Eight regions across the world experience post-summer rainfall like India, says study
Rainfall during retreating monsoon, which parts of South India experiences every year, is not a local anomaly and is global in nature and scale, according to a recent study by the University of Sydney.
In India, retreating monsoon is the withdrawal of south-west monsoon winds from North India. The Global Coherent Pattern of Autumn Monsoon Precipitation by Nandini Ramesh, Quentin Nicolas and William R Boos 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.
Ramesh told DownToEarth:
The discovery that these are part of a global pattern and not one-off occurances means they can be systematically studied, which will help us understand how these communities could be affected by climate change.
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Peninsular India and parts of SouthEast Asia are among the eight regions examined in the study. The diagram shows the "fraction of the annual rainfall received from September to February”.
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, as the diagram illustrates.
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 favourable from September to February because of the role played by sea surface temperature or water temperature.
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, 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."
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