Delay in Monsoon Withdrawal

Criteria for the declaration of monsoon withdrawal

• Monsoon is said to withdraw from a region when the following criteria are met: 
  • Rainfall activity ceases over the area for five continuous days; an anticyclone establishes in the lower troposphere around 1.5 kilometres above sea level, and moisture content reduces.
  • Similar criteria are followed for the declaration of monsoon withdrawal from the country. The wind patterns over the country change from a south-westerly direction to a more westerly direction when the monsoon retreats.

El Nino Southern Oscillation (ENSO):

•  ENSO is a global phenomenon, which affects the temperature and precipitation of many regions around the globe, including the Indian subcontinent. The most important factor with the ENSO is the ability of the scientists to predict its occurrence, thereby predicting the amount of rainfall and temperature in the region. ENSO includes three phenomena:
• El-Nino: It leads to warming in the surface temperature of the eastern Pacific Ocean region. This is associated with an increase in rainfall along the western coast of South America and a decrease in rainfall over the Indian subcontinent. It also leads to westerly winds, instead of the generally occurring easterly winds.
• La-Nina: It is the reverse phenomenon of El-Nino. The temperatures over the eastern Pacific Ocean increases, along with a decrease in rainfall. Also, the rainfall over the Indian subcontinent increases, along with augmentation of the strength of surface-level easterly winds.
• Neutral: These years are not associated with any of the above phenomena.
• Southern Oscillation: This is the interconnection that was established between the El-Nino and the decrease in rainfall over the Indian subcontinent. Although the fishermen and later the scientists knew both the phenomena, the two were thought to be independent of each other. It was quite later that interconnection was established between the two and it was named Southern Oscillation.

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 also called southwest monsoon, which derives its name from winds that blow from a south-westerly direction in the Indian subcontinent.
  • It reaches India from June to September and significantly decides the state of agricultural productivity in India, which in turn, decides the state of the economy.
  • Major factors responsible for the onset of summer monsoon in India are Mascarene High, Coriolis Force, Indian summer, El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (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)