John Guckenheimer

The Pacific pendulum swings irregularly. Sometimes it barely trembles near the bottom, and sometimes it suddenly makes a giant sweep that disrupts weather across the planet. El Niño, the warm current in the equatorial Pacific, recurs every few years, but truly strong events happen once every 10–20 years, and until now no one could explain why they appear so irregularly.

In 2014, forecasters around the world were expecting a powerful El Niño. The ocean had warmed, the trade winds had weakened, all the signs were there. But the event only came a year later. The Pacific seemed to change its mind.

In 2014, forecasters expected a strong El Niño, but it arrived only a year later. The ocean seemed to change its mind. Researchers studied a simple model of the tropical Pacific and found that the system has two distinct "personalities": one regularly generates powerful El Niños, while the other oscillates chaotically without them. This model resembles a pendulum that sometimes jumps to a different swing.