A study on the factors that influence extreme rainfall over the South West coast of India in the monsoons carried out by researchers at the Cochin University of Science and Technology has showed a systematically evolving chain of events, culminating in the extreme rainfall event.
The research was undertaken by a team at the Advanced Centre for Atmospheric Radar Research (ACARR) using meteorological datasets ranging over the last three decades. “The work draws attention in the aftermath of the recent unprecedented pattern of rainfall activity and consequent torrential rains and deluge, especially in Kerala during the monsoons of 2018, 2019, 2020 and 2021. The study shows a statistically significant increase in trend of extreme rains over the South West coast,” said Dr. Ajil Kottayil, scientist at ACARR.
“Monsoon depressions develop over the Bay of Bengal, which propagate in the north-westward direction, intensifying the convection and vorticity over the north-west Bay of Bengal. As a result, the meridional gradient between central India and South west coast is steepened due to which the monsoon low level jet streams intensify. The increase in the humidity flux over the South west coast causes deep convective activity that end up as torrential rains. Usually, the formation of vortices is normal during monsoons but how these get modified due to depressions determine the strength of rainfall,” he said.
The study, which was published in the International Journal of Climatology, found that a heavy rainfall event during monsoon is neither random nor sudden but is the consequence of pre-conditioning that can be quantified by factors that strengthen over a five-day period prior to the day of heavy rainfall.
The variables identified include large scale convection, lower tropospheric cyclonic circulation, monsoon low level jet streams, humidity and meridional mean sea level pressure gradient. Moreover, these incidents are not a part of the active phase of the monsoon but are often associated with north westward propagating monsoon depressions over the Bay of Bengal, it said.
