Waves, Kelvin

KELVIN WAVES AFFECTING weather and climate occur in both the oceans and the atmosphere. These low-frequency, gravity-driven waves propagate vertically and parallel to boundaries (e.g., equator, coastline, air masses, and topography). Kelvin waves are nondispersive and carry energy from one point to another. The height or amplitude of a Kelvin wave is highest near the boundary where it propagates; the wave height decreases as the wave moves farther away from the boundary. In the Northern Hemisphere, the waves propagate anticlockwise, and in the Southern Hemisphere, the waves propagate clockwise. The flow of the Kelvin wave balances pressure perpendicular to the boundary by the forces of gravity and the Coriolis effect.

Kelvin waves in the ocean form as coastal waves or equatorial waves, both of which are caused by external forces—often a shift in the trade winds or resulting from temperature variations—and the water inside the Kelvin waves is usually a few degrees warmer than the surrounding water. Kelvin waves may be called external (or barotropic) Kelvin waves if the ocean is homogenous, and internal (or baroclinic) Kelvin waves if the ocean is stratified.

In the Northern Hemisphere, the equatorial waves propagate parallel to the equator and to the east, and the coastal Kelvin waves propagate in a counterclockwise direction, using the coastline for direction. These waves can be between 5 and 10 cm. high and hundreds of km. wide. Kelvin waves tend to move quickly, with a typical speed of approximately 250 km. (155 mi.)/day and can cross the Pacific in approximately 2 months.

The tidal cycles can cause Kelvin waves by the mechanism of a progressive tide wave moving from open ocean into and out of a narrowed body of water. Because of the Earths rotation, resulting in an anticlockwise direction of current flow inside the channel, flood tides will be greater on the right side of the channel.

The effect on climate results from the Kelvin waves causing a variation in the depth of the oceanic ther-mocline (the boundary between warm waters in the upper ocean and cold waters in the deep ocean). Because of this variation, Kelvin waves can be used to predict and monitor El Nino activity. In comparison with Rossby waves, which carry water back toward the western Pacific and take as long as a decade to move from the eastern Pacific to the western Pacific, the faster-moving Kelvin waves carry warm water eastward in approximately 2 months.

In the atmosphere near the equator, Kelvin waves travel eastward and may propagate upward to higher altitudes. The formation of Kelvin waves is triggered by mountains, thunderstorm updrafts, and anything that interrupts the normal flow of stable air. The trigger forces the air upward, and the stable air sinks by gravity instead of just returning to normal; when air sinks farther, it causes the wave motion. Kelvin waves cannot happen in unstable air because the motion would just allow the movement of air to continue upward to higher altitudes.

Kelvin waves may propagate in the lower and upper stratosphere and mésosphère. In the lower stratosphere, the eastward-moving Kelvin wave is associated with periods of 10 to 30 days, and in the upper stratosphere, the Kelvin wave is associated with periods of 5 to 7 days. In the mésosphère, the Kelvin wave is associated with periods of 3 to 4 days. These Kelvin waves transport energy and eastward momentum upward and contribute to the maintenance of the eastward flow.

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