Every few years, the water in the Pacific Ocean becomes warmer than usual, triggering floods in some countries and droughts in others. This is El Niño, a phenomenon that recurs irregularly, every 3–7 years. Scientists want to predict it, but complex ocean-atmosphere models require enormous computing power. Simple models with a couple of equations would be more convenient, but previously they were devised by intuition. Now researchers have shown how to derive such simple models from the complex system rigorously, without guesswork.
In the complex model, the ocean and atmosphere are coupled, and random wind gusts add noise. The scientists identified special directions along which the system oscillates independently, much like finding the principal axes in a dance. It turns out that two such directions, with a period of about 4.6 years, explain almost everything. Projecting the noise onto these directions yields two simple equations. These equations reproduce El Niño's frequency, its strength, and even rare but intense events. The correlation with real observations exceeds 0.85.
Previously, simple models were like hand-drawn sketches, but now they are derived from the full picture. This means we better understand why El Niño behaves the way it does. The resulting models can be used for rapid forecasting and for testing new ideas in more complex climate models. Thus, order emerges from chaos, and from a symphony, a simple melody arises.
The phase shift between sea surface temperature in the east and thermocline depth in the west is 122 degrees, not 90, as one might expect for a simple harmonic oscillator. This detail is important for correctly reproducing the time delay in the delayed oscillator.