A Kelvin wave is a low-frequency gravity wave whose motion is concentrated near a vertical boundary (e.g., a shore) or the equator.[1] It occurs in the ocean and the atmosphere,[2] as well as in large lakes.[3] Waves along a coast are called coastal Kelvin waves; those along the equator are equatorial Kelvin waves. In a coastal Kelvin wave, a pressure gradient across the flow balances the Earth's Coriolis force, while the fluid moves along the boundary.[1] Kelvin wave retains its shape as it moves (being, in the shallow water model, non-dispersive[4]), and therefore does not curl over.[5] The direction of the wave is fixed: the coastal wave moves towards the poles along the eastern coasts (like California[a]) and towards the equator along the western coasts (like Japan and Australia). Around a closed basin, coastal Kelvin waves travel counterclockwise in the Northern Hemisphere[1] and clockwise in the Southern Hemisphere.[2] Equatorial Kelvin waves always travel eastward.[2]
Kelvin waves are regular events. Both equatorial[6][7] and coastal[8] waves are observed multiple times per year. Observations of the equatorial Kelvin wave are important for predictions about El Niño–Southern Oscillation (ENSO), thus the phenomenon attracts attention of the mainstream media.[5] Coastal Kelvin waves are an important component of tide waves.[3]
The Kelvin waves have planetary scale, sometimes thousands of miles long.[5] The equatorial waves move large amounts of heat and are known to temporarily reverse some currents in the upper ocean. At the same time, the height of the equatorial wave (observed changes in the ocean level) is very small: in the Pacific, downwelling Kelvin waves raise sea level by just a few centimetres.[9]
During the meteorological or oceanographical analysis, it can be assumed that one velocity component of a Kelvin wave vanishes (for example, for an equatorial wave there is no flow in the north–south direction), thus making the momentum and continuity equations much simpler). This wave is named after its discoverer, Lord Kelvin (1879).[10][11]