The Pacific pendulum swings irregularly. Sometimes it barely trembles near the bottom, and sometimes it suddenly makes a giant sweep that disrupts weather across the planet. El Niño, the warm current in the equatorial Pacific, recurs every few years, but truly strong events happen once every 10–20 years, and until now no one could explain why they appear so irregularly. New work shows that the ocean behaves like a system with mixed-mode oscillations, where small cycles alternate with large ones, and to transition to a large cycle you must cross a point of no return.
The authors took a three-dimensional model that describes the temperature of the eastern and western parts of the equatorial Pacific and the depth of the layer of sharp temperature change. After a change of variables, the equations became similar to a pendulum with two time scales: one variable changes quickly, while the other two change slowly. They then applied geometric analysis that allows one to see how trajectories of the system are attracted to slow manifolds and how they jump off them in fast excursions. It turned out that the manifold has a fold, and on it a special point that acts like a funnel: once caught in it, the system starts to rotate and gradually builds up amplitude.
Two mechanisms create small oscillations before the big sweep. The first is rotation around a saddle-focus, a special point where trajectories spiral. The second is the twisting of slow manifolds around a folded node, which causes the amplitude to first decrease and then increase. In both cases, the system accumulates energy until it reaches the threshold, after which a strong El Niño inevitably develops. This threshold can be computed in advance, and it acts as a boundary beyond which the pendulum cannot be stopped.
Previously, the irregularity of El Niño was explained by random noise or chaos, but now it is clear that behind the bursts lies a robust mechanism. The pendulum is not just jiggling under random pushes; it is inherently structured so that from time to time it passes the point of no return and makes a giant sweep. This knowledge could help in forecasting: if we monitor how the system approaches the threshold, we can see in advance the signs of a future extreme event. The model is simplified, but its geometry provides a clear criterion that is worth looking for in real data and more complete climate models.
The name 'El Niño' (Spanish for 'boy') was given by Peruvian fishermen who noticed that a warm current sometimes appears around Christmas. In the model, the point of no return for the development of a strong El Niño corresponds to the western Pacific temperature dropping just a few degrees below the climatic norm.
The idea of a critical threshold after which the system irreversibly transitions to an extreme state resonates with the concept of 'tipping points' in climate fiction, for example, in Kim Stanley Robinson's novel 'The Ministry for the Future', where climate tipping points trigger a cascade of events.