My OperaKelvin Wave
Wednesday, December 13, 2006 6:05:56 PM
In the Pacific Ocean the trade winds blow from Ecuador to Indonesia. The strong, steady winds push surface waters westward, and in fact, the sea surface height in the western Pacific near Indonesia is about 45 centimeters higher on average than the height in the eastern Pacific off the coast of Ecuador. The Indonesian waters are also warmer because as the winds move sun-warmed surface waters away from South America, colder, deeper ocean water wells up to the surface. See figure 1. At Indonesia the warm water layer is about 200 m thick. At Ecuador the warm water layer is only about 50 m thick.
Occasionally the trade winds weaken (El Niño event), and the water piled up in the western Pacific slides gently back across the Pacific. See figure 2. Notice the build up of warm water at the South American coast.
The pulse of returning warm water is called a Kelvin wave. Kelvin waves are gravity driven. They are often excited by an abrupt change in the overlying wind field, such as the shift in the trade winds at the start of El Niño. They are however not restricted to the Pacific or to El Niño events. There are two types of Kelvin waves, coastal and equatorial. Some day I may write more about Kelvin Waves and other similar wave types.A Kelvin wave will cross the Pacific Ocean in about 70 days.
In mid-November 2006, a Kelvin wave was gliding its way across the Pacific and was captured in the image of figure 3 from the NASA/French Jason-1 satellite, which measures sea surface height using a laser altimeter. Because warm water takes up a little more space than cooler water, higher-than-normal sea surface height (positive sea level anomaly) indicates warmer-than-normal sea surface temperatures. Along the equator, the red sea surface heights equate to sea surface temperature departures greater than one to two degrees Celsius.Persistent breakdowns of the trade winds result in a conveyor belt of Kelvin waves that deposit warm waters in the eastern Pacific; the resulting build up of warm water in the usually cool eastern Pacific is the hallmark of El Niño. El Niño events influence weather patterns around the globe. The American West, which has been struggling with drought, often receives more rain during Niño events. But in southeastern Australia, Niño may increase the possibility for a severe summer fire season.
- Main (but not sole) source: http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17484
See also my blog on El Niño Established.
PS: Upwelling is of ecological importance. In regions of upwelling, the supply of nutrients for phytoplankton is continually being replenished from depths (of over 50 meters), and so the production of pytoplankton is high. Higher up the food chain this is important for fish, and therefore for the fishing industry. In an El Niño year upwelling ceases along the eastern Pacific. The local fishing industry along the affected coastline can suffer during long-lasting El Niño events. The world's largest fishery collapsed due to overfishing during the 1972 El Niño Peruvian anchoveta reduction.
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