A swing does not start rocking right after a push; it responds with a delay. The ocean is the same: the wind blows today, but the surface temperature changes several months later. According to the authors, it is this memory of the ocean that makes El Niño and La Niña arrive at the same time of year.

The model replaces the entire ocean with a single number: temperature. Its rate of change depends on what the temperature was earlier, not what it is now. Seasonal winds add a rhythm, like a person pushing a swing. The researchers tried different delay lengths and push strengths and found that warm events almost always occur in winter. Cold events, however, occur in summer, even though in nature they also happen in winter.

This means that a simple scheme is not enough to explain La Niña. Another mechanism is needed to shift the cold phase to winter. But the main finding is different: even such a simple system with memory can be very capricious and abruptly change its behavior. That is why predicting El Niño and La Niña is hard, and it will become even harder as the climate warms.