Joanna Slawinska

An orchestra without a conductor: the instruments set their own rhythm. That is how one might describe the climate system of the Indo-Pacific region, where sea surface temperatures oscillate across many time scales, from seasonal to multi-decadal. Previously, to hear a single instrument, you had to pre-tune filters, cutting off everything else.

In 1972, anchovies vanished off the coast of Peru. Fishermen were left empty-handed, and the world learned the word El Niño. Since then, scientists have been searching for ways to predict when that warm current will once again upend the planet's weather.

It takes eight years for a wave to circle Antarctica. It carries warmth and cold in turn, and sea ice advances and retreats with its crests. Previously, researchers caught this wave using different methods and saw either a propagating signal or a frozen dipole. Now it's clear: the dipole is just a snapshot of the wave when it stretches between two oceans.

The ocean remembers its weather. Each month, its surface temperature forms a vast map, and if you find a similar map in the past, you can see what happened next. That is exactly how the new method for forecasting El Niño works: the warm current that, every few years, alters the climate of half the planet.

The ocean off the coast of Peru warms and cools, and with it the whole planet runs a fever: rains flood deserts, and tropical forests dry out. This is El Niño, the most noticeable climate oscillation after the changing seasons. For half a century, it has been described by a single number: the water temperature in a small square of ocean.

A meteorologist stares at a pressure map, trying to figure out where the cyclone will go. They have a model, but it is flawed, and fresh measurements, but they are incomplete. Merging the two so that the forecast remains physically meaningful is harder than it seems: classical methods sometimes produce negative temperatures or impossible probabilities.