Cold Atlantic water disrupts monsoon rainfall in India
doi:10.1038/nindia.2020.187 Published online 17 December 2020
Cold water in the North Atlantic Ocean may derail the Indian monsoon, resulting in droughts that can have devastating effects on agriculture and the economy across much of India, a team of climatologists has revealed1.
The finding offers an avenue for improved predictability of Indian summer monsoon rainfall (ISMR), the researchers say.
El Nino, a periodic warming of the sea-surface temperature in the Pacific, has been linked to variability of ISMR and droughts. However, it remains unknown what triggers droughts, causing reduced rainfall even in the absence of El Nino.
To find out, scientists from the Indian Institute of Science in Bangalore and Cotton University in Guwahati, both in India, analysed the past patterns of ISMR and droughts across India. They focused on the droughts that occurred in the absence of warm waters in the Pacific.
The researchers identified a spell of drought without El Nino in late August. They linked this drought to a strong, upper-level cyclonic circulation to the west, in the North Atlantic region. These upper-level winds interact with a specific North Atlantic vorticity anomaly, creating a wavetrain that moves towards East Asia, leaving a prominent signature over the Indian region. This wavetrain then disrupts the monsoon there.
Almost half of all sub-seasonal droughts with a steep decline in late-season rainfall appears to be linked to the wavetrain and coincide with years when the North Atlantic was anomalously cold, the researchers found.
1. Borah, P. J. et al. Indian monsoon derailed by a North Atlantic wavetrain. Science. 370, 1335-1338 (2020)