Kelvin Wave

If El Niño is a flood of warm water eastward across the Pacific, Kelvin waves are the individual deliveries. An oceanic Kelvin wave is a subsurface pulse, trapped along the equator by the Earth's rotation, that travels west-to-east across the basin in roughly two months.

The trigger is usually the wind. A westerly wind burst — a spell of days when the far-western Pacific's winds blow opposite to the trades, sometimes courtesy of the Madden–Julian Oscillation — shoves surface water eastward and squeezes the thermocline downward. That depression propagates east as a "downwelling" Kelvin wave, carrying a bulge of warm water beneath the surface. When it arrives in the central and eastern Pacific weeks later, it deepens the thermocline there, shuts down the supply of cold upwelled water, and warms the surface.

One Kelvin wave makes a warm blip. A sequence of them — each burst of westerlies reinforcing the last, each warming making the next burst likelier — is how El Niños are built. Satellite altimeters and the Pacific's buoy array track the waves in real time, which is why forecasters could watch the spring 2026 deliveries stacking up before the surface committed (the May story). Their cumulative result is the number on the dashboard.