Kelvin and Rossby Waves

The interaction of the wind and waves results in the phenomenon which we call El Niño, and scientists have identified two sets of waves that appear to contain the information that starts and stops an El Niño. 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.

The ocean is full of waves, but you might not know how many kinds of waves there are. There's one called a Rossby wave that is quite unlike the waves you see when you visit the beach. It's more like a distant cousin to a tidal wave. The difference is that a tidal wave goes very quickly, with all the water moving pretty much in the same direction. In a Rossby wave, the upper part of the ocean, say the top 100 meters or so, will be lesirely sliding one way, while the lower part, starting at 100 meters and going on down, will be slowly moving the other way. After a while they switch directions. Everything happens very slowly and inside the ocean, and you can't even see them on the surface. These things are so slow, they can take months or years to cross the oceans. If you had the patience to sit there while one was going by, you'd hardly notice it; the water would be moving 100 times slower than walking speed. But they are large, hundreds or thousands of kilometers in length (not height! Remember, you can hardly see them on the surface), so they can have an effect on things. Another wave you rarely hear about is called a Kelvin wave, and it has some characteristics in common with Rossby waves, but is somewhat faster and can only exist close to the equator (say, within about 5 degrees of latitude around the equator).

El Ninos often start with a Kelvin wave propagating from the western Pacific over towards South America. Perhaps you saw, on the TV news, the movie for the El Nino of 1997/98? It showed a whitish blob (indicating a sea level some centimeters higher than usual) moving along the equator from Australia to South America. That's one of the hallmarks of a Kelvin wave, the early part of the El Nino process.

When an El Niño gets going in the middle or eastern part of the Pacific, it creates Rossby waves that drift slowly towards southeast Asia. After several months of travelling, they finally get near the coast and reflect back. The changes in interior ocean temperature that these waves carry with it "cancel out" the original temperature changes that made the El Niño in the first place. The main point is that it shuts off when the these funny interior-ocean waves travel all the way over to the coast of Asia, get reflected, and travel back, a process that can take many months.

This sequence of 3 TOPEX/Poseidon "snapshots" show sea surface height data in the Pacific Ocean. White and red indicated higher than average levels; purple and magenta indicate lower than average levels. These scenes show eastward-moving Kelvin waves andwestward-moving Rossby waves. Superimposed black circles show how the elevated sea surface moves east in April (top), then west as Rossby waves during July (middle) and December (bottom).

The 2 small black circles off the coast of the Americas correspond to sea level highs on the trailing edge of Kelvin waves. The leading edge of this wave group has bounced off the coastline, creating Rossby waves whose rising and falling sea levels are marked by solid and dashed lines, respectively.

In the July image, the circles are moving west with the Rossby waves. The "X" marks a relative sea level low caused by a Kelvin wave moving east.

In December, the Rossby waves continue to move westward. The shapes of the solid and dashed lines indicate that the Rossby waves are moving away from the Americas faster at the equator than at higher latitudes.