Kelvin wave

Kelvin wave, in oceanography and atmospheric science, an extremely long wave that occurs within a stratified (or layered) fluid, such as a layer of air or water. Kelvin waves are commonly associated with long ocean waves that propagate eastward in the Pacific and along the western coastline of the Americas, where they cause the upper ocean layer of relatively warm water to thicken, generating a temporary rise in local sea level. Although Kelvin waves generally occur in the Pacific Ocean and contribute to the development of El Niño conditions, they also appear in the Indian Ocean in conjunction with seasonal shifts in the monsoon. The Kelvin wave is named for Scottish engineer, mathematician, and physicist William Thomson, Baron Kelvin (later known as Lord Kelvin), who first discovered and described Kelvin waves in 1879.

Characteristics

Kelvin waves are gravity waves, in that gravity acts upon them to return them to their equilibrium state—that is, mean sea level. Whereas the periods of other water waves—that is, the time it takes for the wave to complete one cycle—are measured in seconds (wind waves), minutes (tsunamis), or hours (waves associated with tides), the periods of Kelvin waves may last several weeks. Unlike smaller, shorter water waves that reach the shoreline, Kelvin waves can be thousands of miles long, and they do not break. Rather, they keep their shape as they move slowly along the coast. In addition, they are associated with regions of warm water that elevate the ocean’s surface (as a result of thermal expansion) relative to adjacent cooler water (see also ocean warming). As the wave travels across the ocean or along the coast, its passing can raise the average height of an area of ocean by some 15–30 cm (6–12 inches).

Development and association with El Niño

Kelvin waves often appear toward the end of the year preceding an El Niño event, when an area of unusually intense tropical storm activity over Indonesia migrates toward the equatorial Pacific west of the International Date Line. The eastward migration of the center of tropical storm activity begins as trade winds weaken and experience brief reversals, which allows warm water concentrated in the western Pacific near Indonesia and northern Australia to flow toward the central Pacific and South America. Warm water at the surface also heats the surface layer of air, which drives the development of low-pressure cells and storms.

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El Niño events are triggered by a number of interacting atmospheric and oceanic factors, and Kelvin waves are one of the significant catalysts. The eastward movement of warm water along the Equator forces the thermocline (the oceanic water layer in which water temperature decreases rapidly with increasing depth) deeper, which prevents colder water from reaching the surface. This change reduces cold-water upwelling near South America, slowing the delivery of critical nutrients to phytoplankton near the surface (see also marine ecosystem: upwelling). It also raises the temperature of the surface layer in the central Pacific (a key indicator of the onset of El Niño conditions) and of the water near the South American coast. The stored heat energy in the water is transferred from the ocean to the atmosphere above, which increases convection and storm development. As additional warm water builds up in this part of the Pacific, the Kelvin wave spreads northward and southward along the seaboard, elevating coastal water levels.