Shouhong Wang

A seesaw with two children rarely balances perfectly. It tips either left or right, pausing at the extremes for a moment before swinging back. The tropical Pacific behaves similarly: it either warms up off the coast of Peru (El Niño) or cools down (La Niña), and these states can last for months, sometimes over a year.

In the 1920s, British meteorologist Gilbert Walker, trying to forecast monsoons in India, noticed a strange connection: when air pressure rises over the eastern Pacific, it falls over the western Pacific, and vice versa. It was as if a giant air pendulum were swinging along the equator.