A barometer on the island of Tahiti and a barometer in the Australian city of Darwin show different pressures. Subtract one from the other, and you get a number that climatologists call the Southern Oscillation Index. It changes every day, but within these daily jumps hide steady rhythms, just as the beats of a metronome hide within the noise of an orchestra.

Each period finds an explanation in atmospheric and oceanic physics. 24 days is the time for a disturbance wave born in the tropics to circle the Earth. 27 days is the lunar cycle: tidal forces slightly alter the pressure. 37 days is the Madden-Julian oscillation, a large cluster of clouds and rain that slowly crawls over the Indian and Pacific Oceans. 76 days corresponds to intraseasonal oscillations of the tropical atmosphere. And 100 days likely results from the beating of two other rhythms: 27 and 37 days interfere like two strings tuned almost in unison.

If you add all these oscillations with the correct amplitudes, you get a signal reconstruction that predicts quite well what the index will be nine months later. Forecast quality was assessed by the area under the ROC curve: AUC = 0.73. For binary classification of the index sign, this is a good result.

This means that rare events, such as strong El Niño, do not occur independently of one another. The memory of past disturbances persists in the system, and the next extreme may arrive earlier or later than if everything were left to chance.

The barometers on Tahiti and in Darwin continue to measure pressure. Now their daily readings can be read not as noise, but as a score in which short notes hint at what the next measure will be.