The ocean is like a drum beaten by the random hammer of the atmosphere. El Niño is a warm patch in the Pacific Ocean that appears every few years and changes weather worldwide. Scientists have long debated whether it can be predicted precisely, like an eclipse, or whether it is random, like a coin toss. A new study put this to the test using 138 years of data.

Researchers took water temperatures in the Niño 3 region (the part of the ocean where El Niño is strongest). First, they subtracted the usual annual cycle: summers warmer, winters colder. What remained looked like random noise, similar to the jitter of a dust particle in a glass of water. Then the scientists built a computer model of the ocean. Without random kicks, the model behaved like a precise mechanism, but when they added noise mimicking wind gusts, the model began to resemble real data.

Previously, it was unclear whether El Niño is a precise mechanism or a random process. Now it is clear: after removing the seasonal cycle, it is indistinguishable from randomness. This means El Niño forecasts should be probabilistic, like month-long weather forecasts. The ocean drum sounds from random strikes, and within that lies its own order, a statistical one.