What is a normal Monsoon and what factors lead to the weakening of the Monsoon?

Context: According to the private weather forecasting agency Skymet, India is likely to get “below normal” monsoon rains in 2023 with an increased likelihood of El Nino, which typically brings dry weather to Asia.

About Normal Monsoon

  • The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark “long period average” (LPA). 
  • According to the IMD, the “LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) average over a long period like 30 years, 50 years, etc”.
  • The IMD’s prediction of a normal monsoon was based on the LPA of the 1971-2020 period, during which India received 87 cm of rain for the entire country on average.
  • The IMD maintains five rainfall distribution categories on an all-India scale. These are:
    • * Normal or near normal, when the percentage departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA;
    • * Below normal, when departure of actual rainfall is less than 10% of LPA, that is 90-96% of LPA;
    • * Above normal, when actual rainfall is 104-110% of LPA;
    • * Deficient, when departure of actual rainfall is less than 90% of LPA; and
    • * Excess, when the departure of actual rainfall is more than 110% of LPA.

Factors which affect the Monsoon

El Niño

  • The warming in the tropical Pacific Ocean because of El Niño weakens the southeast trade winds flowing to the intertropical convergence zone over India
  • Since these winds are the main driving force of the Indian summer monsoon, El Niño events are associated with weak monsoons and lower than average rainfall. 
  • The location of the El Niño event also influences its effects on the Indian monsoon – warming in the central Pacific Ocean affects the Indian monsoon more than if the warming is in the eastern Pacific Ocean.
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La Niña

  • The La Niña has the opposite effect to the El Niño and is responsible for stronger monsoons and above-average rainfall.
  • Equatorial Indian Ocean Oscillation (EQUINOO): 
  • It is thought to arise as an effect of the Indian Ocean Dipole(IOD) and refers to increased and decreased cloud formation between the western and eastern equatorial Indian Ocean. 

Although the IOD was discovered only in 1999, and the EQUINOO in 2002, both have been recognised as important modulators of the Indian summer monsoon. Positive Indian Ocean Dipole(IOD) and EQUINOO events are associated with more rainfall as these events increase moisture transport from the southeastern parts of the Indian Ocean.

Indian Ocean Dipole(IOD):

  • IOD measures differences in sea surface temperatures between the western and eastern parts of the Indian Ocean.
  • Indian Ocean Dipole (IOD) is basically similar to the El Nino weather system that develops in the Pacific Ocean. It is characterized by an irregular oscillation of sea-surface temperatures in the eastern and western Indian Ocean

Atlantic sea surface temperature(SST) variability

  • The Atlantic SST variability affects the Indian summer monsoon in the same way that the ENSO does. 
  • A warming of the surface of the Atlantic Ocean weakens the monsoon, just as cooling of the ocean’s surface has the opposite effect. 
  • The effects of this phenomenon, also known as the Atlantic Niño on the Indian summer monsoon is thought to be mediated through perturbations in the jet streams above India.

Mascarene High

  • The Mascarene High is a semi-permanent high-pressure zone in the south Indian Ocean, about 4,000 km from India, near the Mascarene Islands. 
  • The Mascarene High begins developing in mid-April and is a major factor in driving the circulation between the northern and southern hemispheres that powers the summer monsoon winds towards the Indian subcontinent from the Indian Ocean.
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  • One of the rather surprising local factors that affects the Indian monsoon, is irrigation. 
  • According to a 2019 study in the journal Climate Dynamics, the trend of decreasing rainfall over the Indo-Gangetic plains could be due to the extensive irrigation in this area. 
  • This is likely because irrigation affects soil moisture levels and temperature simultaneously, which affects atmospheric stability. Since the irrigation is over vast tracts of land, these changes shift the moisture convergence zone to the south, during the active phase of the monsoon.
  • The study finds that winter irrigation (November-March) actually strengthens the monsoon rains over the region for the following year and also reduces intra-seasonal variations in rainfall. 
  • However, with year-round irrigation, there is a noticeable decrease in the summer monsoon rainfall (June-September).

Aerosols and dust

  • Aerosols and dust are other local factors that have been shown to affect the monsoon rainfall in India. 
  • In a recent study in 2022, researchers at IIT Bhubaneswar have shown that dust transported to the Arabian sea from the Middle-Eastern deserts (the Sahara and the Sinai) could increase rainfall in India and south Asia over short time scales of one or two weeks. 
  • In a similar study in 2014, scientists showed that dust aerosols likely heat up the atmosphere over north Africa and west Asia, which increases the flow of moisture over India. This results in rainfall, usually within a week of the event, over central India.

Monsoon Impact

  • The monsoon is critical for a healthy rural economy as 51% of the country’s farmed area, accounting for 40% of production, is rain-fed, according to the agriculture ministry
  • As much as 47% of the country’s population is dependent on agriculture, one of the mainstays of India’s economy, for livelihood, according to Economic Survey 2022-23.
  • It spurs farm produce and improves rural spending besides impacting inflation, jobs, and industrial demand, for ex. Good farm output keeps food inflation under check and ample harvests raise rural incomes and help inject demand into the economy.

Mains Question

Q. Drought has been recognised as a disaster in view of its party expense, temporal duration, slow onset and lasting effect on various vulnerable sections. With a focus on the September 2010 guidelines from the National disaster management authority, discuss the mechanism for preparedness to deal with the El Nino and La Nina fallouts in India.(2014)

Q. Most of the unusual climatic happenings are explained as an outcome of the El-Nino effect. Do you agree?(2014)

Q. How far do you agree that the behaviour of the Indian monsoon has been changing due to humanizing landscapes? Discuss.(2015)


  1. With reference to ‘Indian Ocean Dipole (IOD)’ sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct?
  2. The IOD phenomenon is characterised by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.
  3. An IOD phenomenon can influence an El Nino’s impact on the monsoon.

Select the correct answer using the code given below: 

(a)  1 only 

(b) 2 only

(c) Both 1 and 2

(d) Neither 1 nor 2

Ans. (b)

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