Wind first, then water.
An El Niño is built out of short, violent gusts. A westerly wind burst — a few days of wind blowing the wrong way across the western Pacific — shoves warm water east as an undersea bulge called a Kelvin wave, which surfaces weeks later off South America. Stack the equator's days top to bottom and each burst, and the warming it launches, shows up as a streak you can follow.
Time runs down the page; longitude runs across, from Indonesia (left) to Ecuador (right). The same waves seen from below: the Pacific Cutaway →
The wall
The equatorial Pacific, one row per day
Each panel is the 5°S–5°N average (0–300 m heat: 2°S–2°N) from 120°E to 80°W. Black boxes on the wind panel mark the westerly wind bursts found by the rule below; black bars on the left edge of the other panels mark the same dates. Hover (or touch) for the date, longitude and every panel's value there.
Every burst, by rule
| # | Dates | Days | Longitudes | Peak |
|---|
The rule
Why 850 hPa (about 1.5 km up)? It is the level forecasters use for the Madden–Julian Oscillation, the pulse of tropical storminess that often triggers bursts, and the reanalysis wind there runs daily back to 1948, so 1997 and 2015 can be measured the same way. Studies that use the surface wind find the same big events; the count depends on the threshold, so compare counts only within this page, where one rule is applied to every year.
1997 and 2015 at the same stage
How to read the wall
What a Hovmöller diagram is
A map squeezed to one line. Each row is one day's strip of the equator, averaged over a band a few hundred kilometres wide; the next day goes underneath. Anything that moves along the equator draws a slanted streak: eastward motion slants down and to the right. Named after the Danish meteorologist Ernest Hovmöller, who drew the first one in 1949.
Westerly wind bursts
The trade winds normally blow from east to west and pile warm water up near Indonesia. A burst reverses them for a few days over a stretch of ocean a thousand kilometres or more wide, usually beside a cluster of storms (the teal patches in the cloud panel) or a pair of tropical cyclones straddling the equator. On the wind panel, bursts are the green blobs in the west.
Kelvin waves
Each burst pushes a bulge of warm water east along the equator at about 2.5 m/s, crossing the Pacific in roughly two months. It is invisible from above at first — it travels 100–200 m down — and shows up in the heat panel as a warm streak slanting down and to the right, then warms the surface in the east when it arrives. The streak's slope is its speed.
Why it matters
A strong El Niño needs this feedback to keep going: warmer water in the east shifts the storms east, which weakens the trades and invites more bursts. Where the heat store is on the recharge–discharge loop is on the ENSO Wheel; the thermocline itself is on the Pacific Cutaway.