Weakening Trend of Summer Monsoons in India

Weakening Trend of Summer Monsoons in India

Question: Summer monsoon has shown a weakening trend over the past century in India. Elaborate.

• Summer monsoon has shown a lower trend over the past 100 years with less rainfall over large regions of the Indian subcontinent

• Monsoon is caused when land heats up more than ocean and warm air over it rises and results in low pressure

• The low pressure belt attracts rain bearing winds from a cooler ocean to blow on to the land and leads to rainfall

• Strong thermal contrast between land and ocean results in strong monsoon and vice versa.

• Study by Dr. R.M. All and scientists at Centre for Climate Change Research, Indian Institute of Tropical Meteorology have found that this thermal contrast has been lessening in recent times

• Land has become cooler and ocean warmer as the monsoon has shown a decreasing trend over the past 100 years

• Under global warming scenario, land temperature should increase substantially in the hot summers and drive the monsoon

• Observed data from 1870s from the IMD and other resources was studied and it was found that from 1901 to 2012, rainfall has been lessening over central South Asia from south of Pakistan through Central India to Bangladesh

• Decrease is of considerable significance in C. India where agriculture is mostly rain fed

• Several studies have found extreme rainfall events have increased over some regions while the frequency of moderate to heavy rainfall has decreased over the Indian subcontinent

• Reduction in land sea temperature contrast has been associated with strong warming on the Indian Ocean in a multi decadal scale due to global warming induced climate change

• Decrease in thermal contrast surface temperature trends is also seen within the upper atmosphere such as warming trends in the ocean surface being transferred to the atmosphere above via convective processes

• Decrease in land sea temperature difference is also the result of suppressed warming over the Indian land area due to rising aerosol levels

• Aerosols in the atmosphere reflect the sun’s heat into space and create a cooling effect

• Warming Indian ocean has also weakened monsoon circulation and large scale upward motion of warm moist air over the equatorial ocean

• This is compensated by subsidence of dry air over the Indian subcontinent, eliminating convection and rainfall in India

• Warming India ocean has caused more rain in the water than on land leading to the drying up of the Indian subcontinent over a span of time

Facts and Stats

• There has been a reduction of 10 to 20 percent in mean rainfall in central India

• Monsoon rainfall is weakening over South Asia during the past century since 1950s

• Surface warming in the Indian Ocean especially western regions have reached values of 1.2 degrees celsius during the past century, much lower than warming trends in other tropical oceans
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