Reasons behind rainfall deficit in some areas and excess in Assam this monsoon

• Assam received 1,891.9 mm of rainfall from March 1 to June 24, just 347.5 mm less than the annual precipitation the State receives.
• The outcome has been devastating.
• Meteorologists and climate change specialists attribute the high pre-monsoon and monsoon rains to several factors.

Position of monsoon this year

• The country received 2% less rain this year than it usually does between June 1 and June 23 every year.
• The total rainfall was brought down by 34% over central India and 15% over peninsular India compared to the 32% more received by the east and northeast and 7% more by northwest India.
• During the monsoons, whenever moisture-laden southerly or south-westerly winds from the Bay of Bengal hit the region’s east-west oriented mountain ranges, Arunachal Pradesh, Assam, and Meghalaya receive more rainfall in comparison to other States of the north-eastern region.
• The recent episode of heavy rainfall underlined the presence of the east-west trough in the lower levels of the atmosphere over the region and the incursion of large-scale moisture due to strong southerly and south-westerly winds from the Bay of Bengal.

Factors which determine rainfall pattern

• Ecological and climate differences from one place to another
• Climate change is said to have increased the water and surface temperature of the Arabian Sea and the Bay of Bengal by up to 2 degrees, causing the frequent formation of low-pressure areas and cyclonic circulations, resulting in heavy rains.
• Aerosols, including black carbon, released by biomass burning, influence the western part of northeast India close to the Indo-Gangetic Plain the most.
• Rising black carbon emissions lead to a decrease in low-intensity rainfall while pushing up severe rain in the pre-monsoon season in northeast India.

Reasons behind monsoon getting late this year

• The seasonal monsoon winds are an extremely complex and intricate combination of physical processes that operate not only in the atmosphere but also involve land and ocean.
• In India, June 1 is regarded as the date of arrival of the monsoon, which accounts for about 80% of the rainfall in the country.
• The monsoon landed early in Kerala this year, three days ahead of the normal date of June 1, but then it turned sluggish on its western branch’s upward journey.
• The most important synoptic disturbances during the monsoons over India are disturbances (lows, depressions, etc.) that form mostly over the Bay of Bengal, move westwards or west north-westwards along the monsoon trough, and produce a large volume of rainfall.
• The other synoptic disturbance which affects monsoon rainfall significantly is the position of offshore trough or vortex along the west coast of India.
• Monsoon rainfall in India is known to be affected by global phenomena such as El Nino or La Nina — large-scale warming or cooling events of the sea surface.
• Other factors such as the Indian Ocean Dipole and Madden-Julian Oscillation also influence monsoon rainfall.

Exam Track

Prelims takeaway

• El Nino and La Nina
• ENSO
• Monsoon & its types

Mains track

Q. Explain the reasons behind rainfall deficit in some areas and excess in Assam this monsoon