A swing picks up speed when you push it in time. But sometimes the person on the swing responds not to every push but to every third one, and the amplitude grows slowly yet inexorably. Similar behavior has been found in Earth's global temperature: after removing the slow warming from data for 1880–2009, what remains are oscillations almost entirely governed by a three-year rhythm, not an annual one.
The author explains this shift as subharmonic resonance. A nonlinear oscillator, such as a pendulum with a stiff spring, can respond at a frequency that is a fraction of the driving frequency. The annual push becomes a three-year cycle if the system's natural frequency is close to one-third of a year. In the climate, Rossby waves, giant planetary waves that take years to cross the ocean, may play the role of such a pendulum.
Previously, three-year oscillations were attributed to random noise or external factors like solar activity. Now it is clear that the climate system itself transforms one period into another, without additional external signals. This changes the view on the origin of interannual variability and may explain why El Niño has a characteristic quasi-three-year rhythm.
Third-order subharmonic resonance means that the system responds at a frequency three times lower than the forcing frequency: an annual push generates a three-year cycle. Such behavior is familiar in nonlinear electrical circuits and mechanical oscillators, but in climate it appears unexpected.