Kelvin waves can cross the Pacific in two months. They can only exist near the equator due to the Earth’s rotation. The amplitude of the Kelvin wave is several tens of meters along the thermocline, and the length of the wave is thousands of kilometers (1° longitude = 111 km)
The figure at the right shows Kelvin waves inside the ocean, computed with temperature data from moored buoys operated by NOAA. It shows the depth of the 20°C temperature level as a series of standard latitude-longitude plots stacked in time. The latitude width of each time step is 4° (2°S to 2°N) and time is from March 1996 at the top to March 1998 at the bottom.
Notice the yellow/orange lines which slope across the Pacific beginning in January 1997 (marked by thick lines). These are Kelvin waves. Eastward movement is indicated by the slope in time from west to east. These waves set up a change in the warm water thickness in the eastern Pacific beginning in March. Other Kelvin waves are visible after El Niño developed; the first two, however, were early indicators that an El Niño would probably occur this year.
Figure courtesy of NOAA PMEL