Rise in extreme rainfall events due to an unusual behaviour of the Arabian Sea

An unusual behaviour of the Arabian Sea has triggered a three-fold rise in “widespread extreme rainfall” in central India since the 1950s, scientists report.
 
In the last 65 years, there has been a three-fold rise in "widespread extreme rainfall” events over the entire central belt of India from Gujarat and Maharashtra in the west coast to Odisha and Assam in the east, leading to a steady rise in the number of flash floods with significant socioeconomic losses.
 
The rise in extreme rainfall events also occurred along some parts of the Western Ghats including south Kerala, north Karnataka and Goa.
 
It has long been a scientific puzzle to figure out where the much-needed moisture is coming from since the total monsoon rainfall and local moisture availability decreased in the recent decades.
 
Indian weather scientists in collaboration with their colleagues from USA and France now cracked the code. They demonstrated that the extra moisture was flowing in from the Arabian Sea, which changed its behaviour possibly due to global warming.
 
“Due to rapid warming in the Arabian Sea, the monsoon winds are becoming unstable. They are exhibiting a lot of fluctuations - so some days they are weak, but a few days they are very strong. This results in sudden surges of moisture transport to India,” team leader Roxy Mathew Koll from the Indian Institute of Tropical Meteorology, Pune told DH.
 
Floods attributed to extreme rain events in India alone amounted to losses of about $ 3 billion per year, which is 10% of the global economic losses. There have been 268 reported flooding events in India over 1950–2015 affecting about 825 million people, leaving 17 million homeless and killing 69,000 people
 
It was presumed that many of these heavy rainfall spells result from low-pressure systems that develop in the Bay of Bengal and move northwestward bringing moisture into the central India.
 
But with observed records showing a decline in the number of these low-pressure systems, it was a puzzle for the scientists to figure out how the extreme rains are on a rise despite a weakened monsoon and a decrease in the number of low-pressure systems over central India.
 
“The findings suggest the ocean-atmospheric conditions which are precursors to these extreme rainfall events and occur more than a week ahead. This will help in forecasting future extreme events,” he said.
 
The findings have been published in the journal Nature Communications on Tuesday.
 
The study demonstrates the monsoon winds (westerlies) over the northern Arabian Sea exhibit increased variability (large fluctuations), driving surges of moisture supply, leading to extreme rain episodes across the entire central Indian belt. This is due to the increased warming north of the Arabian Sea, as a result of increased human activities including carbon dioxide emissions.
 
The warm ocean temperatures in the northern Arabian Sea and the adjacent northwest India and Pakistan, resulting in increased moisture and also large fluctuations of the monsoon westerly wind.
 
Asked how it's different from previous studies that also predicted an increase in the number of extreme rainfall events in India, Koll said, “There are several differences - (a) This is the only study which shows the "widespread" nature of extreme events across central India, which can lead to large-scale floods. (b) This is the only study which has provided the mechanism which has been causing these extremes (c) Most of the earlier studies point to the link between local temperatures and extremes, while we show that non-local sources such as the Arabian Sea are important.”