Why in the News?

This year’s La Niña forecasts from the leading international agencies were significantly off the mark. In this context, it is essential to analyze the potential impacts of the delayed onset of La Niña and understand why global weather models failed in their predictions.

What’s in Today’s Article?

• Phenomenon of the El Niño Southern Oscillation (ENSO)?
• Normal Conditions
• What is El Nino and La Nina?
• Comparing La Nina,El Nino with Normal Conditions
• Latest Predictions of La Nina
• Impact of Delayed Onset on the Indian Monsoon

What is the El Niño Southern Oscillation (ENSO) Phenomenon?

• ENSO is a phenomenon characterised by variations in sea surface temperatures in the tropical Pacific Ocean, driven by atmospheric fluctuations above.
• ENSO has three phases: warm (El Niño), cool (La Niña), and neutral/normal, which occur in irregular cycles lasting two to seven years.
• As it disrupts global air circulation and influences weather patterns around the world, ENSO is a key factor in the variability of the natural climate on a global scale.

Normal Condition:

• In the neutral phase (neither El Niño nor La Niña), trade winds blow from east to west across the surface of the tropical Pacific Ocean. This movement pushes warm, moist air and warmer surface waters toward the western Pacific, keeping the central Pacific Ocean relatively cooler.
• In the western Pacific, the warm sea surface temperatures inject heat and moisture into the atmosphere above. This warm air rises through a process called atmospheric convection, and if the air is sufficiently moist, it leads to the formation of towering cumulonimbus clouds and rainfall.
• This pattern of rising air in the west, sinking air in the east, and westward-moving air at the surface is known as the Walker Circulation.

What is El Nino?

El Niño:

EL Nino means Little Boy in Spanish

El Niño is a climate pattern characterized by the unusual warming of surface waters in the eastern tropical Pacific Ocean. It represents the “warm phase” of the larger El Niño-Southern Oscillation (ENSO) phenomenon and occurs more frequently than La Niña.

Impacts:

• Impact on Oceans: El Niño affects ocean temperatures, alters the speed and strength of ocean currents, impacts the health of coastal fisheries, and influences local weather patterns from Australia to South America and beyond.
• Increased Rainfall: The warmer surface waters increase convection, leading to significantly higher precipitation. This results in heavy rainfall across South America, contributing to coastal flooding and erosion.
• Diseases from Floods and Droughts: El Niño-related flooding can lead to outbreaks of diseases such as cholera, dengue, and malaria in some regions, while drought conditions can cause wildfires that pose respiratory health risks.
• Positive Impact: El Niño can sometimes have positive effects, such as reducing the frequency of hurricanes in the Atlantic.
• South America: While El Niño brings increased rainfall to South America, it often leads to droughts in Indonesia and Australia. These droughts can threaten water supplies, dry up reservoirs, and reduce river flows, negatively impacting agriculture reliant on irrigation.
• Western Pacific: The warm trade winds associated with El Niño push warm surface water toward the western Pacific, near Asia and Australia. As a result, the sea surface is typically about 0.5 meters higher and 4-5°F warmer in Indonesia compared to Ecuador. This westward movement of warm water causes cooler waters to upwell along the coasts of Ecuador, Peru, and Chile, a process known as upwelling. Upwelling brings cold, nutrient-rich water to the ocean’s upper layer, known as the euphotic zone.

 What is La Nina?

• La Nina:
  • La Niña, which translates to “The Little Girl” in Spanish, is also known as El Viejo, anti-El Niño, or simply “a cold event.”
  • La Nina events represent periods of below-average sea surface temperatures across the east-central Equatorial Pacific.
    • It is indicated by sea-surface temperature decreased by more than 0.9℉ for at least five successive three-month seasons.
  • La Nina event is observed when the water temperature in the Eastern Pacific gets comparatively colder than normal, as a consequence of which, there is a strong high pressure over the eastern equatorial Pacific.
  • La Niña is recognized as the cold phase of ENSO, characterized by a band of cooler water extending from east to west across the equatorial Pacific Ocean.
  • Impacts:

Europe: In Europe, El Niño reduces the frequency of autumnal hurricanes. Conversely, La Niña typically results in milder winters in Northern Europe (especially the UK) and colder winters in southern and western Europe, which can lead to snowfall in the Mediterranean region.

North America: The effects of La Niña are most pronounced in continental North America, including:

• Stronger winds in the equatorial region, particularly in the Pacific.
• Enhanced conditions for hurricanes in the Caribbean and central Atlantic.
• Increased instances of tornadoes in various U.S. states.

South America: La Niña often causes drought in Peru and Ecuador. However, it usually benefits the fishing industry along the western coast of South America.

Western Pacific: In the western Pacific, La Niña increases the likelihood of landfall in vulnerable areas, including continental Asia and China. It also leads to heavy flooding in Australia and raises temperatures in the Western Pacific, the Indian Ocean, and off the coast of Somalia.

Comparison of El Nino, La Nina with Normal Conditions:

During the neutral or normal phase, the eastern Pacific (off the northwestern coast of South America) is cooler than the western Pacific (around the Philippines and Indonesia).

• This is due to prevailing trade winds, driven by Earth’s rotation between 30 degrees north and south of the equator, which blow from east to west.
• These winds push warmer surface water westward, allowing cooler waters from below to rise and replace the displaced water.

 In the El Niño phase, these trade winds weaken, reducing the displacement of warmer waters from the American coasts, causing the eastern Pacific to become warmer than usual.

In contrast, during the La Niña phase, trade winds strengthen, pushing larger amounts of warm water to the western Pacific.

In India, El Niño is associated with reduced rainfall during the monsoon season, while La Niña typically enhances monsoon activity.

The impacts of El Niño and La Niña, such as extreme temperatures, heavy rainfall, and droughts, have been intensified by human-induced climate change.

Recent Predictions for La Niña and the Effects of a Delayed Onset on the Indian Monsoon:

Latest Predictions of La Niña:

• One of the longest recorded La Niña episodes spanned from 2020 to 2023, with the subsequent El Niño event occurring from June 2023 to May 2024, after which ENSO shifted to a neutral phase.
• Recent predictions indicate that the first signs of La Niña onset are likely to appear by the end of September or early October.
• La Niña is expected to peak in November and persist throughout the winter in the northern hemisphere.
• Initial forecasts had projected that La Niña conditions would begin around July.
  • The discrepancy in predicting La Niña’s onset is attributed to weather models being more accurate at detecting signals in strong La Niña or El Niño phases, rather than weaker ones.

Impact of Delayed Onset on the Indian Climate:

• Southwest Monsoon (June – September):

  • A delayed onset of La Niña does not necessarily imply a poor monsoon performance. For instance, the IMD has forecasted a 109% ‘above normal’ rainfall over most areas during this monsoon season.
  • However, precipitation varies significantly at the regional level, with states in the east and northeast experiencing deficient rainfall.

• Northeast Monsoon Season (October – December):

  • Known as the winter monsoon, this season primarily affects Tamil Nadu, coastal Andhra Pradesh, Rayalaseema, southern interior Karnataka, and Kerala.
  • While La Niña generally does not support increased northeast monsoon rainfall, there have been exceptions in the past.

Cyclogenesis:

• The north Indian Ocean basin, which includes the Bay of Bengal and the Arabian Sea, sees its highest cyclone activity in May and November.
• In La Niña years, there is an increased likelihood of frequent cyclogenesis, with storms generally being more intense and having longer durations than usual.

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