In 2014, forecasters around the world were expecting a powerful El Niño. The ocean had warmed, the trade winds had weakened, all the signs were there. But the event only came a year later. The Pacific seemed to change its mind.
The new study shows that the ocean can behave like a pendulum with two distinct modes of swinging. In one mode, the pendulum regularly swings high, which corresponds to years with strong El Niños. In the other, it jitters with small amplitude, and no strong events occur. The tricky part is that the switch between modes happens without any obvious cause, triggered by minuscule perturbations.
The researchers built a simplified mathematical model of the ocean. In it, sea surface temperature and the depth of the warm layer change according to three equations. When the parameters were tuned in a certain way, the model revealed two coexisting states. The first is a cycle of about 12 years with one powerful El Niño per period. The second is chaotic oscillations of smaller amplitude, with no strong events.
Even a weak seasonal forcing, a thousand times smaller than the main forces, makes the model jump between regimes. In a 6000-year simulation, runs of strong El Niños could last from a single cycle up to 24 in a row, while quiet periods without strong events could stretch for two centuries. All this without any external push, purely from internal dynamics.
The Pacific pendulum swings on its own, and its behavior is fundamentally unpredictable over the long term. That doesn't mean forecasts are useless, but they must be probabilistic: not "will there be an El Niño?" but "what is the probability of a strong event?" And that probability can change at any moment, like the ocean's mood.
In a 6000-year simulation of the model, an epoch without strong El Niño could last more than two centuries, then suddenly give way to a series of intense events, all without any external forcing other than a weak seasonal cycle.