Every few years, the warm El Niño current heats up the tropical Pacific. That heating acts like a conductor, setting the tempo for the mid-latitude atmosphere. But the atmosphere doesn't respond in just one way; it responds in two. One response is steady and predictable, like an orchestra playing from the score. The other is erratic, yet punctuated by powerful low notes that emerge once every few decades.
Scientists built a computer model with two components: a tropical one that captures El Niño itself, and an extratropical one that describes the mid-latitude atmosphere and ocean. They ran the model hundreds of times from different initial conditions, each run extending fifteen thousand years into the future. It turned out that the system can be attracted to one of two regimes. The first regime changes rapidly and has almost no slow internal oscillations: its variability is locked to El Niño's rhythm. The second regime changes slowly, with a period of about 58 years, and can be predicted twice as far ahead as the first.
This finding matters for weather and climate prediction. Forecasters typically average over many model runs to get the most likely scenario. But if the system can jump between two regimes, the arithmetic mean describes neither of them. It's like averaging the sound of two orchestras playing in different keys: you'd get cacophony. So reliable forecasts require very long runs and a large ensemble of possibilities, so as not to miss the second regime.
In the chaotic regime, the oceanic trajectories of PBA2 align into three distinct bundles, shifted in phase by 35 years. This resembles a troop of soldiers marching in lockstep, even though each one follows its own chaotic trajectory.
The idea of multiple climate regimes, between which the system can jump, echoes Kim Stanley Robinson's novel 'Forty Signs of Rain', where climatologists confront unexpected and abrupt shifts in the climate system that do not fit linear forecasts.