DH Deciphers | What is El Niño and why does it matter?

With the threat of an El Nino hanging over the 2023 southwest monsoon, Kalyan Ray explains what is the climate phenomenon that is being feared so much. The fear comes from the fact that almost all drought years since independence was associated with an El Nino event.

What is El Nino?

El Nino (means little boy in Spanish) is a climate pattern that affects weather and consequently economies worldwide. This and its complementary phenomenon La Nina are part of an ocean-atmospheric coupled system known as ENSO or El Nino / Southern Oscillation. While the SO is a large-scale atmospheric event that causes a "see-saw" effect in surface air pressure between the eastern and western tropical Pacific, El Nino and La Nina (little girl) are associated with sea-surface temperature of the equatorial Pacific Ocean.

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What happened during El Nino and La Nina?

El Nino is marked by warming of the sea surface temperatures in the central and eastern equatorial Pacific Ocean, typically beginning around Christmas. It is characterised by five consecutive three-month running SST anomalies in an area of the equatorial Pacific (known as Nino 3.4) with a temperature above the threshold of 0.5 degrees Celsius. The warming leads to reduced rainfall over Indonesia and increased precipitation over the tropical Pacific Ocean. During an El Nino, the low-level surface winds that normally blow from east to west along the equator weaken or even reverse direction.

La Nina is just the opposite. It is characterized by cooling of the ocean surface, resulting in below-average sea surface temperatures in the central and eastern tropical Pacific Ocean. During La Niña events, rainfall tends to increase over Indonesia while decreasing over the central tropical Pacific Ocean. The normal easterly winds along the equator become stronger.

How does an El Nino impact the Indian summer monsoon rainfall?

El Nino is generally associated with a weak summer monsoon in India, though it is not a one-to-one relation. There have been 18 drought years since 1901, of which 13 were El Nino years, suggesting that 72 per cent of the drought years are associated with this phenomenon.

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Between 1951 and 2022 there were 16 El Niño years. Of these 16 years, there were six weak El Nino years, three moderate and seven strong El Nino years. The all-India seasonal rainfall was in the deficient (less than 10 per cent) category during two of the six weak El Nino years; in all the three moderate El Nino years and four of the seven strong years. This means in nine of the 16 El Nino years India had deficient monsoon rainfall, suggesting El Nino can be a major deciding factor for the monsoon but not the sole one.

Also it should be noted that in 12 of the 16 El Nino years, the September rainfall was below normal. In eight of these 12 years, it was less than 20 per cent below the long period average.

What’s the El Nino forecast for this year?

According to the World Meteorological Organisation, the likelihood of El Nino developing later this year is increasing. There is a 60 per cent chance for a transition from the current ENSO-neutral to El Nino during May-July 2023. This will increase to about 70 per cent in June-August and 80 per cent between July and September, says the WMO. A senior scientist from the India Meteorological Department says there is a 90 per cent probability of El Nino developing during the monsoon season and continuing till 2024 winter. However, at this stage there is no indication of the strength or duration of El Nino, as per the WMO.

Why does the IMD issue a normal monsoon forecast if there is an impending El nino threat?

IMD’s forecast primarily relies on two factors – (1) no 1:1 relation between El Nino and Indian summer monsoon and (2) a positive Indian Ocean Dipole, a temperature see-saw between the equatorial Arabian Sea and Bay of Bengal. While a positive IOD is indeed beneficial for the monsoon (seen in 1997 when rainfall was normal despite being an El Nino year), experts point out that IOD is not such a reliable forcing parameter for monsoon unlike the El Nino.