The T/P data show a Kelvin wave in the central Pacific in January, while the SST data indicate cooler than normal surface waters in the region. By February sea-level and SST were both higher than normal in the eastern Pacific after the Kelvin wave reached the coast. In March, a larger Kelvin wave began forming and can be seen in the T/P data; again, it is not observable in the SSTs.
The second Kelvin wave moved across the Pacific in April and May, as observed by T/P. By May, there was a tongue of warm water extending westward past 250°E. The T/P data, sensitive to heat changes at depth, shows that the warming approached below the surface from the west. The SST data on the other hand shows the surface warming moved west from the coast after the Kelvin waves reached the eastern Pacific and depressed the thermocline.
By July, El Niño had fully developed. Note the similarities in sea-level and SST at this point, since the heating has mixed throughout the water. There are significant differences, though. Peak sea-level was to the west of peak SST, and there were much larger sea-level changes in the western Pacific than SST changes. All of these differences were due mainly to sub-surface heat changes which the SST data cannot detect.
Both sea-level and SST continued to rise through the Fall. Both peaked in early December. Note the “lobes” of high sea-level forming north and south of the equator in December (indicated by arrows). These are indicative of another type of internal wave, a Rossby wave. Just like Kelvin waves were created when the ocean was adjusting to the formation of El Niño, Rossby waves are created as it dissipates. These waves move westward, and help re-adjust the thermocline in the west and east to normal.