Stalled Monsoon in 2021

In News

Recently, it has been highlighted that the southwest monsoon, one of the most stable weather systems on the planet, has gone for a toss in 2021.

Major Highlights

• The monsoon onset happened almost on time (3^rd June) and it made swift progress, with ample rains all over the country in the first three weeks of June.
  • The monsoon had a timely onset because the cyclone dragged the monsoon trough onto Kerala and it met the definition of the onset.
    • However, the large-scale conditions are not really favourable to keep the trough moving.
  • The progress and rains happened because of two back to back cyclones, Tauktae in the Arabian Sea and Yaas in the Bay of Bengal, just before the onset of monsoon rains.
• The core monsoon zone actually has excess rain thus far but because of the cyclones and everyone is missing this key understanding, according to scientists, who worry about the patterns of the rainfall deficits.
  • In the July beginning, the country-wide deficit of monsoon rainfall stands at seven per cent below normal. This is when 75 per cent of meteorological subdivisions till mid-June had reported normal, excess or large excess rains.
    • This is a major cause of concern for the millions of farmers in India who are still dependent on the annual rainfall season for their agricultural activities.

• Researchers held that attention should be paid to the large-scale dynamics that stretch northwestward from the northwestern tropical Pacific.
  • These are often conducive to the 10-20 oscillations that come from the east towards India. These oscillations determine the further progress of the trough.
  • As of mid-July, the monsoon winds over India have remained stalled for 24 days, which is one of the longest such hiatus in the progress of the monsoon season over India in recent time.
  • Even if the monsoon revives, it may remain weak for the following week or so.
• Heatwaves Due to Monsoon Stalling
  • It is also one of the reasons for the spate of heat waves in north and north west India.
  • While on the eastern side, there have not been any low pressure areas in the Bay of Bengal which usually aid the progress of the monsoon season, on the western side there is a constant flow of westerlies flowing in from North Africa and the Arabian Peninsula.
  • These westerlies are dry and contain dust which does not allow the formation of clouds which means that the sun has a direct line of sight towards the land, heating it up and causing heat waves.
    • The westerly winds coming in are due to an active Atlantic Intertropical Convergence Zone (ITCZ) which is a monsoon-like trough that exists between the eastern Atlantic Ocean and North Africa.
• According to India Meteorological Department’s (IMD) own monsoon progress map, the northern limit of the monsoon trough still remains stationary and there are a multitude of factors that may have led to this long hiatus.

Factors Responsible

• Temperature Anomalies in North
  • The monsoon is delayed due to lower-than-usual temperatures in northern Pakistan and northern India because it takes a longer time to heat these regions to monsoon temperature, which led to the absence of the formation of the low-pressure system over the north Bay of Bengal.
  • Suitable conditions occur for the appearance of synoptic-scale Rossby waves in between the African Jet and mid-latitude westerly winds that led to the longest break and heatwaves in Delhi.
    • The midlatitude Rossby waves are unlikely to have affected the Indian monsoon but the overall warming of the northern hemisphere will have to be looked at carefully to see if it affected the monsoon trough.
  • When temperatures in northern Pakistan reach the monsoon temperature in Delhi, these Rossby waves disappear and the monsoon begins in Delhi.
• Pre-monsoon Rainfall in May
  • It has disorganised the onset of the monsoon and caused alternating premature rainfall and dry spells, in turn, leading to a longer transition to the monsoon.
  • When one part of the Indian subcontinent is warming, another is cooling, which leads to an erratic transition to the monsoon and a risk of dry spells.
• Unfavourable Madden Julian Oscillation and Indian Ocean Dipole
  • It is another reason for the break in monsoon progress that all scientists have highlighted.
  • The Madden Julian Oscillation (MJO) is an eastward moving pulse of cloud and rainfall that recurs every 30 to 60 days.
    • It causes major fluctuations in the monsoon over India.
  • The Indian Ocean Dipole (IOD) is a climatic system formed by the difference in temperatures in the two different sides of the Indian Ocean.
    • In its negative phase, when the eastern Indian Ocean is warmer and has more rainfall, it sometimes stalls the progress of the monsoon winds.

Monsoon

• It is a seasonal change in the direction of the prevailing or strongest winds of a region.
• It always blows from cold to warm regions and causes wet and dry seasons throughout much of the tropics.
• Monsoons are most often associated with the Indian Ocean.
  • The summer monsoon and the winter monsoon determine the climate for most of India and Southeast Asia.
• Summer Monsoon
  • It is associated with heavy rainfall and usually happens between April and September.
  • As winter ends, warm, moist air from the southwest Indian Ocean blows toward countries like India, Sri Lanka, Bangladesh and Myanmar and brings a humid climate and torrential rainfall to these areas.
  • Major factors responsible for the onset of summer monsoon in India are Mascarene High, Coriolis Force, Indian summer, El Nino Southern Oscillation (ENSO) and IOD.
    • Compared to the roles of Mascarene High, Coriolis Force, India’s summer season and ENSO, IOD’s role has been discovered recently.
    • However, the relation between IOD and monsoon rainfall is still being debated and has not been fully comprehended.
  • Significance
    • India and Southeast Asia depend on the summer monsoon as agriculture relies on the yearly rain.
    • Many areas in these countries do not have large irrigation systems surrounding lakes, rivers, or snowmelt areas so the summer monsoon fills wells and aquifers for the rest of the year.
    • Dairy farms, which help make India the largest milk producer in the world, also depend on the monsoon rains to keep cows healthy and well-fed.
    • Industry in India and Southeast Asia also relies on it as a great deal of electricity in the region is produced by hydroelectric power plants.
• Winter Monsoon
  • The Indian Oceans winter monsoon, which lasts from October to April, is less well-known than its rainy summer equivalent.
  • The dry winter monsoon blows from the northeast and winds start in the air above Mongolia and northwestern China.
  • Winter monsoons are less powerful than summer monsoons in Southeast Asia, in part because the Himalaya Mountains prevent much of the wind and moisture of the monsoons from reaching the coast.
  • These are sometimes associated with droughts, however, not all winter monsoons are dry.
  • Unlike the western part of Southeast Asia, the eastern, Pacific coast of Southeast Asia experiences its rainy season in the winter.
    • The winter monsoon brings moist air from the South China Sea to areas like Indonesia and Malaysia.

(Image Courtesy: Britannica)

Source: DTE