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Environment & Ecology May 29, 2026 4 min read Daily brief · #5 of 27

Explained | El Niño, La Niña and changing weather patterns

The planet recently exited the first "triple-dip" La Niña of the 21st century — a rare event in which La Niña conditions persisted for three consecutive year...

GS Paper 1 (Physical Geography Climate) GS Paper 3 (Disaster Management)

What Happened

  • The planet recently exited the first "triple-dip" La Niña of the 21st century — a rare event in which La Niña conditions persisted for three consecutive years (2020–2021, 2021–2022, 2022–2023) without reverting to neutral or El Niño between cycles.
  • The triple-dip event, which lacked the typical precursor of a strong preceding El Niño, has prompted climate scientists to revise current models of ENSO dynamics.
  • Following the triple-dip La Niña, ENSO conditions are now shifting toward El Niño, with global forecasts indicating a high probability of El Niño development through 2026 — directly relevant to India's upcoming monsoon season.
  • Changing weather patterns linked to the ENSO cycle are intensifying extreme events: the 2020–2023 La Niña produced record floods in Australia, prolonged droughts in East Africa, and above-average Atlantic hurricane seasons.

Static Topic Bridges

ENSO — El Niño–Southern Oscillation

ENSO is a recurring coupled ocean–atmosphere phenomenon centred in the tropical Pacific. It oscillates between three phases — El Niño (warm), La Niña (cool), and Neutral — on an irregular 2–7 year cycle. The phenomenon was first systematically studied by Sir Gilbert Walker in the early 20th century while he investigated the causes of Indian monsoon failures.

  • El Niño: Trade winds weaken → warm surface water pools in the central-eastern Pacific → Walker Circulation weakens → suppressed convection over the Indian Ocean.
  • La Niña: Trade winds strengthen → cold upwelling intensifies in the eastern Pacific → Walker Circulation strengthens → enhanced convection over the Indian Ocean.
  • ENSO explains approximately 29% of the interannual variability of the Indian Summer Monsoon (ISM).
  • Monitoring index: Oceanic Niño Index (ONI), measured using SST anomaly in the Niño 3.4 region (5°N–5°S, 120°W–170°W); threshold ±0.5°C for 5 consecutive overlapping 3-month periods.
  • The "Southern Oscillation" component is measured by the Southern Oscillation Index (SOI) — difference in sea-level pressure between Tahiti and Darwin, Australia.

Connection to this news: The triple-dip La Niña (2020–2023) is the first of its kind in the 21st century and has challenged the conventional understanding that La Niña events reset after a single cycle; its transition into El Niño conditions is now driving India's 2026 monsoon deficit risk.

La Niña — Mechanism and Global Impacts

La Niña is the cool phase of ENSO, characterised by anomalously cold SSTs in the central and eastern equatorial Pacific. During La Niña, intensified trade winds pile warm water in the western Pacific and enhance upwelling of cold deep water off the South American coast.

  • Impacts on India: La Niña generally strengthens Indian monsoon rainfall, contributing to above-normal seasons.
  • Global impacts: Increased rainfall and floods in Australia and Southeast Asia; drought in the southwestern United States and parts of South America; heightened Atlantic hurricane activity; drought and food insecurity in East Africa.
  • The 2020–2023 triple-dip event reached a minimum ONI of −1.3°C during October–December 2020.
  • The triple-dip was unusual in being sustained by persistent easterly wind anomalies driven by extratropical forcing from the Southern Hemisphere, rather than by a recharge of oceanic heat content.

Connection to this news: Understanding why the 2020–2023 triple-dip La Niña was anomalous — and how its ending transitions into El Niño — is central to understanding the current shift toward below-normal monsoon conditions for India.

Indian Ocean Dipole (IOD) — Modulator of ENSO Impact

The Indian Ocean Dipole (IOD) is an independent climate mode that measures the anomalous SST gradient between the western Indian Ocean (off East Africa) and the eastern Indian Ocean (off Sumatra/Indonesia). A positive IOD (warmer west, cooler east) enhances rainfall over India and can counterbalance the monsoon-suppressing effect of El Niño. A negative IOD compounds El Niño's drying effect.

  • Measured by the Dipole Mode Index (DMI).
  • IOD events co-occur with but are distinct from ENSO.
  • In 2019, a strongly positive IOD significantly cushioned the Indian monsoon despite moderate El Niño conditions.
  • In 2002 and 2015, negative IOD combined with El Niño produced severe monsoon deficits.

Connection to this news: The IOD state during the 2026 monsoon season will be a critical secondary factor determining whether El Niño's suppressive effect on rainfall is partially offset or amplified.

Key Facts & Data

  • Triple-dip La Niña duration: 2020–2021, 2021–2022, 2022–2023 — the first of the 21st century
  • ONI minimum during 2020–2023 event: −1.3°C (October–December 2020)
  • ENSO cycle duration: typically 2–7 years per full cycle; individual phases last 9–12 months
  • ENSO accounts for ~29% of interannual variability in the Indian Summer Monsoon
  • Niño 3.4 region: 5°N–5°S, 120°W–170°W (SST monitoring zone)
  • SOI measured between Tahiti and Darwin; negative SOI indicates El Niño conditions
  • Sir Gilbert Walker first documented the Southern Oscillation in the early 1900s studying Indian monsoon failures
  • El Niño probability for 2026 SWM season: 92% (per IMD)
On this page
  1. What Happened
  2. Static Topic Bridges
  3. ENSO — El Niño–Southern Oscillation
  4. La Niña — Mechanism and Global Impacts
  5. Indian Ocean Dipole (IOD) — Modulator of ENSO Impact
  6. Key Facts & Data
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