Before describing how altimetry can monitor El Niņo, I would like to discuss how El Niņo begins, and why El Niņo warming causes a sea-level variation.
During normal conditions, winds blow from east to west, due to differences in the atmospheric pressure. Normally, a high pressure system sits over the eastern Pacific, while a low pressure system sits over the western Pacific. Because of the low pressure system in the west, there is increased upward convection which puts water vapor into the atmosphere and results in more rainfall in the west than in the east.
At the same time, the surface currents along the equator generally move east to west. This transports water warmed year round by the sun to the western Pacific, where it tends to pile up before flowing north and south as other currents. This pushes down the thermocline, or the region where the temperature change with depth is the greatest. In the east, cold water upwells, or rises up from great depths to
replace the warm water which flowed west, and the thermocline
All of this will cause a certain sea-level signature, since
sea-level is a measure of the integrated water density. Warmer
water has a lower density than colder water, and takes up a
greater volume. Thus, sea-level is higher where the
thermocline is depressed and the upper waters
are warm, and sea-level is lower where the
thermocline is raised and upper water is cool.
Changes in the atmosphere over the
western Pacific cause all of this to change.
Figure courtesy of NOAA PMEL