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thefrog

Kelvin Waves ... Atmospheric

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Hello,

I'd like to compare oceanic and kelvin waves,

I know that in oceanic kelvin waves, the phase speed is given by c = sqrt(gH) where H is the depth and g the acceleration due to gravity.

Is the same true for atmospheric kelvin waves? What is their phase speed?

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Also, are atmospheric Kelvin waves affected at all by the coastline? Of course the oceanic ones will be, as the coastline is a boundary for the ocean, but what about the atmospheric ones?

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The derivation of Kelvin Wave equations is way beyond me but as far as I'm aware the phase speed relative to the ground in the equatorial lower stratosphere is about 25 m s-1 moving eastward. This compared to a Mixed Rossby gravity wave of about 23m s-1 moving westward.

There are members on here who know far more about this than me who could come up with a much fuller explanation. It might be an idea to state what your definition of Kelvin Waves is to make sure we all singing from the same hymn sheet.

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Kelvin Waves are eastward propogating waves near the atmosphere. They have a vertical scale of about 10 km ... so clearly their speed is less than the phase speed for gravity waves given in my first post. Why is this? They are gravity waves, so do they have a 'reduced gravity' effecting them?

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I’m afraid I’m getting a little confused here and I suspect one of the problems is I’m not quite understanding what you are getting at. Gravity waves are definitely confusing the issue. Without going into any detail atmospheric gravity waves can be generated by many sources like jet streams, tidal waves, tropical cyclones, hurricanes, volcanic eruptions, nuclear explosions and thunderstorms. They propagate vertically and horizontally, dissipate, interact nonlinearly, and profoundly influence the momentum, energy, and the constituents in the atmosphere. But I have a feeling you are talking about equatorial waves and if so there are two classes of well established equatorial waves, Kelvin waves and mixed Rossby gravity waves and I wouldn’t class Kelvin waves as gravity waves. These two equatorially trapped wave modes are believed to play a crucial role in forcing the quasi-biennial oscillation (QBO) of the lower tropical stratosphere.

A few comments on Kelvin waves.

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 nondispersive, i.e., the phase sped of the wave crest 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. 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.

Equatorial Kelvin Waves.

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. This wave always propagates eastward and exist only on the equator. Equatorial Kelvin waves are often associated with anomalies in surface wind stress.

Equatorial waves propagate towards the east in the northern hemisphere, using the equator as a wave guide. Coastal Kelvin waves propagate around the northern hemisphere oceans in a counterclockwise direction using the coastline as a wave guide. These waves especially the surface waves are very fast moving, typically with speeds of about 2.8ms-1, or about 250 km day-1.

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.

Courtesy Stratosphere Troposphere Interactions, K. Mohanakumar

This is a very complex subject, my understatement for the day, and a don’t pretend for one moment to have a great knowledge of it.

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