Because a Kelvin wave is associated with density fluctuations inside the ocean, it can be seen in the sea-level measurements made by altimeters, although with a reduced magnitude. The figure to the right shows sea-level anomalies determined from TOPEX/Poseidon. Notice the two Kelvin waves, with amplitudes of 10 and 15 cm in sea-level compared to 30 and 40 meters in thermocline change. Note that a depression in the thermocline (from the previous picture) is associated with an increase in sea-level.
The Kelvin waves travel east and set up changes in the eastern Pacific that lead to El Niņo by depressing the thermocline there. There are smaller Kelvin waves in 1995 and 1996. These are seasonal, and do not cause El Niņo events. However, large Kelvin waves, such as those observed in early 1997 almost always lead to El Niņo events.
A larger version of this image is available from