El Niño & ENSO glossary

The vocabulary of ENSO in plain language. Definitions are original wording based on NOAA, Bureau of Meteorology, JMA and WMO documentation.

ENSO (El Niño–Southern Oscillation): The coupled ocean–atmosphere climate cycle in the tropical Pacific with three phases: El Niño (warm), La Niña (cool) and Neutral. It is the dominant source of year-to-year climate variability on Earth.
El Niño: The warm phase of ENSO: anomalously warm sea surface temperatures in the central and eastern equatorial Pacific, weakened trade winds, and an eastward shift of tropical rainfall.
La Niña: The cool phase of ENSO: cooler-than-average central/eastern equatorial Pacific, stronger trade winds, and enhanced rainfall over Indonesia and northern Australia. Roughly the mirror image of El Niño.
Southern Oscillation: The atmospheric half of ENSO — a see-saw in surface air pressure between the eastern tropical Pacific (Tahiti) and the Indo-Australian region (Darwin), identified by Sir Gilbert Walker in the 1920s.
Niño 3.4 region: The equatorial Pacific box from 5°N–5°S and 170°W–120°W. Sea surface temperature anomalies here are the standard measure of ENSO state because they correlate best with global impacts.
ONI (Oceanic Niño Index): NOAA's official ENSO index: the three-month running mean of sea surface temperature anomalies in the Niño 3.4 region. El Niño conditions require ONI ≥ +0.5 °C for five consecutive overlapping seasons.
SOI (Southern Oscillation Index): An atmospheric ENSO index based on the standardized pressure difference between Tahiti and Darwin. Sustained values below about −7 indicate El Niño; above +7 indicate La Niña (Bureau of Meteorology scale).
Trade winds: The persistent easterly winds blowing along the tropical Pacific from the Americas toward Asia. They pile warm water into the western Pacific; their weakening is both a trigger and a symptom of El Niño.
Walker Circulation: The east–west atmospheric loop over the tropical Pacific: air rises over the warm western Pacific, flows east aloft, sinks over the cooler eastern Pacific, and returns west as the trade winds. Named after Gilbert Walker by Jacob Bjerknes.
Bjerknes feedback: The self-reinforcing loop that amplifies El Niño: warmer eastern Pacific water weakens the trade winds, which lets the east warm further, which weakens the winds more. Named after meteorologist Jacob Bjerknes.
Thermocline: The sharp boundary layer between warm surface water and cold deep water. Normally deep (~150–200 m) in the western Pacific and shallow (~30–50 m) in the east; during El Niño the eastern thermocline deepens, cutting off cold upwelling.
Upwelling: The wind-driven rise of cold, nutrient-rich deep water to the surface, as along the coast of Peru. It sustains rich fisheries — and is suppressed during El Niño, with severe consequences for marine life.
Kelvin wave (oceanic): A large-scale wave of warm, thick surface water that travels eastward along the equator, crossing the Pacific in about two to three months. Downwelling Kelvin waves deepen the eastern thermocline and can herald El Niño.
Westerly wind burst: A days-to-weeks episode of west-to-east winds over the western Pacific that can trigger downwelling Kelvin waves and help kick off an El Niño. Often associated with the Madden–Julian Oscillation.
Warm pool: The vast reservoir of very warm water (28–30 °C) around Indonesia and the western tropical Pacific — the warmest large area of open ocean on Earth and the engine room of tropical rainfall.
El Niño Modoki (Central Pacific El Niño): An El Niño 'flavor' in which the strongest warming sits near the International Date Line rather than off South America. From the Japanese for 'similar but different'; its global effects differ from a classic eastern Pacific event.
El Niño Costero (Coastal El Niño): A warming confined to the far-eastern Pacific off Peru and Ecuador (Niño 1+2 region), tracked by Peru's ENFEN committee. It can bring torrential coastal rains even without a basin-wide event.
Teleconnection: A remote climate effect: shifts in tropical Pacific rainfall launch planetary atmospheric waves that redirect jet streams and alter weather thousands of kilometres away — the mechanism behind El Niño's global reach.
Spring predictability barrier: The well-documented drop in ENSO forecast skill for predictions made during Northern Hemisphere spring, when the cycle is transitioning and weather noise can tip the system either way.
Indian Ocean Dipole (IOD): An ENSO-like see-saw in Indian Ocean sea surface temperatures. A positive IOD often co-occurs with El Niño and reinforces drought over Indonesia and Australia while wetting East Africa.
Oceanic heat content (warm water volume): The amount of warm water stored in the equatorial Pacific's upper ocean. It 'recharges' before El Niño and 'discharges' during it, and its slow evolution is why El Niño can be forecast months ahead.
TAO/TRITON array: The network of about 70 moored buoys spanning the equatorial Pacific, built between 1985 and 1994 after the unforecast 1982–83 El Niño. It measures winds, surface and subsurface temperatures in real time and underpins ENSO monitoring.
ENSO-neutral: The state in which neither El Niño nor La Niña criteria are met and tropical Pacific conditions are near their long-term average.
Triple-dip La Niña: A La Niña that persists across three consecutive Northern Hemisphere winters, as in 2020–2023 — a rare occurrence in the modern record.