El Niño-Southern Oscillation (ENSO)
Mechanism and Phases
ENSO is a coupled ocean-atmosphere phenomenon in the tropical Pacific that alternates between three phases: El Niño (warm), La Niña (cool), and neutral. It is the single largest source of year-to-year natural climate variability on the planet, affecting rainfall, temperature, and extreme weather across most of the globe.
- Under normal conditions, trade winds blow westward along the equator, pushing warm surface water into the western Pacific (near Indonesia/Australia) and causing upwelling of cold, nutrient-rich water off the coast of Peru and Ecuador.
- During El Niño, trade winds weaken or reverse, allowing the warm water pool to spread eastward across the central and eastern Pacific. This suppresses upwelling and raises SSTs in the Niño 3.4 region (5°N–5°S, 170°W–120°W) by more than 0.5°C for at least five consecutive overlapping three-month periods.
- During La Niña, trade winds strengthen, amplifying the normal pattern — warmer western Pacific, colder eastern Pacific — with opposite climatic effects.
- The Southern Oscillation is the atmospheric pressure see-saw between the eastern Pacific (Tahiti) and the western Pacific (Darwin, Australia), measured by the Southern Oscillation Index (SOI). El Niño corresponds to a negative SOI.
- ENSO cycles typically recur every 2–7 years and last 9–12 months, though major events can persist for 18 months or more.
● Tracked since May 06, 2026 · last seen July 04, 2026 · updates as the daily brief publishes
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