Over the tropical Pacific Ocean, air moves in a loop: near the surface it flows westward, rises near Indonesia, returns eastward, and sinks near South America. This loop is called the Walker circulation. Sometimes it weakens, and the ocean off Peru becomes warmer than usual—that's El Niño. Other times it strengthens, and the water cools—that's La Niña. But why do these states alternate irregularly rather than on a schedule?

Mathematicians built a model of the atmosphere and found an answer: the system has two stable dimples, like in a child's game with a ball and holes. As long as the ball sits in a dimple, it doesn't move. Likewise, the atmosphere can remain in an El Niño or La Niña phase for years. To shift to the other state, a nudge is needed—for instance, a random weather disturbance.

Previously, scientists thought El Niño was simply an oscillation, like a pendulum. Now it's clear that it's more like a system with two resting positions. Understanding this helps better predict when the next switch will occur and what to expect from weather in different parts of the world.