Rainfall during retreating monsoon, which parts of South India experience every year, is not a local anomaly and is global in nature and scale, according to a recent study by the University of Sydney.

What is Retreating Monsoon?

• In India, retreating monsoon is the withdrawal of south-west monsoon winds from North India.
• The withdrawal is gradual and takes about three months.
• With the retreat of the monsoons, the clouds disappear and the sky becomes clear. The day temperature starts falling steeply.
• Monsoon rains weaken all over India except few southeastern states.
• It is helpful in Rabi crop cultivation.

What has the research found?

• The research has identified regions in the northern hemisphere that receive the bulk of the rainfall during September, October and November and southern hemisphere that receive most of the rainfall from March to August.
• The discovery that these are part of a global pattern and not one-off occurrences means they can be systematically studied, which will help understand how these communities could be affected by climate change.
• Peninsular India and parts of South-East Asia are among the eight regions examined in the study.

Factors affecting the retreat

• The eight global regions identified by the study that receive most of their rainfall after summer, have several things in common.
• They lie on the eastern fringes of landmasses and are in close proximity to mountain ranges with modest heights.

Two predominant factors cause the phenomenon:

• First, the low mountain range in each region runs from north to south, shielding it from west-bound winds that trigger summer monsoon.
• After summer, the range aids in the ‘orographic lift’ or rising of east-bound air mass from a lower to higher elevation, forming clouds and resulting in rain.
• The second factor is atmospheric convection or vertical movement of air.
• As the earth is heated by the sun, different surfaces absorb different amounts of energy and convection may occur where the surface heats up very rapidly.
• As the surface warms, it heats the overlying air, which gradually becomes less dense than the surrounding air and begins to rise.
• This condition is more favorable from September to February because of the role played by sea surface temperature or water temperature.

Answer this PYQ in the comment box:

Q.The seasonal reversal of winds is the typical characteristic of:

(a) Equatorial climate

(b) Mediterranean climate

(c) Monsoon climate

(d) All of the above climates

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