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Early Southwest Monsoon

  • Posted By
    10Pointer
  • Categories
    Geography
  • Published
    18th Jun, 2021
  • Context

    Just about 10 days after it broke over the Kerala coast two days behind schedule, the southwest monsoon has progressed rapidly covering many parts of east, west, south and central India before the normal date.

  • Background

    • In India, June to September is the period for the Southwest monsoon and is considered as the main rainy season for the Indian subcontinent.
    • However, this year, things are not in their usual trend.
    • The second half of May kept the entire weather community on its toes, as two cyclones brought record-breaking rains to many parts of India.
      • One of them was the rarest of the rarest—Cyclone Tauktae—which formed in the Arabian Sea, travelled parallel to India’s west coast, before making landfall in Gujarat and then curving east towards Delhi. It changed the entire pre-monsoon rainfall scenario, especially over West and Central India.
      • A week later, Cyclone Yaas did the same over East India, resulting in a bumper pre-monsoon season with a pan-India 18% excess rainfall.
    • Noteworthy are the massive positive departures over West, Northwest and Central India.

    Rainfall trend in India

    • India has been witnessing increasing events of ‘extremely heavy’ and ‘very heavy’ rainfall since 2012, data from the Ministry of Earth Sciences showed.
      • In 2012, 185 stations reported ‘extremely heavy’ rainfall while in 2020, this increased to 341, nearly 85 per cent jump, as per the data.
      • Further, 2019 had been an exceptional year as 554 stations reported ‘extremely heavy’ rainfall, the highest since 2012.
      • Also, at least 3,056 stations reported ‘very heavy’ rainfall, also the highest since 2012.
    • Classification: The rainfall recorded:
      • below 15 mm is considered ‘light’
      • between 15 and 64.5 mm is ‘moderate’
      • between 64.5 mm and 115.5 mm is ‘heavy’
      • between 115.6 mm and 204.4 mm is ‘very heavy’
    • Anything above 204.4 mm is considered as ‘extremely heavy’ rainfall.
  • Analysis

    • Monsoon dynamics, in a nutshell

    What is Monsoon?

    • Monsoon is the phenomenon associated with the seasonal reversal of winds.
    • With the apparent shift of sunrays, the pressure belts shift leading to a reversal of the direction of trade winds.
    • In India, this phenomenon is responsible for rainfall and another climatic phenomenon. The monsoonal rainfall is a feature of the Indian subcontinent.
  • What drives monsoon?

    • Monsoons are predominantly driven by the two winds: southwesterly and northeasterly, which defines the two monsoon patterns i.e., southwest and northeast.
      • South-West Monsoon: The South-West monsoonal rainfall is the type of rainfall that occurs in almost all parts of India during summers. During this, the southwesterly winds blow from East Africa to India.
      • North-East Monsoon: The North-East Monsoonal rainfall is associated with the rainfall in the southern states of Tamil Nadu, Andhra Pradesh, Karnataka and Kerala during winters. During the northeast, the winds completely reverse with respect to the change in the atmospheric conditions and blow from India to Africa.
  • What makes the winds change direction (seasonal reversal)?

    • It all depends on the differential heating and cooling of the surrounding landscape and oceans.
    • During the summer months, the land is significantly hotter due to the impact of the Sun's radiation.
      • As a result, low-pressure forms in the inland areas of the country, simultaneously, high-pressure conditions develop over the surrounding Indian Ocean.
      • This in turn, drives winds from sea to land.
      • As the moisture-laden winds rush inward they bring bountiful rains.
    • During winters, the pattern reverses, changing the wind directions to the northeast.

    The retreating monsoon winds gather moisture from the Bay of Bengal and pour it over the southern peninsula during the northeast monsoon months.   

    Indian Ocean Dipole

    • The story of monsoon is never complete without talking about the Indian Ocean Dipole.
    •  It refers to the difference in sea-surface temperatures in opposite parts of the Indian Ocean.
    • It is often called the "Indian Niño" because of its similarity to its Pacific equivalent.
  • Where does monsoon form?

    • A monsoon forms in the-
      • tropics (between 0 and 23.5 degrees latitude north and south)
      • subtropics (between 23.5 degrees and 35 degrees latitude north and south)
    • The strongest monsoons tend to occur in India and South Asia in the north and Australia and Malaysia in the south.
    • Monsoons also occur in southern parts of North America, in Central America, northern areas of South America, and in western Africa.
  • What contributed to early monsoon?

    • Cyclone Yaas: Cyclone Yaas, formed in the Bay of Bengal during May, helped the monsoon make a timely arrival over the Andaman Sea.
    • Other reasons include-
      • strong westerly winds from the Arabian Sea
      • off-shore trough prevailing for a week between Maharashtra and Kerala
      • formation of a low-pressure system over the North Bay of Bengal

    In the last one decade since 2011, the monsoon has covered the entire country in June itself on four occasions — 2020 (June 1–26), 2018 (May 28–June 29), 2015 (June 5–26) and 2013 (June 1–16).

  • Why monsoon is crucial for India?

    • India receives about 70% of its annual rainfall during the four-month season that is crucial for the country’s farm-dependent economy and for rice, soybeans, and cotton cultivation.
    • A normal monsoon significantly helps the agriculture sector. Good rains have been a prime reason for the farm sector’s resilience for two years despite the pandemic.
    • India has over 150 million farmers and nearly half of Indians are dependent on a farm-based income.
    • As much as 60% of India’s net-sown area does not have access to irrigation.
  • The monsoon ahead

    The southwesterly cross equatorial flow is likely to strengthen and deepen in the coming days. The convection is likely to get enhanced over the Andaman Sea and adjoining East Central Bay of Bengal, favouring the formation of a cyclone.