The ocean beneath the event.
Slice the Pacific along the equator and look at it side-on. Normally a thick lens of warm water sits in the west and the cold deep water almost reaches the surface off South America. An El Niño is that lens sloshing east: the boundary between warm and cold water — the thermocline — flattens, and the east warms from below before it warms at the surface.
Monthly ocean analysis from NOAA's GODAS, 2°S–2°N, down to 400 m. The winds that drive it: the Hovmöller Wall →
The slice, month by month
Temperature along the equator, surface to 400 m
Colour: how much warmer (red) or colder (blue) than the 1991–2020 normal for that month. Lines: the temperature itself every 2 °C; the thick black line is 20 °C, the usual marker of the thermocline. The dashed line is where 20 °C normally sits that month. Hover for values.
The same month in 1997 and 2015
The tilt of the thermocline
Depth of the 20 °C isotherm along the equator for the chosen month. Normally it slopes from 150–180 m deep in the west to 30–60 m in the east; in a strong El Niño the eastern end sinks and the slope flattens.
Heat in the top 300 m, month by month
0–300 m mean temperature anomaly · 2°S–2°NKelvin waves as slanted streaks
Each row is a month; each column a longitude. A Kelvin wave carries a pulse of heat east in about two months, so it draws a warm streak slanting down to the right (the 5-day version is on the Hovmöller Wall). The chosen month is outlined; click a row to jump to it.
What you are looking at
The thermocline
The tropical ocean is two layers: a warm, light layer on top and cold water below, with a sharp step between them. The trade winds drag the warm layer west, so it is thick near Indonesia (the 20 °C line near 150–200 m) and thin off Ecuador (near 50 m), where cold water wells up and cools the surface. That tilt is the Pacific's normal state.
Kelvin waves
When a westerly wind burst weakens the trades, the warm layer surges east as a Kelvin wave: a bulge that pushes the thermocline down as it passes, 100–200 m below the surface, at about 2.5 m/s. In this slice it shows as a red blob sliding right from month to month; when it reaches the east it stops the cold upwelling and the surface warms.
Recharge and discharge
Before an El Niño the equatorial band stores extra warm water (recharge); during the event the heat spreads east and then drains toward the poles (discharge), which is why a big El Niño so often flips into La Niña. The heat panel above shows the store filling and moving; the ENSO Wheel follows the same store against Niño-3.4 for every event since 1980.