In 1997, even before the monsoon began, waters of the equatorial Pacific Ocean had warmed sharply, leading to one of the most powerful El Nino events in the last century. As such a phenomenon typically suppresses monsoon rains over India, a severe drought was widely predicted. As it turned out, the monsoon that year ended with above average rains. Just five years later, in 2002, a moderate El Nino unexpectedly wrecked the monsoon and produced a massive drought.

With another El Nino developing in the Pacific, there is considerable worry over its impact on this year’s monsoon. With the onset of rains over Kerala delayed by a few days and the monsoon’s subsequent northward progression stymied, those concerns are bound to escalate.

Although not every El Nino retards the monsoon, the Pacific becoming exceptionally warm greatly heightens the risk of a monsoon turning deficient. When the Pacific is neither unusually warm nor cool, there is only a 16 per cent chance of a monsoon ending in a drought. Rainfall data for 126 years indicates that the odds of a drought jump to over 40 per cent when there is an El Nino.

Almost a century has passed since Sir Gilbert Walker, then Director-General of Observatories in India, found indications that what happens far away in the Pacific affects the monsoon. Scientific understanding of what is known as the ‘El Nino Southern Oscillation’ (ENSO) has grown in leaps and bounds over recent decades.

Nevertheless, predicting how an El Nino will shape up and, more importantly for India, forecasting what might thereby happen to the monsoon are still challenges.

There could be several factors influencing the interplay between an El Nino and the monsoon. For one thing, which part of the Pacific warms has an impact on the monsoon.

El Ninos come in two ‘flavours,’ noted K. Krishna Kumar, who was then with the Indian Institute of the Tropical Meteorology (IITM) in Pune, along with a group of other scientists in a paper published in Science in 2006.

In 1997, the eastern Pacific had become exceptionally warm, thereby limiting the atmospheric circulation changes that adversely affected the monsoon. It was when the sea surface temperature anomalies were highest in the central Pacific that an El Nino had drought-producing effects over India.

Central Pacific El Ninos had appeared in 2002 as well as in 2004 and 2009, with all three years ending in drought, said Dr. Krishna Kumar, currently a consultant with the Qatar Meteorology Department.

Not clear

As this point in time, it was difficult to say which sort of El Nino would manifest this year, he told this correspondent. “The current generation of climate models do not have the capacity to distinguish whether a central or eastern Pacific El Nino will evolve.”

Besides, what happens in the Indian Ocean also shapes the course of the monsoon.

Toshio Yamagata’s research group at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has studied the ‘Indian Ocean Dipole’ (IOD) and its effect on rains over India. During a ‘positive IOD’, the eastern equatorial Indian Ocean off Sumatra in Indonesia becomes colder than normal while the western tropical part of the ocean near the African coast becomes unusually warm. Such an event has been found to be beneficial for the monsoon. On the other hand, a ‘negative IOD,’ when temperatures at either end of the Indian Ocean swing in the opposite direction, hampers the monsoon.

An IOD can counter or worsen an El Nino’s impact on the monsoon, according to a paper by K. Ashok, currently at IITM in Pune, along with Dr. Yamagata that was published in Geophysical Research Letters in 2001.

A positive IOD had facilitated normal or excess rainfall over India in 1983, 1994 and 1997 despite an El Nino in those years. But during years such as 1992, a negative IOD and El Nino had cooperatively produced deficit rainfall.

The latest prediction from the JAMSTEC group suggests a ‘very high’ probability of a negative IOD turning up this year. Sulochana Gadgil, a much respected atmospheric scientist who was with the Indian Instititute of Science (IISc) in Bangalore, has along with colleagues been examining wind patterns over the equatorial Indian Ocean that are associated with changes in cloud formation.

During the positive phase of the ‘Equatorial Indian Ocean Oscillation (EQUINOO),’ there is enhanced cloud formation and rainfall in western part of the equatorial ocean near the African coast while such activity is suppressed near Sumatra.

This phase is associated with good rains over India. Its negative phase, when cloud formation and rainfall flares up near Indonesia, retards rains over India.

While EQUINOO and IOD go in step during strong positive IOD events, such as in 1994 and 1997, they do not always do so, according to Prof. Gadgil. The severe drought of 2002, for instance, occurred when a moderate El Nino as well as strong negative EQUINOO together took a toll on the monsoon; that year, the IOD was slightly positive.

The fate of the monsoon depends to a large extent on the Pacific Ocean system and EQUINOO, she argues. While climate models can generate reasonable predictions of events in the Pacific and its impact on the monsoon, they are not able to do the same for EQUINOO.

“The monsoon has a mind of its own,” cautioned Raghu Murtugudde, professor of atmospheric and oceanic sciences at the University of Maryland in the U.S. It was not a one-way street with an El Nino affecting the monsoon. What happened to the monsoon in the key months of July and August might determine how the El Nino evolved.

“We need to be able to forecast the monsoon without relying totally on the predictability of El Nino.”

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