Why in news?

• 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.

What is El Niño Southern Oscillations (ENSO)?

• 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.
• 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.

Evaluating the likely impact

• 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%.
• 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.
• 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 Australia”. It said that the persistence of La Niña was most likely to result in a worsening of the drought in Africa.

Reference:

1. https://indianexpress.com/article/explained/explained-climate/india-monsoon-the-triple-dip-la-nina-8162945/