- La Niña basically refers to an abnormal cooling of the central and eastern Pacific Ocean waters off the coasts of Ecuador and Peru.
- Such cooling (sea surface temperatures i.e. SSTs falling) is a result of strong trade winds blowing west along the equator, taking warm water from South America towards Asia.
- The warming of the western equatorial Pacific, then, leads to increased evaporation and concentrated cloud-formation activity around that region, whose effects may spread to India as well.
Latest La Nina event:
- The latest La Nina event was one of the longest ever, lasting from July-September 2020 to December-February 2022-23. And it brought copious rains to India – just as two previous “strong” La Nina in 2007-08 and 2010-11, followed by one “moderate” episode in 2011-12, had done.
- It is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean.
- El Niño is the “warm phase” of a larger phenomenon called the El Niño-Southern Oscillation (ENSO). La Niña, the “cool phase” of ENSO, is a pattern that describes the unusual cooling of the region’s surface waters. El Niño and La Niña are considered the ocean part of ENSO, while the Southern Oscillation is its atmospheric changes.
- During El Niño, the trade winds weaken or even reverse:
- Instead of blowing from east (South America) to west (Indonesia), they could turn into westerlies.
- As the winds blow from the west to east, they cause the masses of warm water to move into the central and eastern equatorial Pacific Ocean.
- The rise in SSTs there, thus, produces increased rainfall along western Latin America, the Caribbean and US Gulf Coast, while depriving Southeast Asia, Australia and India of convective currents.
- El Niño occurs simultaneously with the Southern Oscillation. The Southern Oscillation is a change in air pressure over the tropical Pacific Ocean. When coastal waters become warmer in the eastern tropical Pacific (El Niño), the atmospheric pressure above the ocean decreases.
- An El Niño event can be identified by the variations in sea surface temperature (SST) over the equatorial Central Pacific (the Nino 3.4 region).
- El Niño events are classified as weak, moderate, strong, and very strong depending on the strength of the positive SST variations.
- An El Niño event is announced when monthly Nino 3.4 SST deviations reach +0.50 °C, along with consistent atmospheric features, and when these anomalies persist for three consecutive months.
Impact on Indian Monsoon:
- El Nino may result in a weakening of the Indian monsoon, leading to drier conditions and reduced rainfall.
- On the other hand, La Niña events, which bring cooling to the Pacific Ocean, can result in stronger monsoons and more precipitation in India.
El Niño years and its trends in India
- According to statistics, about 60 per cent of the time there will be a probability of drought in India during an El Niño year.
- Chances of below-normal rain will be 30 per cent, while the prospect of normal rain remains very rare at 10 per cent.
- However, El Niño conditions have been known to be unpredictable as well. For instance, even the strongest El Niño has given normal Monsoon rains of 102 per cent in 1997, while weak El Niño conditions resulted in severe drought in 2004 to the tune of 86 per cent.
- Drought years:
- Statistics from the year 2000 till 2019 show that there have been four instances of drought years.
- In 2002 and 2009, the countrywide deficiency was 19 per cent and 22 per cent, respectively, which were considered severe drought years.
- While in 2004 and 2015 the deficiency stood at 14 per cent each, which was again a drought.
- There has been only one instance in the last 25 years, since 1997, when the country saw surplus rain of 2 per cent despite El Niño.
- Recent research indicates that the frequency of extreme El Niño events increases linearly with the global mean temperature, and that the number of such events might double (one event every 10 years) under 1.5°C of global warming.
- This pattern is projected to persist for a century after stabilisation at 1.5°C, indicating continuing high risks.
- Changes to the frequency of extreme El Niño and La Niña events may also increase the frequency of droughts and floods in South Pacific islands.