A Kelvin wave is a wave in the ocean, a large lake or the atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. 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 as it moves in the alongshore direction over time. A Kelvin wave (fluid dynamics) is also a long scale perturbation mode of a vortex in superfluid dynamics; in terms of the meteorological or oceanographical derivation, one may assume that the meridional velocity component vanishes (i.e. there is no flow in the north–south direction, thus making the momentum and continuity equations much simpler). This wave is named after the discoverer, Lord Kelvin (1879).
Coastal Kelvin wave In a stratified ocean of mean depth H, whose height is perturbed by some amount η (a function of position and time), free waves propagate along coastal boundaries (and hence become trapped in the vicinity of the coast itself) in the form of Kelvin waves. These waves are called coastal Kelvin waves. Using the assumption that the cross-shore velocity v is zero at the coast, v = 0, one may solve a frequency relation for the phase speed of coastal Kelvin waves, which are among the class of waves called boundary waves, edge waves, trapped waves, or surface waves (similar to the Lamb waves). Assuming that the depth H is constant, the (linearised) primitive equations then become the following:
the continuity equation (accounting for the effects of horizontal convergence and divergence): ∂ u ∂ x + ∂ v ∂ y = − 1 H ∂ η ∂ t {\displaystyle {\frac {\partial u}{\partial x}}+{\frac {\partial v}{\partial y}}={\frac {-1}{H}}{\frac {\partial \eta }{\partial t}}}
the u-momentum equation: ∂ u ∂ t = − g ∂ η ∂ x + f v {\displaystyle {\frac {\partial u}{\partial t}}=-g{\frac {\partial \eta }{\partial x}}+fv}
the v-momentum equation: ∂ v ∂ t = − g ∂ η ∂ y − f u . {\displaystyle {\frac {\partial v}{\partial t}}=-g{\frac {\partial \eta }{\partial y}}-fu.}
in which f is the Coriolis coefficient, which depends on the latitude φ:
f = 2 Ω sin ϕ {\displaystyle f=2\,\Omega \,\sin \phi }
where Ω ≈ 2π / (86164 sec) ≈ 7.292×10−5 rad/s is the angular speed of rotation of the earth. If one assumes that u, the flow perpendicular to the coast, is zero, then the primitive equations become the following:
the continuity equation: ∂ v ∂ y = − 1 H ∂ η ∂ t {\displaystyle {\frac {\partial v}{\partial y}}={\frac {-1}{H}}{\frac {\partial \eta }{\partial t}}}
the u-momentum equation: g ∂ η ∂ x = f v {\displaystyle g{\frac {\partial \eta }{\partial x}}=fv}
the v-momentum equation: ∂ v ∂ t = − g ∂ η ∂ y {\displaystyle {\frac {\partial v}{\partial t}}=-g{\frac {\partial \eta }{\partial y}}}
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