With the classical El Niño, the tropical eastern Pacific near the South American coast becomes warmer than usual while the western side of the ocean cools.
This year, the monsoon has got off to an unpropitious start, with last month's nationwide rainfall showing a deficit of 43 per cent. Since 1901, the June rainfall had a shortfall of over 40 per cent in only four previous years. The last time this occurred was in 2009 when rains were poor in the following three months as well and the monsoon ended in a drought.
Some scientists are seeing similarities between the El Niño, the exceptional warming of the tropical Pacific Ocean, that is occurring this year and one that turned up five years back.
With the classical El Niño, the tropical eastern Pacific close to the coast of South America becomes warmer than usual while the western side of the ocean, near Indonesia, cools. In recently years, scientists have drawn a distinction between this sort of El Niño and ones where the warming is principally in the central Pacific. The latter, it is argued, has a greater impact on the monsoon, reducing rains over India, than the former.
But the El Niño that manifested in 2009 was unique, according to K. Ashok of the Indian Institute of Tropical Meteorology (IITM) in Pune. That year, from around June to almost October, the entire Pacific basin turned abnormally warm, with no cooling anywhere.
In an assessment issued a week back, the World Meteorological Organization noted that this year's developing El Niño has a “somewhat unusual pattern” with sea surface temperatures that are above average across virtually the entire tropical Pacific, not just in the eastern and central portions.
In a paper published in Geophysical Research Letters in 2012, Dr. Ashok, along with T. P. Sabin, and P. Swapna, both of them also at IITM, as well as Raghu Murtugudde, professor of atmospheric and oceanic sciences at the University of Maryland in the U.S, examined the effect that basin-wide warming in the Pacific could have. A climate model run with the Pacific sea surface temperatures of 2009 reproduced many features seen that year, including reduced rainfall over India.
“The tricky question is how the El Niño will evolve this year, whether the basin-wide warming will persist in the coming months and the impact that will have on the monsoon,” remarked Dr. Murtugudde.
During the evolution of a typical El Niño, as the western side of tropical Pacific cools and eastern part warms, trade winds, which blow from east to west over that ocean, weaken considerably and sometimes even reverse direction. This shift in wind pattern aids the growth of the El Niño.
In an El Niño with basin-wide warming, the development of such a sea-surface temperature gradient and the accompanying change in winds was disrupted. Consequently, the El Niño may not strengthen as it ordinarily would, he pointed out. However, even a weaker-than-expected El Niño might wreck the monsoon.
What matters most for the Indian monsoon are conditions over equatorial central Pacific, according to Sulochana Gadgil, a leading atmospheric scientist who was at the Indian Institute of Science in Bangalore.
Atmospheric conditions over that part of the ocean have been adverse for the monsoon this year.
Moreover, “a developing El Niño can have a major impact as seen in 2002 and 2004.”
The evolving El Niño was the primary factor keeping the Indian monsoon suppressed this year, said D.R. Sikka, a distinguished meteorologist who retired as director of IITM and was the first to establish the link between El Niño and droughts over India.
Weather models indicated that rainfall in July could also be somewhat below normal. “We are hoping that some recovery will take place in the month of August.”