The Indian Monsoon

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Notes

The Indian Monsoon 

  • Monsoon winds have a significant impact on India's climate. Historically, sailors who came to India were among the first to notice the phenomenon of the monsoon. They benefited from the wind system's reversal as they arrived by sailing ships at the mercy of the winds. The Arabs, who had also come to India as traders, named this seasonal reversal of the wind system 'monsoon'.
  • The monsoons are experienced in the tropical area roughly between 20° N and 20° S. To understand the mechanism of the monsoons, the following facts are important.
    a. The differential heating and cooling of land and water results in low pressure over the landmass of India, while the seas around it experience relatively high pressure.
    b. The shift of the position of Inter Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain (this is the equatorial trough normally positioned about 5°N of the equator. It is also known as the monsoon trough during the monsoon season).
    c. The presence of a high-pressure area over the Indian Ocean east of Madagascar, approximately at 20°S. The Indian Monsoon is affected by the intensity and location of this high-pressure area.
    d. During the summer, the Tibetan plateau becomes extremely hot, resulting in strong vertical air currents and the formation of low pressure over the plateau at about 9 km above sea level.
    e. The presence of the tropical easterly jet stream over the Indian peninsula during summer and the movement of the westerly jet stream to the north of the Himalayas.
  • Inter Tropical Convergence Zone: The Inter Tropical Convergence Zone (ITCZ,) is a broad trough of low pressure in equatorial latitudes. This is where the northeast and the southeast trade winds converge. This convergence zone lies more or less parallel to the equator but moves north or south with the apparent movement of the sun.
  • Apart from this, it has also been noticed that changes in the pressure conditions over the southern oceans also affect the monsoons. Normally when the tropical eastern south Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. This periodic change in pressure conditions is known as the Southern Oscillation or SO.
  • The pressure difference between Tahiti (Pacific Ocean, 18°S/149°W) and Darwin in northern Australia (Indian Ocean, 12°30'S/131°E) is calculated to forecast the intensity of the monsoons. If the pressure differences were negative, it would indicate late and below-average monsoons. 
  • SO is the El Nino phenomenon in which a warm ocean current flows past the Peruvian Coast, in place of the cold Peruvian current, every 2 to 5 years. The changes in pressure conditions are connected to El Nino. Hence, the phenomenon is referred to as ENSO (El Nino Southern Oscillations).
  • El Nino: This is a name given to the periodic development of a warm ocean current along the coast of Peru as a temporary replacement for the cold Peruvian current. ‘El Nino’ is a Spanish word meaning ‘the child’, and refers to the baby Christ, as this current starts flowing during Christmas. The presence of El Nino leads to an increase in sea-surface temperatures and a weakening of the trade winds in the region.
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