Indian Monsoon: new parameters

Monsoon wind, that decides the ups and downs of Indian economy, is itself guided by various dynamic factors. The complexity of its mechanism is increasing day by day with the global warming and fast changing climate of the world.

The common factors include

  1. Thermal contrast between Indian peninsula and surrounding NIO, which include Arabian Sea, Bay of Bengal and north Indian Ocean.
  • This contrast develops due to difference of specific heat capacity of land and sea.
  • Due to low specific heat capacity of land, it gets heated up quickly; the air above it becomes light and rises up.
  • The rising air column expands and cools.
  • The cooling air loses holding capacity, thus increasing relative humidity.
  • With 100% relative humidity, condensation starts, transforming vapour to liquid water droplets.
  • Droplets of water coalesce to form bigger drops; increasing their weight and so they start falling under the force of gravity.
  • When the air column over land rises, the surrounding moisture laden air from the ocean supplement it, thus enhancing precipitation.
  1. Shift of ITCZ Inter tropical convergence zone is a low pressure zone found near 50 N to 50S latitude where trade winds of two hemisphere coverage.
  • During summer due to Northward shift of vertical rays of sun, ITCZ shifts northwards, with maximum shift to 20 0
  • Trade winds of southern hemisphere have to move beyond the equator to reach this low pressure zone of ITCZ.
  • So trade wind turns to their right while crossing equator in North hemisphere and blows as equatorial westerly.
  • These equatorial westerly results in the origin of South West Monsoon
  1. Southern oscillation and Walker Cell: It is the periodic change in the pressure condition in Southern Pacific Ocean belt between Tahiti in the Eastern Pacific and Darwin in the western Pacific. Negative oscillation means low pressure in east Pacific and this weakens the South West Monsoon wind.
  2. IOD: Positive IOD that refers to the condition when eastern Indian Ocean has low temperature condition than western Indian Ocean has positive impact on Indian Monsoon.
  3. Recurving cyclone:
    1. Recurving cyclone:It refers to the change in the direction of Cyclones of West Pacific Ocean from westward to north-eastward. This distracts the westward compression over SW Monsoon and reduces rainfall over Indian Peninsula.
  • During the monsoon months, cyclones in the Western Pacific move westwards towards India and aid rain-bearing systems over the sub-continent.
  • But during some years they ‘recurve’, or start to swing north-east, and do not give as much of a push to the rains as they do in the good monsoon years.
  • This re-curving frequently happens during the El Nino years, but not necessarily.

 

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