivers in China, Bangladesh, etc.

Know about El Nino and La Nina

• Normal Conditions: During normal conditions in the Pacific ocean, trade winds blow west along the equator, taking warm water from South America towards Asia. 
  • To replace that warm water, cold water rises from the depths — a process called upwelling. 
• El Niño and La Niña are two opposing climate patterns that break these normal conditions.
• El Nino: During El Niño, trade winds weaken. Warm water is pushed back east, toward the west coast of the Americas and as a result cold water is pushed towards Asia.
  • El Niño means Little Boy in Spanish. South American fishermen first noticed periods of unusually warm water in the Pacific Ocean in the 1600s. The full name they used was El Niño de Navidad, because El Niño typically peaks around December.

• La Niña: It means Little Girl in Spanish. La Niña is also sometimes called El Viejo, anti-El Niño, or simply “a cold event.” La Niña has the opposite effect of El Niño. During La Niña events, trade winds are even stronger than usual, pushing more warm water toward Asia. 

Effects on the Indian Ocean

• A transition from a La Niña winter to an El Niño summer has historically tended to produce a largest deficit in the monsoon. If an El Niño emerges by summer, India is likely to experience a deficit monsoon. The monsoon deficit will be accompanied by extreme wet and dry events. 
• The vertical shear, which is the change in intensity of winds from the surface to the upper atmosphere, tends to be weaker as well. This in turn can favour enhanced cyclogenesis, i.e. cyclone formation.
• The La Nina phenomenon on the other hand can lead to the above normal rainfall and decreased temperature in India.

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