July 25, 2021: Not far from its literal sense, the retreating monsoon phenomena refers to the withdrawal of the erratic rainfall pattern simmering down as it departs specifically in the months of October and November in North India. The unprecedented rainfall akin to different parts of the Indian subcontinent, do not record any local meteorological disturbance, rather are global and natural, both in magnitude and scale as per the latest study by the University of Sidney.
Latest study on retreating monsoon
Peninsular India, popular for its inconsistent monsoon rains, witnesses the retreating monsoon phenomena as the south-west monsoon winds depart from North India.
A study termed The Global Coherent Pattern of Autumn Monsoon Precipitation by Nandini Ramesh, Quentin Nicolas and William R Boos pin-pointed that 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.
A global phenomenon
One of the co-authors said, “The discovery that retreating monsoon are part of a global pattern and not one-off occurrences means they can be systematically studied, which will help us understand how these communities could be affected by climate change.”
Including India and parts of South-East Asia, this phenomenon was observed and studied in eight regions.
These eight global regions under the spectrum of the study, receive most of their rainfall after summer. This is backed by many commonalities in their geographical locations and topography like all of them lie on the eastern fringes of landmasses and are in close proximity to mountain ranges with modest heights.
The study’s inference highlighted that India’s east coastal region receives the biggest share of its annual rainfall not during the monsoon season but exactly after in the months of September, October and November.
Retreating monsoon season
The season featuring retreating monsoon is marked by the beginning of the withdrawal of the south-west monsoon (mid-September – November) and persists till early January.
The process of ranging from the peninsula in the month of October and from the extreme south-eastern tip by December covers a span of 3-months.The south-west monsoons withdraw from the Coromandel coast in mid-December.
Also, in Punjab, it is observed that the south-west monsoons withdraw from there in the second week of September. Interestingly this withdrawal of the retreating monsoon stretches eventually and takes a longer duration compared to the advancing one.
Unlike the erratic climate during the actual monsoon rains, the onset of retreating monsoon, causes clearer skies as the clouds disappear.
However, the disappearance of clouds makes the climate of various places hotter gradually. It is also hinted by severe tropical cyclones emerging from the Bay of Bengal. The month of October-November is prone to severe cyclones.
Temperature and pressure stats
With the withdrawal of winds and showers in the retreating monsoon, the temperature comes down sharply.
When the monsoon starts retreating towards the south, the pressure gradient gets low. Local pressure conditions generally become directly proportional to the wind flow during that time.
Usually for the south eastern coast of India, it is during the retreating monsoon season that rainfall becomes abundant – tropical cyclones also occur during this time. The state of Tamil Nadu receives almost half of its annual rainfall during this time, hence even termed as the winter monsoon or the northeast monsoons.
The retreating monsoon brings scattered amounts of rainfall to different places across India.
Heavy rainfall is experienced in the western part of Western Ghats (200-400cm) and North-eastern India (Assam, Arunachal Pradesh, Sikkim), while Karnataka, Gujarat and Maharashtra receive low rainfall from the retreating monsoon.
As seen in the study in almost all the eight regions, there are two deciding factors causing the phenomenon.
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.
When the surface temperature rises, it heats the overlying air, which gradually becomes less dense than the surrounding air and begins to rise. This factor is especially noticed from September to February because of the role played by sea surface temperature or water temperature.
The discovery that these autumn monsoon regions are segments of a global pattern holds high value as it allows study into how global factors like climate change might affect them, said Ramesh.
“To predict what could happen to these areas during climate change, we need to understand the fundamental processes that give rise to the autumn monsoons. This study is a first step towards that goal.”