The Pacific Ocean sometimes warms more than usual: that's El Niño. And sometimes it cools instead: that's La Niña. Both affect weather around the globe, but they behave oddly: El Niño arrives abruptly and leaves quickly, while La Niña can drag on for months. Scientists long couldn't figure out why.

New research describes the ocean as a pendulum. If a pendulum is well lubricated, it swings long and steadily. But if friction is strong, each push quickly dies out, and the motion becomes jerky. The ocean, it turns out, behaves like a heavily damped pendulum: it loses energy in about 20 months. It is rocked by random wind gusts, which get stronger when the ocean is already warm.

For the pendulum to behave like the real ocean, two small 'tilts' are needed. One makes the warm phase shorter and stronger, and the cold phase longer and weaker. The second acts like a spring that, after a big swing, slowly pulls the pendulum back, keeping it in the cold phase. That second tilt is what explains why La Niña sometimes lasts two years in a row. Now this simple model can be used to test more complex climate models and improve weather forecasts.