‘North Atlantic air currents trigger droughts in India’

Air currents in faraway North Atlantic region can trigger adverse weather events, particularly droughts, during the Indian monsoon season, found a team of Indian researchers, helping meteorologists improve forecasting the timing and severity of Indian droughts in future.

Between 1900 and 2015, India suffered 23 droughts but only 13 were associated with El Nino, unusual heating of the waters in the equatorial Pacific Ocean. What triggered the other 10, including the 2014 drought, remained a mystery.

Significantly, an El Nino drought and a non-El Nino drought follow distinctly different patterns. During an El Nino drought, the rainfall deficit — departure from a long-term average — sets in early around mid-June and becomes progressively worse. By mid-August, the deficit spreads across the country and triggers a drought. The non-El Nino drought, on the other hand, is a sub-seasonal drought which sets in the middle of August.

Noticing this, a team of researchers led by V Venugopal and Jai Sukhatme at the Centre for Atmospheric and Oceanic Sciences (CAOS) of the Indian Institute of Science, Bengaluru, wanted to explore this further.

The Rossby wave

The CAOS researchers noticed an unusual atmospheric disturbance in the midlatitudes. It emerged from winds in the upper atmosphere interacting with a deep cyclonic circulation above abnormally cold North Atlantic waters. The resulting wave of air currents, called a Rossby wave, curved down from the North Atlantic — squeezed in by the Tibetan plateau — and hit the Indian sub-continent around mid-August, suppressing rainfall and throwing off the monsoon that was trying to recover from the June slump.

Their study appeared in the prestigious journal Science last week.

“The influence of midlatitudes on Indian monsoon variability has been known. However, that it can trigger or result in a drought is a new finding. In addition, people also knew that droughts occurred even though the equatorial Pacific waters were not anomalously warm (i.e., no El Niño),” said Venugopal.

Besides, they were also the first team to discover the distinct “flavours” of evolution of the two droughts – subseasonal deficit in non El Niño droughts vs the “classical” notion of seasonal deficit in El Niño droughts.

“The roles of Indian ocean and Pacific ocean dominate most discussions surrounding monsoon forecast, perhaps for justifiable reasons. Our work suggests that midlatitudes also have a role to play and deserve attention,” Venugopal told BusinessLine .

Sukhatme, an associate professor at CAOS, said there are two main findings from the study. Firstly, droughts can be subseasonal in character, especially in the absence of El Niño.

Secondly, late-August breaks in the monsoon can be can be traced to the Rossby wave.

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