The ocean is like a drum beaten by the random hammer of the atmosphere. El Niño is a warm patch in the tropical Pacific that arrives every few years and alters weather across the globe. Scientists have long debated whether El Niño works like a precise clockwork that can be predicted, or like a roll of the dice where each throw is independent of the last. A new study tested this using 138 years of data and got an unexpected answer.

The researchers took sea surface temperature records from the Niño 3 region (a rectangle in the eastern Pacific where El Niño is most pronounced). First, they removed the seasonal cycle from the data, the rhythm that makes the ocean warmer in summer and cooler in winter. The remaining signal, in theory, should reveal whether El Niño has a hidden pattern. The result looked like noise: after subtracting the seasonality, the temperature series became indistinguishable from a random process, akin to the Brownian motion of a dust particle in a glass of water.

To be sure, the scientists compared real data with model output. First, they built a model where the ocean and atmosphere exchange heat according to strict rules, with no random kicks. After removing seasonality, this model behaved like deterministic chaos, meaning predictable, though complex. But once they added random noise to the model, mimicking wind gusts and pressure fluctuations, the model series became as random as the observations. This suggests that El Niño is not a precise mechanism but rather the ocean's response to the atmosphere's erratic blows.

Previously, it was unclear whether El Niño is low-dimensional chaos driven by the seasonal cycle or a stochastic process caused by atmospheric noise. Now it is shown that after removing the seasonal cycle, the Niño 3 series is statistically indistinguishable from a random process. For forecasting, this means El Niño should be predicted probabilistically: instead of a single scenario, one should provide a range of possible outcomes, as in a month-ahead weather forecast. The ocean drum does not play from a score but from random strikes, and within this chaos there is its own order, a statistical one.