Retreating monsoon rains a global phenomenon

Context: According to a recent study by the University of Sydney, it was found that Rainfall during retreating monsoon, which parts of South India experiences every year, is not a local anomaly and is global in nature and scale.

• The discovery that these autumn monsoon regions are part of a global pattern is significant since it allows study into how global factors like climate change might affect them.

Key Findings

• Peninsular India and parts of SouthEast Asia are among the eight regions examined in the study.
• India’s east coastal region receives the biggest share of its annual rainfall not during the monsoon season but right after in the months of September, October and November.
• 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.

About Retreating Monsoon

• In simple words, retreating means withdrawal. So, withdrawal of south-west monsoon winds from skies of north India during months of October and November is known as retreating monsoon. The withdrawal is gradually and takes about three months.

Difference between retreating monsoon and north east monsoon
Retreating monsoon

• With retreat of the monsoons, the clouds disappear and the sky becomes clear. The day temperature starts falling steeply.
• Monsoon rains weakens all over India except few south eastern states.
• Monsoon trough weakens and gradually shifts south wards
• Most severe and devastating tropical cyclones originate in the Indian seas especially in the Bay of Bengal due to retreating monsoons.
• Direction of winds is from North west to south east and Winds blow from surface to sea there by carrying no moisture.
• It is helpful in Rabi crop cultivation.

North-eastern monsoon

• Northeast Monsoon affects only five sub-divisions of Tamil Nadu, Kerala, South Interior Karnataka, Rayalaseema and Coastal Andhra Pradesh
• On-set of north east monsoon coincides with retreating monsoon winds.
• Unlike retreating monsoon, onset of north eastern monsoon is not clearly defined.
• Winds change its direction to north easterly beginning with Tamil Nadu, along with substantial increase in rainfall intensity and spread beginning with Tamil Nadu and Andhra Pradesh.
• Rainfall variation is huge in north-east monsoon.
• El-Niño has no effect on north-eastern monsoon winds unlike south-western monsoon winds.
• Winds blow from sea to land there by causing rainfall. For instance, monsoon winds absorb moisture while passing through Bay of Bengal and cause rainfall in parts of Andhra and Tamil Nadu.
• The rainfall helps in preparing land for next cropping season.