Chapter 5 · Try it yourself
ENSO Record Reader
Since 1950, scientists have kept one number that tracks the Pacific’s mood swings: the Oceanic Niño Index. It measures how much warmer or cooler than normal the sea surface is along the middle of the equatorial Pacific. Red bars are El Niño winters, blue bars are La Niña. Pick a winter and see what the rest of the planet was going through.
Every winter since 1950
Tap or click a bar to pick that winter. Each bar is the November–January average, in °C above or below normal.
1997–98
Arrow keys move one winter at a time; Page Up and Page Down jump ten.
Jump to a famous winter
Winter 1997–98
ONI that winter
+2.4 °C
Peak of the event
+2.4 °C
El Niño
Season by season
Each bar is a three-month average, April–June to March–May. It takes five in a row past ±0.5 °C to count as an official El Niño or La Niña.
Around the world
Data: NOAA Climate Prediction Center, Oceanic Niño Index (ONI, based on ERSST.v5), file oni.ascii.txt, retrieved 21 September 2026; the numbers are stored in this page. Simplified model of the record: each winter is shown by one number, its November–January average in the Niño 3.4 region. Strength words (weak 0.5–0.9, moderate 1.0–1.4, strong 1.5–1.9, very strong 2.0 or more) are a common convention, not an official NOAA scale. The effects listed are typical, not guaranteed, and plenty of other things shape each year’s weather.