On the island of Tahiti and in the Australian city of Darwin, barometers stand. They measure air pressure every day. The difference between these two measurements is called the Southern Oscillation Index. In the daily records of this index, as in the noise of an orchestra, steady rhythms are hidden.
If you add all these rhythms together, you can predict what the index will be nine months from now. The forecast accuracy was assessed as 0.73 on the AUC scale. This is a good result for predicting whether the pressure will be above or below average.
After removing the rhythms, the remaining random fluctuations turned out to be not entirely random. Rare strong events such as El Niño do not occur independently of one another but with a memory of the past. This means that after one strong event, the next may come sooner than if pure chance were at play.
The barometers on Tahiti and in Darwin continue to measure pressure. Now their daily readings can be read like a score, where short notes hint at what the next measure will be.
The 100-day period may arise as a beat between 37 and 27 days: 1/(1/27 - 1/37) ≈ 100 days. This is an example of how the interaction of two oscillations generates a third, slower one.