Rainfall Pattern in Assam

What is the issue?

This monsoon, some areas received deficit rainfall while Assam received excess rainfall.

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 three days ahead of the normal date in Kerala this year, but it turned sluggish on its western branch’s upward journey.
The central India suffered a deficit while the east and north-eastern parts battled excess rain leading to widespread floods in Assam and Meghalaya.
The key monsoon months are July and August and they bring nearly two-thirds of the monsoon rains.

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.
Meteorologists said the recent episode of heavy rainfall underlined the
- Presence of the east-west trough in the lower levels of the atmosphere over the region
- Incursion of large-scale moisture due to strong southerly and south-westerly winds from the Bay of Bengal
According to the IMD, Assam received 41% above normal rainfall during the pre-monsoon season (March to May), received 71% more than normal rainfall up to June 25.
A 2018 study by Indian Institute of Tropical Meteorology revealed that Assam had been witnessing a decreasing trend in the average monsoon rainfall since 1870 while experiencing sudden downpour days leading to frequent flooding.

Ecological and climate difference- Assam’s valleys experience both excessive and insufficient rainfall from time to time due to ecological and climate difference from one place to another.
Climate change- Climate change increases the water and surface temperature of Arabian Sea and Bay of Bengal by up to 2 degrees, forming low-pressure areas resulting in heavy rains.
Black carbon emissions- Aerosols, including black carbon, released by biomass burning, leads to a decrease in low-intensity rainfall while pushing up severe rain in the pre-monsoon season in northeast India.

Depressions- The important synoptic disturbances during the monsoons over India are lows, depressions, etc. that form mostly over the Bay of Bengal and produce a large volume of rainfall.
Position of offshore trough- The other synoptic disturbance which affects monsoon rainfall significantly is the position of offshore trough or vortex along the west coast of India.
Global phenomena- Monsoon rainfall in India is known to be affected by global phenomena such as El Nino or La Nina.
Other factors such as the Indian Ocean Dipole and Madden-Julian Oscillation also influence monsoon rainfall.

References

Quick facts

El Niño- El Niño is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean.
- In India, El Nino has been found to have strong links in suppressing the monsoon rainfall.
La Nina- La Nina refers to the periodic cooling of ocean surface temperatures in the central and east-central equatorial Pacific.
- La Nina has been found to be helpful in bringing good rainfall to India.
Indian Ocean Dipole- IOD is defined by the difference in sea surface temperature between two areas (or poles) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia.
- Positive event- warmer sea surface temperatures in the western Indian Ocean relative to the east bringing more rainfall to India
- Negative event- cooler sea surface temperatures in the western Indian Ocean relative to the east bringing less rainfall to India
Madden-Julian Oscillation (MJO)- The MJO is a moving system of wind, cloud and pressure that brings rain as it circles around the equator.
- MJO goes around the globe in 30-60 days on average. Sometimes, it can take 90 days.
- In the active phase, MJO results in more than average rainfall for that time of the year, while in the suppressed phase, the area receives less than average rainfall.