El Niño and La Niña

In Context

• India is experiencing a colder winter than normal with the La Niña is going on for a record-breaking third consecutive year also known as the ‘Triple dip’ La Nina.
  • Forecasts for the 2023 fall and winter are predicting that El Niño will occur with more than a 50% probability.

Significance 

• Increase in the planet’s average surface temperature:
  • An El Niño year creates a global-warming crisis in miniature:
    • This is because the warm water spreading across the tropical Pacific and releases a large amount of heat into the atmosphere.
  • An El Niño this year could increase the planet’s average surface temperature by more than 1.5° C from pre-industrial levels.
• Transition of La Niña to El Niño:
  • A transition from a La Niña winter – which we are in currently – to an El Niño summer has historically tended to produce the largest deficit in the monsoon. 
  • This means that pre-monsoon and monsoon circulations tend to be weaker in an El Niño year.
• Weaker vertical shear:
  • The vertical shear, which is the change in intensity of winds from the surface to the upper atmosphere, tends to be weaker as well. 
  • This in turn can favour enhanced cyclogenesis, i.e. cyclone formation.
• Monsoon Deficit & dry events:
  • If an El Niño state does emerge by summer, India is likely to experience a deficit monsoon in 2023. 
  • The monsoon deficit will be accompanied by extreme wet and dry events. 
  • While overall seasonal total could be deficient, there are likely to be isolated pockets of heavy or very heavy rainfall.
• Affecting weather worldwide:
  • Both phenomena affect the weather worldwide and can have drastic effects on economies that depend on rainfall.

More about the El Niño, La Niña & ENSO

• El Niño:
  • About:
    • El Niño is the warming of seawater in the central-east Equatorial Pacific that occurs every few years. 
    • During El Niño, surface temperatures in the equatorial Pacific rise, and trade winds — east-west winds that blow near the Equator — weaken. 
    • Normally, easterly trade winds blow from the Americas towards Asia. Due to El Niño, they falter and change direction to turn into westerlies, bringing warm water from the western Pacific towards the Americas. 
  • Outcomes:
    • Disruptions in the food chain:
      • The phenomena of upwelling, where nutrient-rich waters rise towards the surface, is reduced under El Niño. This in turn reduces phytoplankton. 
      • Thus, fish that eat phytoplankton are affected, followed by other organisms higher up the food chain. 
    • Disruptions in the overall ecosystem:
      • Warm waters also carry tropical species towards colder areas, disrupting multiple ecosystems. 
    • Alterations in wind & weather patterns:
      • Since the Pacific covers almost one-third of the earth, changes in its temperature and subsequent alteration of wind patterns disrupt global weather patterns. 
      • El Niño causes dry, warm winters in Northern U.S. and Canada and increases the risk of flooding in the U.S. gulf coast and southeastern U.S. It also brings drought to Indonesia and Australia.

• La Niña:
  • About:
    • La Niña is the opposite of El Niño. La Niña sees cooler than average sea surface temperature (SST) in the equatorial Pacific region. 
    • Trade winds are stronger than usual, pushing warmer water towards Asia.
  • Outcomes:
    • On the American west coast, upwelling increases, bringing nutrient-rich water to the surface. 
    • Pacific cold waters close to the Americas push jet streams — narrow bands of strong winds in the upper atmosphere — northwards. 
    • This leads to drier conditions in Southern U.S., and heavy rainfall in Canada.
    • La Niña has also been associated with heavy floods in Australia. Two successive La Niña events in the last two years caused intense flooding in Australia, resulting in significant damage.

• ENSO:
  • The combination of El Niño, La Niña, and the neutral state between the two opposite effects is called the El Niño Southern Oscillation (ENSO). 
  • Southern oscillations are large-scale changes in sea level pressure in the tropical Pacific region.

• Effect of El Niño & La Niña on India’s monsoons:
  • In India, El Niño causes weak rainfall and more heat, while La Niña intensifies rainfall across South Asia, particularly in India’s northwest and Bangladesh during the monsoon. 
  • At present, India, like the rest of the globe, is witnessing an extended ‘triple dip’ La Niña. 
  • This is why India saw surplus rain in September, a month that usually sees the monsoon retreat, for the third year in a row.

Source: TH