How a super El Niño is born, and how to read this site.

In a normal year the trade winds blow from east to west along the equator. They push the sun-warmed surface water toward Indonesia, where it piles up into the warm pool, the warmest large body of ocean on Earth. Along South America the same winds drag surface water away and cold water wells up from below: the cold tongue.

Under the surface, warm water sits on top of cold, separated by the thermocline, a layer where temperature drops fast (oceanographers often track the 20 °C line). Because the winds pile warm water in the west, the thermocline is deep there and shallow in the east, close enough to the surface that upwelling brings cold water straight up.

The air answers the ocean. Moist air rises over the warm pool in towering thunderstorms, flows east high up, sinks over the cool east and returns west as the trade winds: the Walker circulation.¹

Jacob Bjerknes saw in the 1960s that the two halves drive each other.^2,3 If the east warms a little, the east–west temperature contrast weakens, so the trade winds weaken, so less cold water wells up and the thermocline in the east sinks, so the east warms more. That loop, the Bjerknes feedback, runs the other way too: a cooler east strengthens the trades, which cool it further. Warm is El Niño; cool is La Niña.

The feedback needs a push, and it needs fuel. The fuel is warm water stored below the surface along the equator, built up over the years before (section 3). The push usually comes as a westerly wind burst: days to a few weeks of winds blowing the wrong way, from west to east, over the western Pacific, often kicked off by a pulse of tropical thunderstorms (the Madden–Julian Oscillation) or by a pair of tropical cyclones either side of the equator.⁴

A burst sets off a downwelling Kelvin wave: a bulge of warm water, invisible at the surface, that rides east along the equator and pushes the thermocline down as it goes. It takes two to three months to cross the Pacific.⁵ When it arrives in the east, upwelling brings up warmer water than usual, the surface warms, the trades weaken, and the Bjerknes feedback takes over. Each round of warming invites more westerly bursts.

A super El Niño (1982–83, 1997–98, 2015–16) needs everything at once: a well-recharged ocean, repeated bursts through spring and summer, and no interruption. In 2014 the Pacific looked ready, but a strong easterly burst in July stalled it; the warm water it left behind gave the 2015 event a head start.⁶ El Niños tend to peak near the end of the calendar year.⁷

See it in the data: the Kelvin waves of this year on the Hovmöller Wall, and the thermocline itself on the Pacific Cutaway.

Think of the equatorial Pacific as a battery. During La Niña and neutral years the strong trades pump warm water toward the equator and store it below the surface: the warm water volume (the water above the 20 °C line between 5°N and 5°S) grows. That is the recharge.

An El Niño spends the charge. As it peaks, the weakened winds let the stored warm water drain away from the equator toward higher latitudes, and the thermocline rises across the basin: the discharge. With the battery flat, the east cools, and the stage is set for La Niña, which in turn slowly recharges the ocean. Fei-Fei Jin wrote this down as the recharge oscillator;⁸ buoy measurements show warm water volume leading Niño-3.4 temperature by several months,⁹ which is why forecasters watch it.¹⁰

The chart plots the real thing: each dot is a month, Niño-3.4 temperature across, warm water volume up. The cycle runs clockwise: recharged (top), El Niño (right), discharged (bottom), La Niña (left). The ENSO Wheel follows it since 1980.

When the thunderstorms of the warm pool shift east during El Niño, they heat the atmosphere over the central Pacific instead. That sends waves of high and low pressure arcing out to the higher latitudes, like a rock in a stream. These long-distance links are teleconnections.^12,13

Over North America, El Niño stretches the Pacific jet stream east and pulls it south, so winter storms ride in along the southern tier, roughly the line of Interstate 10: wetter and cooler from California to Florida, milder and often drier across the north. The pressure pattern that goes with it is the positive phase of the Pacific–North American pattern (PNA): low pressure south of Alaska, high over western Canada, low over the Southeast.¹⁴ La Niña tends toward the opposite.

These are odds, not promises. El Niño tilts the dice; the weather still rolls them. Across the record El Niño explains at most about a third of the year-to-year swings in West Coast winter rain, and in 2015–16 Southern California stayed dry.¹⁵ A sudden stratospheric warming can also override El Niño's mild north (the stratosphere).