Floods in June, 2023 devastated the state of Assam in northeast India. Credit: Biju Boro/AFP via Getty Images
The summer monsoon season in northeast India is at least a month longer than in central India, the core monsoon zone1. But increasing greenhouse gas emissions are likely to shorten the northeast rainy season by five days and trigger frequent, extreme rainfall.
The Indian summer monsoon, from June to September, brings most of the yearly rainfall. Scientists at Cotton University, Guwahati, suggest that the monsoon season in northeast India begins in mid-May and could last up to 155 days, surpassing the 120-day season seen in central India.
To define the onset, withdrawal and length of the rainy season in northeast India, researchers used the rate of temperature change with altitude in the troposphere as a proxy for the heat source over the region. They looked at the yearly rainfall cycle using observational and reanalysis data from 1901 to 2015, and CMIP6 model data sets from 1850 to 2014 for historical simulations.
They found that the El Niño and Southern Oscillation (ENSO) significantly affects the duration of the rainy season over northeast India. In a warming environment, El Niño, the warm phase of ENSO, shortens and dries rainfall, while La Niña, the cool phase of ENSO, lengthens and intensifies it.
The predictability of the northeast rainy season depends on the accuracy of ENSO forecasts and the combined effect of other climate phenomena, such as the Indian Ocean Dipole and the Madden-Julian Oscillation.
