Two pendulums hang side by side, with a spring stretched between them. One might stay still on its own, but if the other swings, the spring transmits a nudge, and the first one starts swinging too.
Two regions of the Pacific Ocean work the same way. They exchange heat through waves that travel from one shore to the other. One wave is fast, the other slow, and this time difference acts like a spring. The scientists captured this in an equation and used a computer to see what happens for different coupling strengths.
Previously, scientists thought the influence of a neighboring region was random noise, like gusts of wind. Now it turns out to be a regular mechanism: one region nudges the other, and that changes the whole picture. Instead of a single threshold of 0.5 °C, they now suggest looking at a range: from 0.5 to 1.6 °C in the first region and from -0.06 to 2 °C in the second.
So two pendulums on a wall teach each other to swing, and the Pacific Ocean follows the same rules.
Kelvin and Rossby waves, which carry heat across the Pacific, travel at different speeds: the fast Kelvin wave crosses 13,350 km in 110 days, while the slow Rossby wave takes 329 days. This difference creates the delay needed for oscillations.
In Kim Stanley Robinson's novel 'Forty Signs of Rain', climate models predict abrupt changes in ocean currents. Here a similar idea appears: small changes in initial conditions can switch the system between states, as in scenarios of climate surprises.