In 1981, the ocean off the coast of Peru behaved strangely. The warm current that usually arrives every few years changed its rhythm, and scientists still argue about what exactly shifted. A new study offers a simple answer: the ocean sloshes like water in a bowl that is rocked by tides and Earth's rotation.

The model reduces ocean dynamics to a single oscillation equation. Instead of complex calculations of currents and temperatures, the author describes how the water level in a basin responds to external pushes. The main push comes from lunar tides, which act with several periods, and the Chandler wobble, a free motion of Earth's poles with a period of about 6.46 years. A biennial modulation is added, which changes the strength of the restoring force.

The key point is the phase inversion of the biennial modulation in 1981–1996. Without this shift, the model would not reproduce the observed Southern Oscillation Index. With it, the correlation reaches 0.8, which is very high for climate data. Solar activity did not improve the fit, so it was excluded. The machine learning tool Eureqa independently confirmed the choice of factors: the quasi-biennial oscillation and the Chandler wobble.

Previously, El Niño was considered either a random process or too complex for a simple description. Now it is clear that the main features of the phenomenon are reproduced by a wave equation with periodic forcings. If rare regime shifts, such as the 1981 shift, can be predicted, the model will provide forecasts years ahead.

The ocean continues to slosh in its bowl. We are only beginning to understand which hands are rocking it.