What is a Kelvin wave?

Kelvin waves are slow-moving, relatively flat, warm water waves that stretch thousands of miles across the Pacific Ocean. These waves affect the upper surface of the equatorial ocean — between the surface and the deeper thermocline (the transition layer below the surface where temperatures get colder) at depths between 660 and 3,300 feet. Kelvin waves move slowly — at about five to ten miles per hour — and can take about two to three months to cross the Pacific Ocean before reaching the South American coast. Due to their wide and flat structure, they do not break on shore like other waves.

Kelvin waves can form along the equator in the western Pacific Ocean and travel west to east in response to the weakening of the trade winds, as a result of El Niño. As the water flows eastward in the northern hemisphere, the coriolis force (due to the rotation of the Earth) pushes water to the right (towards the equator); and in the southern hemisphere, pushes water to the left (also towards the equator). When they reach the South American coast along the equator, the wave splits: about half of the wave heads south toward the pole and the other half heads north along the west coast of the Americas. Kelvin waves that travel alongside the coastline are called “coastally trapped” as they are held against the coast due to the Coriolis effect. During an El Niño event, there can be several Kelvin waves that form, often as a series of successive waves that occur over the span of many months.

Kelvin waves can make surface water temperatures warmer than normal as they push down the cooler waters of the thermocline. These waves also temporarily raise sea level, by about 6 to 12 inches or more, due to the larger volume of warm water occupying more space than the colder water.

Kelvin waves are unique from other kinds of waves.

• Typical ocean waves are faster moving at speeds averaging 20 miles per hour, with short wavelengths between 10 to 100 feet from peak to peak, and crash ashore when they approach land.
• Oceanic and atmospheric Rossby waves, also called planetary waves, are massive slow-moving waves that form due to the rotation of the Earth. Oceanic Rossby waves can contribute to high tides and coastal flooding in some parts of the world. Atmospheric Rossby waves are mainly formed from the Earth’s geography and can result in long, enduring weather patterns.
• Tsunamis form from sudden events such as earthquakes or underwater landslides and move rapidly at speeds up to 500 miles per hour.