Before describing how altimetry can monitor El Niño, I would like to discuss how El Niño begins, and why El Niño 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 greatest. In the east, cold water upwells, or rises up from the great depths to replace the warm water which flowed west, and the thermocline is shallow.

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.

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