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The just concluded (March to May) pre-monsoon season may have helped entrench an undeniable trend of nagging rainfall deficits over North-East India even when the rest of the country witnesses a rainfall normal or even surplus season.
This pre-monsoon left behind deficits of (minus) 55 per cent in Nagaland-Manipur-Mizoram-Tripura, (minus) 30 per cent in Assam and Meghalaya that houses some of the world’s wettest places, and (minus) 27 per cent in Arunachal Pradesh during March-May when the country as a whole had a rare +18 per cent surplus.
Also read: First monsoon low likely next week: IMD
Natural ‘lift’ to clouds
Undulating terrain and lofty peaks here provide natural lift to rain-bearing clouds from the Bay of Bengal to pour down in torrents. But rainfall trends in the past few years have just failed to convert. The second long-range monsoon forecast issued by India Meteorological Department on Tuesday too signals deficit.
Annual rainfall (January-December) also was not much to write home about both in 2019 and 2020 when the country as a whole received a surplus of 10 per cent in the two consecutive years.
Traditionally, there is hardly any area in the region where annual monsoon rainfall amounts to less than 1,000 mm. Not any longer. This pre-monsoon provides the best indicator of how things may have been reversed over the past decades even as it delivered surplus elsewhere.
High rainfall normal
Researchers note that the sustained rainfall deficits in the region are despite the occasional heavy rainfall and flooding of the Brahmaputra and other rivers. Individual states here have traditionally high normal rainfall averages just as the case is with the country’s South-West coast (Kerala and Coastal Karnataka).
So, any deviation from these high rainfall normals over the hilly terrain, with the Head Bay of Bengal located not too far to the South-West, get magnified in the final analysis. But these deviations have become demonstrably more frequent in the last few years.
Why is this so? The usual suspect is global warming/climate change. But GP Sharma, President, Meteorology and Climate Change, Skymet Weather, is of the view that changes in the behavioural pattern of typically localised meteorological and weather factors may also be playing their roles.
Number of Bay systems
According to him, the frequency of formation of rain-bearing systems in the Bay (low-pressure areas or depressions) and their track matter a lot with respect to rainfall prospects in North-East India. Another important weather determinant in the region is wind and the direction to which it blows.
It can even be said that winds have almost complete sway over weather in the North-East. If they’re westerly or south-westerly (generally the Bay), it means rains. There is another component called ‘valley winds’ (blowing from the opposite direction and continental) which are dry, and herald sunny weather.
Undeniable wind factor
“As an aviation meteorologist, whenever I see ‘Kolkata’ winds or ‘Silchar’ winds on the charts, it means pouring rain. During the formation stage of a system in the Bay, we get good (sunny) weather. After it crosses the coast in the next six hours or so, the winds become westerlies to south-westerlies and bring rain.”
The number of systems forming in the Bay too counts; so too their frequency and track. A landfall point over central Odisha or adjacent West Bengal is most ideal for rains to happen over the North-East. However, during the last three seasons or so, this may not have been the case, Sharma told BusinessLine .
Shifting tracks, a worry
“Most of them passed through the central parts of the East Coast — much southerly tracks and away from the North-East. They did not follow the normal tracks along the monsoon trough across Odisha, Bihar Uttar Pradesh. Instead, they chose to move across Chhattisgarh or Maharshtra or parts of Central India.”
Another major occasion when it rains heavily in the North-East is during the ‘break monsoon’ phase. Here, the monsoon trough shifts north, closer to the foothills of the Himalayas. “But one didn’t find any break monsoon phase last year. Even in 2019, there was no pronounced break period,” Sharma recalled.
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