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Kelvin and Rossby Waves The interaction of the wind and waves results in the phenomenon which we call El Niņo, and scientists have identified two sets of waves that appear to contain the information that starts and stops an El Niņo. Although they are called waves, they are waves in the language of physics rather than waves that break on the seashore.
Kelvin waves are created by winds blowing over the ocean surface from the west along the equator. Before a warm event develops, eastward flowing winds increase over the area of warm sea surface temperature to the east of New Guinea. These so called "westerly wind bursts" produce two effects: (1) they move warm water eastward from the warm pool to the central equatorial Pacific, generating the warm sea surface temperatures observed there, and (2) they also produce Kelvin waves, causing a lowering of the thermocline and an increase in the sea surface temperature over the eastern Pacific. These model animations on this page are from Dr. Steven Myers and RE Media showing what happens when the easterly winds abruptly turn the other way. After one month, the western Pacific has both type of regions. It has deeper regions of the ocean because of the downwelling Kelvin Waves as shown around area 1 of Model Image A. The Western Pacific Ocean also has areas which are shallower due to the upwelling Rossby Wave as shown around area 2. Changes in the thermocline cause small changes in sea level (which can be measured by satellite, specifically the TOPEX/Poseidon Satellite.) Although the thermocline can not be measured directly by satellites, satellites can measure, and proxy information, changes in sea level that have been correlated with changes in the thermocline. Kelvin waves can change the depth of thermocline by 30 meters or more and the sea level by tens of centimeters. More specifically, this changes the volume of warm water in the western Pacific’s warm pool, as the thermocline becomes shallower than normal, while the eastern counterpart’s volume increases. Kelvin waves are the mechanism that is said to be responsible for the popular notion that warm water sloshes back and forth (from west to east and back again) across the equatorial Pacific basin, as water does in a filled bathtub each time the person in the bath moves.
In the first image, the Kelvin has just reached the western coast of South American; this took about 2.5 months, a distance of one-third of the circumference of the Earth. In this image, one can see that the most upper layer in the west is now thinner than normal, as shown by area 1 Model Image B. Also notice that the upper layer in the central ocean is thicker than normal, as shown by area 2 Model Image B. The thermocline, therefore, is now lowered in the east. Once forced, Kelvin waves move eastward, independently of the season. Although it has been centrally implicated in starting off an El Niņo, a Kelvin wave does not necessarily lead to an El Niņo event. When the Kelvin wave "hits" the coast of South America, it generates coastal Kelvin waves that propagate both to the north and to the south along the western coasts of North and South America. It also generates what is known as a Rossby wave, a westward-moving internal wave that travels at one-third the speed of a Kelvin wave. Rossby wave are many miles in length, and have two parts: the top part takes up approximately the top 300 feet of the ocean, and the lower part is below this. One part moves one way, and the other part moves in the opposite direction. At intervals the two parts switch directions. Rossby waves are slow, moving only several miles a day. They can take some nine months to reach Australia and southeast Asia. Rossby waves depress the thermocline in the western Pacific region. Some people think a Rossby wave begins the process of decay of an El Niņo, suggesting that the onset of an El Niņo carries with it seeds of its own destruction. While such internal waves are not visible to the naked eye, there are ways to identify their existence using indirect measure. Now the Kelvin wave is reflected as Rossby wave northwards. The two white circles representated by area 1 of Model Image C are the two Rossby waves. In this image, we can also see that the Kelvin wave is now spreading northward against the Western North American coast. This is called the coastal Kelvin wave. This coastal wave ceases the upwelling near the Californian coast. This causes warmer waters, and thus, fishes then move northward towards Canada in search for cooler water. This wreaks havoc on the Californian fishermen. In this image, the Rossby wave has now reached the central Pacific ocean, as shown by area 1 of Model Image D. Area 2 of Model Image D, the red arcs east of the eyes of the Rossby waves, represents another form of Rossby waves which are usually away from the equator. In the last image, we can see that the Rossby wave has spread towards the western Pacific Ocean, as shown by area 1 of Model Image E. The Kelvin Wave is restoring conditions in the east, as shown by area 2 of Model Image E. The net effect of a series of Kelvin waves is to raise the sea surface temperature systematically over much of the equatorial Pacific. Thunderstorm activity, following the warm water, and usually located over Indonesia and the warm pool, also moves eastward into the central and eastern Pacific. This weakens the trade winds, causing further warming of the SST, and so on. Scientists call this interaction a "positive feedback" mechanism between the ocean and the atmosphere, one that results finally in El Niņo conditions.
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