Monsoon-El Nino relationship status in India: It's complicated

NEW DELHI: In 1997, the strongest El Nino in decades drove up temperatures across the world, making it the century's hottest year. Dire predictions were made for the Indian monsoon. But the 1997 monsoon ended at 2% above normal. Five years later, a mild El Nino, less than half the intensity of the 1997 event, coincided with the worst drought in India in 15 years.

The relationship between El Nino and the Indian summer monsoon is complicated, at the very least. The most severe droughts have all been in El Nino years, but the strongest El Ninos haven't always depressed rains here.

Consider this. Since 1980, the four severest drought years -1982, 1987, 2002 and 2009 - were ones that witnessed El Nino events. Three El Nino years 1994, 1997 and 2006 didn't impact the monsoon at all.Three other droughts during this period, in 1986, 2004 and 2014 were not in 'official' El Nino years. Year 1991 saw an El Nino event, and a near-drought -90.7% rainfall.

A drought, which IMD calls deficient rains, is defined as monsoon rainfall that's at least 10% below the long-term average.



"The connection between El Nino and the Indian summer rains is still a very live issue. The understanding's been complicated by recent complexities wherein at least two types of El Nino have been identified. It's an ongoing debate," says Krishna AchutaRao in IIT Delhi's Centre for Atmospheric Sciences.

During an El Nino, surface waters in east and central equatorial Pacific Ocean warm up abnormally leading to changes in wind patterns that impact weather across large parts of the globe. The region of heavy summer rainfall usually shifts east from Indonesia to central Pacific or to the coast of Peru in South America, disrupting air circulations that affect the India monsoon.

Each El Nino is a unique event, some trends have been identified. For instance, the El Nino-monsoon relationship waxes and wanes. It has weakened in the past 20 years or so.

In a 2006 paper in Science, researchers K Krishna Kumar and Balaji Rajagopalan, with others, showed that the location of warm waters in the Pacific was a key factor in determining if an El Nino would affect the monsoon or not. Since then, El Ninos have been classified into central Pacific (CP or Modoki) that appears to have stronger links with the monsoon, and eastern Pacific (EP or traditional).

What negates the effects of an El Nino on the monsoon? Called the Indian Ocean Dipole (IOD), this factor is the difference in sea surface temperatures between west and east sections of the Indian Ocean. A positive IOD, warmer waters in the west, is thought to have cancelled the adverse impact of El Ninos in 1997 and 2006.

How the monsoon, El Nino and the Indian Ocean Dipole interact and impact each other remains a subject of research. India will continue to live with the uncertainty of the relationship.