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Sunday, Oct 02, 2022

The ‘triple dip’ La Niña, and its likely impact in India

La Niña refers to the ENSO phase in which sea-surface temperatures are cooler than normal. Continuance of La Niña into 2023 is not bad news from the Indian standpoint, but it is not the same for other regions.

In the Indian context, La Niña is associated with good rainfall during the monsoon season.

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 year. 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.

The periodic warming and cooling of surface waters in the equatorial Pacific Ocean — a phenomenon described as El Niño Southern Oscillations, or ENSO — is known to trigger widespread changes in atmospheric conditions, and has a major influence on global weather patterns, including the Indian monsoon. La Niña refers to the ENSO phase in which sea-surface temperatures are cooler than normal. The warmer phase is known as El Niño. A result of interactions between ocean and wind systems, El Niño and La Niña have almost opposite impacts on weather events.

‘Triple dip’ La Niña

El Niño and La Niña episodes typically last for about nine months to a year. They usually develop in the March-June period, and are the strongest during winter (November-January in the northern hemisphere), before weakening or dissipating by March or April of next year.

Occasionally, however, they 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.

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The current La Niña episode has already surpassed that in length. Having started in September 2020, it has 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.

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. 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 quite common. An El Niño is more likely to be a single-year event.

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According to a recent paper published by Chinese researchers, almost half (six out of 13) of all La Niña events since 1950 have stretched for two years, while three, including the current one, have continued for three years. 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.

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. Prolonged La Niña events in those instances could be explained by the fact that the amount of accumulated heat in the oceans was higher, and therefore took a longer time to dissipate. 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

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In the 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. Until now, the monsoon season this year has produced 7% more rain compared to normal. Last year, the seasonal rainfall was almost 100%.

But, even though powerful, ENSO condition is only one of the several factors affecting monsoon rainfall in India. There is no one-on-one correlation 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.

The continuance of La Niña further into 2023 is not bad news from the Indian standpoint. But it is not the same for many other regions where La Niña has very different impacts.

In most parts of the United States, for example, La Niña is associated with very dry winters. In Australia and Indonesia, and generally in the tropical region, La Niña is expected to bring more rainfall.

The widespread drought in the United States and flooding in eastern Australia this year could have been a result of the prolonged La Niña. The excessive rainfall in Pakistan, which is experiencing its worst flooding disaster, can also be blamed in part on La Niña.

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In its latest bulletin, the World Meteorological Organisation (WMO) said the worsening drought in the Horn of Africa and southern United States carried the “hallmarks of La Niña”, as did the “above average rainfall in southeast Asia and Australasia”. It said that the persistence of La Niña was most likely to result in a worsening of the drought in Africa.

Climate change link

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“Every unusual weather event these days is attributed to climate change, but science is not conclusive right now (on the linkage of ENSO events with global warming),” J Srinivasan, a distinguished scientist at the Divecha Centre for Climate Change at the Indian Institute of Science in Bengaluru, said.

The occurrences of El Niño or La Niña are not very regular. Sometimes they emerge every two years, at other times there has been a gap of even seven years. Historical records do not go very far in the past. As a result, the natural variability of ENSO is not understood very clearly. And when the natural variability itself is not clear, the influence of global warming is difficult to quantify.

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There have been suggestions that climate change is increasing the frequency and intensity of El Niño and La Niña episodes, but the question is not entirely settled. A part of the reason is the fact that trade winds 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.

But there is clearer evidence of another kind of linkage with global warming.

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 pointed out by the WMO, this is not enough to reverse or neutralize the impacts of global warming.

“Its (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,” the WMO statement said.

Incidentally, just a few days ago, the National Oceanic and Atmospheric Administration (NOAA) of the United States declared that the month of August this year was the sixth hottest August in the last 143 years. Average global temperatures were about 0.9 degree Celsius higher than the 20th century average.

First published on: 21-09-2022 at 01:00:32 am
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