Walker Circulation

The Walker circulation is the tropical Pacific's atmospheric conveyor belt, named for Sir Gilbert Walker, who inferred it from pressure records a century ago. In its normal state: moist air rises in colossal thunderstorms over the warm pool near Indonesia, flows eastward at high altitude, sinks — dry — over the cooler eastern Pacific, and returns westward at the surface as the trade winds. Rising air means rain (Indonesia's forests); sinking air means desert (coastal Peru).

The loop exists because of the ocean's east–west temperature contrast, and it maintains that contrast — the trade winds it drives are what pile warm water in the west. That two-way dependence is ENSO's engine room.

During El Niño, warm water spreading east erodes the temperature contrast; the Walker circulation weakens and its rising branch migrates toward the central Pacific. The consequences are this site's impacts section in miniature: drought where the rising air used to be (Indonesia, Australia), rain where it lands (the central Pacific, coastal South America), and a cascade of jet-stream effects beyond. The circulation's health is measured by the Southern Oscillation Index — the pressure seesaw tracked on our dashboard.