Kelvin wave

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A Kelvin wave is a wave in the ocean or atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline. A feature of a Kelvin wave is that it is non-dispersive, i.e., the phase speed of the wave crests is equal to the group speed of the wave energy for all frequencies. This means that it retains its shape in the alongshore direction over time.

A Kelvin Wave (fluid dynamics) is also a long scale perturbation mode of a vortex in superfluid dynamics.

Kelvin waves in the ocean always propagate with the shoreline on the right in the northern hemisphere, and with the shoreline on the left in the southern hemisphere.

[edit] Equatorial Kelvin wave

An equatorial Kelvin wave is a special type of Kelvin wave in which the equator acts analogously to a topographic boundary for both the Northern and Southern Hemispheres. When the motion at the equator is to the east, any deviation toward the north is brought back toward the equator because the Coriolis force acts to the right of the direction of motion in the Northern Hemisphere, and any deviation to the south is brought back toward the equator because the Coriolis force acts to the left of the direction of motion in the Southern Hemisphere. Note that for motion toward the west, the Coriolis force would not restore a northward or southward deviation back toward the equator; thus, equatorial Kelvin waves are only possible for eastward motion. Both atmospheric and oceanic equatorial Kelvin waves play an important role in the dynamics of El Nino-Southern Oscillation, by transmitting changes in conditions in the Western Pacific to the Eastern Pacific.

Equatorial Kelvin waves are often associated with anomalies in surface wind stress. The plot below shows wind stress anomalies (deviations from "normal" winds) on the left and on the right the depth at which the water is 20°C known as the 20°C isotherm depth, which defines the lower limit of the surface layer of warm water). Positive (eastward) anomalies in wind stress in the central Pacific excite positive anomalies in 20°C isotherm depth which propagate to the east as equatorial Kelvin waves.