An orchestra falls out of tune before a storm, then plays in unison again. In the Pacific Ocean, there is a small region where water temperature is measured daily at 105 points. Usually, these points change in a coordinated way, like musicians playing the same part. But before El Niño (the warm phase of the climate cycle), the harmony breaks: each point starts to follow its own rhythm.

Scientists have linked all the points with invisible threads. If temperatures at two locations change similarly, the thread is strong. Before El Niño, the threads weaken. This discord can be quantified by a number called the FI index. When FI rises, disorder in the region grows. It turns out that such disorder appears about a year before El Niño, and its magnitude hints at how strong the event will be.

Such forecasts help farmers know whether to expect drought, and cities to prepare for floods. The orchestra, once out of tune, plays in unison again, but now we hear the opening notes much earlier.