Explained | What accounts for deficit in some areas and excess rainfall in Assam this monsoon?

What are the factors determining rainfall pattern? Was the monsoon late this year?

The story so far: 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. According to the India Meteorological Department (IMD), Assam’s annual normal rainfall between 1961 and 2010 has been 2,239.4 mm. The outcome has been devastating. A total of 117 people have died since April, 17 of them in landslides. An arterial railway track linking southern Assam, Manipur, Mizoram and Tripura has been washed away, 55 lakh people have been affected across more than 5,000 villages and a town, Silchar, has been flooded for more than a week. At least 38 people have died in flash floods and landslides in Arunachal Pradesh and Meghalaya since April. Meteorologists and climate change specialists attribute the high pre-monsoon and monsoon rains to several factors.

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. According to the IMD, the rain deficiency — private forecaster Skymet pegs the deficit at 4% till now — is expected to be overcome by the first week of July. 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. 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 and the incursion of large-scale moisture due to strong southerly and south-westerly winds from the Bay of Bengal.

Assam, which receives rainfall beyond the June-September monsoon phase, does not always get above-normal or excess rain. But this year, according to the IMD, it received 41% above normal rainfall during the pre-monsoon season (March to May), and it has received 71% more than normal rainfall up to June 25. A 2018 study based on IMD data conducted by Pune-based Indian Institute of Tropical Meteorology revealed that the State had been witnessing a significant decreasing trend in the average monsoon rainfall since 1870 while experiencing sudden downpour days leading to frequent flooding. It found that the average rainfall deficiency was 5.95 mm per decade between 1981 and 2016. A 2017 study published in the International Journal of Current Microbiology and Applied Sciences said 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 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. A recent study by the Indian Institute of Technology, Guwahati said 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, it said, leads to a decrease in low-intensity rainfall while pushing up severe rain in the pre-monsoon season in northeast India.

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. But if central India suffered a deficit, the east and north-eastern parts battled a diametrically opposite problem — excess rain — leading to widespread floods in Assam and Meghalaya. Historically, June rainfall is patchy and contributes less than 18% of the total monsoon rainfall. The key monsoon months are July and August and they bring nearly two-thirds of the monsoon rains. 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.

With inputs from Jacob Koshy