Xinyang Wang

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.