The ocean is like a pendulum with a memory. A push from seasonal winds returns not immediately but months later, when a wave crosses the Pacific. This delay, the authors argue, is what makes El Niño and La Niña repeatedly peak at the same time of year.
The model reduces the ocean to a single number, surface temperature, and hides the physics in the delay. The rate of temperature change depends on its value in the past, not the present. Seasonal forcing adds a rhythm, like pushing a swing. The equation is solved numerically, sweeping three parameters: delay from zero to two years, ocean-atmosphere coupling strength, and amplitude of the seasonal push.
With weak seasonal forcing, the phase can jump abruptly: the temperature maximum suddenly occurs in summer. This is not a smooth shift but a leap between stable states. For the same parameters, the model can yield dozens of different solutions; in one run, 61 were found, all with a period of ten years.
The boundaries between regimes in the space of initial conditions turn out to be fractal. Change the starting temperature slightly, and the system settles into a different rhythm. As the delay increases, periods grow in jumps: one year, three, five, seven. Between them lie narrow bands of chaos where behavior never repeats.
The work shows that even the simplest delayed model reproduces the winter locking of El Niño but not La Niña. The cold phase requires an additional mechanism, for example another feedback loop with its own delay. This matters for predictions: if the system can abruptly switch regimes, then climate warming could make its behavior even more unexpected.
The name 'El Niño' (Spanish for 'the boy child') comes from the warm current off the coast of Peru often appearing around Christmas, in honor of the baby Jesus.