Why it rains, or does not | La Niña, El Niño: Opposites attract, repulse monsoon

El Niño and La Niña are mutually opposite phenomena, during which an abnormal warming or cooling of sea surface temperatures is observed in the Pacific Ocean along the equator, off the coast of South America.

Written by Amitabh Sinha | Updated: October 3, 2016 9:08 am
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For the third year in a row, the monsoon’s performance has been decided predominantly by ocean phenomena occurring far away in the Pacific, but which are now part of well-known monsoon terminology — El Niño and La Niña.

Thanks to the “Godzilla” El Niño — one of the longest and strongest in recent decades — 2014 and 2015 had witnessed droughts. And the slightly less-than-expected, and forecast, cumulative rain this year — despite a ‘normal’ monsoon overall — is being attributed to the fact that the anticipated La Niña didn’t develop over the Pacific Ocean.

El Niño and La Niña are mutually opposite phenomena, during which an abnormal warming or cooling of sea surface temperatures is observed in the Pacific Ocean along the equator, off the coast of South America. Together they constitute what is known as the El Niño-Southern Oscillation system, or ENSO for short. The two phenomena impact weather and climate events across the world, including the Indian monsoon. The warming of the Pacific — El Niño — is known to suppress rainfall over India, especially in the northwestern parts; La Niña has been observed to help the monsoon.

After the 2014-15 El Niño weakened in the first quarter of 2016, most global climate models, including the one used by the India Meteorological Department (IMD), predicted the setting-in of La Niña by the middle of the year. In its April 12 bulletin, the Bureau of Meteorology of Australia noted that “five of the eight surveyed (climate) models” were suggesting that “La Niña was likely”, while the other three were neutral. But it also said ENSO forecasts made in April “tend(ed) to have a lower accuracy than at other times”.

In forecasting 106% (of normal) rain this monsoon, the IMD model had factored in La Niña. But the La Niña did not develop, and the monsoon season officially ended on Friday with 97% (of normal) rain.

“The chances of La Niña (setting in by) this fall (September) were 75% in June, but they fell to around 55-60% in July, and again in August to 40%. Sea surface temperatures were cooling, but the pace of cooling has slowed,” the National Oceanic and Atmospheric Administration says on its website. The latest NOAA outlook says ENSO neutral conditions are likely to continue until winter.

ENSO not ‘special’

Despite the strong correlation observed between ENSO events and seasonal rainfall in India, similar heating or cooling of sea surfaces in other (non-Pacific) oceans too likely influence the monsoon — but these relationships are not very well understood.

“A recent study links Atlantic Ocean sea surface temperatures to the Indian monsoon. These are active areas of research,” Arpita Mondal of the climate studies programme at IIT Bombay said.

That said, Pacific Ocean sea surface temperatures do seem to have the strongest relationship with the Indian monsoon — as a result of which its influence is sometimes over-emphasised. “When we look for the teleconnection between the Indian monsoon and the world’s oceans, the strongest signal is the equatorial east Pacific (where ENSO conditions play out). In other regions, like the Indian Ocean or the Atlantic Ocean, the teleconnection is not strong and robust,” J Srinivasan of the Divecha Centre of Climate Change at the Indian Institute of Science said.

“But it is also true that most climate models tend to over-emphasise the link to equatorial east Pacific, and do not account for the role of other regions. That’s because we don’t understand the mechanisms by which other regions influence our monsoon,” he said.

The Indian Ocean dipole (IOD), a similar phenomenon nearer home, is also a predictor the IMD uses to make its forecast. “But the IOD develops during the monsoon season itself, and it is not clear if it influences the monsoon or is influenced by it,” Srinivasan said.

Complex interaction

While several factors influence the monsoon and the strongest ones are used as predictors in the forecast, the interaction of these predictors amongst themselves is extremely complex. “El Niño with positive IOD will have a different impact on the monsoon compared to El Niño with a negative IOD. Furthermore, ENSO conditions are observed in different regions within the Pacific Ocean. Each of these affects the monsoon circulation in different ways. To have a good assessment of the impact of El Niño or La Niña, we need to understand their characteristics, position, the strength of other factors, and much more,” said Subimal Ghosh of IIT Bombay.

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