Glossary

ENSO

The El Niño–Southern Oscillation: the coupled ocean-atmosphere cycle whose warm (El Niño), cool (La Niña) and neutral phases steer global weather.

Kelvin Wave

A subsurface ocean pulse that carries warm water eastward along the equator in roughly two months — the delivery mechanism of El Niño onsets.

Niño 3.4

The equatorial Pacific box (5°N–5°S, 170°W–120°W) whose sea-surface-temperature anomaly is the standard measure of El Niño's strength.

ONI (Oceanic Niño Index)

NOAA's official ENSO index: the three-month running mean of Niño 3.4 sea-surface-temperature anomalies. ±0.5°C marks El Niño / La Niña territory.

RONI (Relative Oceanic Niño Index)

The ONI recomputed relative to average tropical ocean warming — a fairer gauge of El Niño's true atmospheric punch in a warming world.

SOI (Southern Oscillation Index)

The standardized Tahiti-minus-Darwin pressure difference: the atmosphere's ENSO gauge. Sustained negative values accompany El Niño.

Spring Predictability Barrier

The seasonal dip in ENSO forecast skill: predictions made (or crossing) February–May are the least reliable of the year.

Teleconnections

Long-distance atmospheric links — jet-stream and wave-train responses — through which tropical Pacific anomalies reshape weather worldwide.

Thermocline

The transition layer between warm surface water and the cold deep. Tilted deep-west/shallow-east in normal years; El Niño flattens it.

Walker Circulation

The Pacific's east–west overturning loop: air rises over the warm west, sinks over the cool east. El Niño weakens and shifts it, moving the rain.