An eastward-propagating oceanic wave along the equator that deepens the thermocline and can trigger El Niño events.A Kelvin wave is a type of ocean wave that travels eastward along the equator, trapped near the coast or the equator by the Earth's rotation. Unlike ordinary waves that move across the ocean surface, a Kelvin wave involves a bulge in the thermocline—the boundary between warm surface water and colder deep water—that propagates along the equator. In the Pacific, these waves are crucial for the onset of El Niño, as they push warm water eastward and deepen the thermocline off South America.
The wave is named after Lord Kelvin, who first analyzed waves trapped at coastlines in the 19th century. The equatorial version was later applied to oceanography in the 1960s, notably by Klaus Wyrtki, who linked it to El Niño dynamics.
The wave's behavior comes from the Coriolis effect, which deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. At the equator, the Coriolis force vanishes, but just north and south it acts in opposite directions, effectively trapping the wave's energy within a narrow band around the equator. The wave propagates eastward because the restoring force is gravity acting on the displaced thermocline, and its speed is set by the local gravity wave speed, typically a few meters per second. When such a wave reaches the eastern Pacific, it depresses the thermocline, reducing upwelling of cold water and allowing sea surface temperatures to rise—a hallmark of El Niño.
Kelvin waves are monitored in real time using satellite altimetry and moored buoys (like the TAO array) to forecast El Niño and La Niña events months in advance.
A single Kelvin wave crossing the Pacific can take about two to three months, and its arrival at the South American coast often signals the start of an El Niño warming.
