Context

• The ongoing La Niña phase of the equatorial Pacific Ocean has just been predicted to persist for at least another six months, making it one of the longest ever La Niña episodes in recorded history. It is also only the third episode since 1950 to stretch into a third consecutive year.
• The World Meteorological Organisation (WMO) on August 31 had stated for the first time this century, La Nina would span three consecutive northern hemisphere winters to become a ‘triple dip’ La Nina.
• This is likely to have wide-ranging implications for weather events across the world in the coming months, and can potentially aggravate both floods and droughts in different regions.

What is ENSO (El Nino and Southern Oscillation)?

• ENSO is one of the most important climate phenomena on earth due to its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe.
• Only El Nino = [Warm water in Eastern Pacific + Cold water in Western Pacific].
• Only SO = [Low Pressure over Eastern Pacific + High Pressure over Western Pacific]
• ENSO = [Warm water in Eastern Pacific + Low Pressure over Eastern Pacific] + [Cold water in Western Pacific + High Pressure over Western Pacific].
• In ENSO formation of an El Niño [Circulation of Water] is linked with Pacific Ocean circulation pattern known as the southern oscillation [circulation of atmospheric pressure]
• Though ENSO is a single climate phenomenon, it has three states, or phases, it can be in:

• A warming of the ocean surface, or above-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean.
• This accumulation of warm water causes the thermocline to drop in the eastern part of Pacific Ocean which cuts off the upwelling of cold deep ocean water along the coast of Peru. Fishing industry is effected
• It is associated with lower than normal monsoon rainfall in India.

• A cooling of the ocean surface, or below-average SSTs, in the central and eastern tropical Pacific Ocean.
• It is associated with a comparatively better monsoon rainfall in India.

• Neither El Nino nor La Nina.
• Often tropical Pacific SSTs are generally close to average.

Origin and duration: El Niño and La Niña episodes usually develop in the March-June period and last for about nine months to a year.

• They are the strongest during winter (November-January in the northern hemisphere), before weakening or dissipating by March or April of next year.

Difference: However, El Niño and La Niña events are not mirror images of each other. They differ in length and strength.

• El Niño episodes occur more frequently and are usually associated with more impactful weather events. An El Niño is more likely to be a single-year
• La Niña, on the other hand, has a longer run. That is why multi-year La Niña events, those that continue for more than 12 months, are more common.

About ‘Triple dip’ La Niña

• Stretched phases: Occasionally, these El Niño and La Niña episodes continue for much longer periods.
  • In recent years, the El Niño of 2015-16, spread over 19 months, was one of the longest on record, and was dubbed ‘Godzilla’ due to its sustained high intensity.
• Present context: ‘Triple Dip’ La Niña is a period where the La Niña period extends for up to three consecutive winters and results in multiyear cooling of the surface temperature of the equatorial Pacific Ocean.
  • For example, the current La Niña episode was started in September 2020, prevailed for the last 24 months, and looks set to continue for another six months, and has thus been classified as a ‘triple dip’ La Niña.
• Duration of La Nina: According to study, almost half (six out of 13) of all La Niña events since 1950 have stretched for two years (double dip La Nina), while three, including the current one, have continued for three years.
  • Comparison with El Nino: In contrast, over 75% of El Niño events (15 out of 20) ended within a year. No El Niño has ever stretched into a third year.
• Distinct current ‘triple dip’ La Niña episode: While prolonged La Niña episodes are not uncommon, the current one differs from the previous two triple-year events in an important aspect.
  • Both the earlier events, one between 1973 and 1976, and the other between 1998 and 2001 were preceded by a strong El Niño and prolonged La Niña events was a result of longer heat dissipation time as the amount of accumulated heat in the oceans was higher.
  • In the absence of a strong El Niño preceding it, the reason for the current La Niña episode is not very clear at the moment.

Evaluating the likely impact on India

• Indian context: La Niña is associated with good rainfall during the monsoon season. This is the opposite of El Niño which is known to suppress monsoon rainfall.
  • Thus, a continued spell of La Niña could lead to expectation of another year of good, or normal, rainfall during the monsoon.
  • This year, India has received 740.3 mm of rainfall, which was quantitatively 7 per cent above the seasonal average till August 30.
• One of the several factors: But, even though powerful, ENSO condition is not the only factor affecting monsoon rainfall in India and no one-on-one correlation exists between the ENSO condition and the amount of rainfall.
  • Also, the influence of ENSO is at a macro level. There are wide variations in rainfall at the local level, which are getting exacerbated by climate change.

Impact on other regions

• In most parts of the United States, for example, La Niña is associated with very dry winters. The recent widespread drought in the United States is also La Nina consequence.
  • Recently, the National Oceanic and Atmospheric Administration (NOAA) of the United States declared that the month of August in 2022 was the sixth hottest August in the last 143 years.
• In Australia and Indonesia, and generally in the tropical region, La Niña is expected to bring more rainfall. For example, flooding in eastern Australia this year.
• The excessive rainfall in Pakistan, which is experiencing its worst flooding disaster, can also be blamed in part on La Niña.
• As per World Meteorological Organisation (WMO), the the persistence of La Niña could most likely result in a worsening of the drought in Africa.
• The experts are also suggesting that as a result of this triple dip, there world will witness an overactive Atlantic Hurricane season in 2022.

Climate change link

• No clear records: Since there are no distant historical records, the natural variability of ENSO is not understood very clearly. Also nothing conclusive has been witnessed on the linkage of ENSO events with global warming.
• Indistinct variability: The irregular occurrences of El Niño or La Niña, for example sometimes emerging every two years and at other times there has been a gap of even seven years leads to unclear natural variability. Hence making it difficult to quantify the influence of global warming.
• Role of trade winds in ENSO: Few experts believe that climate change is increasing the frequency and intensity of El Niño and La Niña episodes, but no conclusive evidence exists.
  • This is because trade winds also play a very important role in triggering ENSO events and the changes in the strength of trade winds are not easily explained by global warming.
• Distinguished linkage: There is evidence of another kind of linkage with global warming and ENSO events as follows:
  • During La Niña years, the colder surfaces allow the oceans to absorb more heat from the atmosphere. Consequently, the air temperatures tend to go down, producing a cooling effect.
  • However, as per WMO, La Niña’s cooling influence is temporarily slowing the rise in global temperatures but it will not halt or reverse the long term warming trend.
  • Incidentally, it was noted that average global temperatures were about 0.9 degree Celsius higher than the 20th century average.