The Nature of Geographic Information

23. Using Radar Altimetry to Monitor El Nino


Remotely sensed data play crucial roles in global change research. One example is data produced by radar altimetry, which are used to monitor the global climatological phenomenon called El Niño. El Niño is the colloquial name for a periodic weakening of the trade winds and warming of the surface layers of the eastern and central equatorial Pacific Ocean. El Niño events occur irregularly at intervals of 2-7 years, and typically last 12-18 months. Poorly understood teleconnections between the ocean and the atmosphere result in altered regional precipitation patterns that sometimes include severe floods. The animation below illustrates the changes in sea surface temperatures that accompanied the 1997-98 El Niño event.

Gif file showing changing sea surface temperature
Figure 8.24.1 Sea surface temperature anomalies from January 1997 through March 1998, estimated from on-site measurements and sea surface elevation data produced by the TOPEX-POSEIDON satellite altimeter. NOTE: To replay the animation, hold down the Shift key and reload the page (NOAA-CRIES Climate Diagnostics Center, 2006).

Active remote sensing provides an alternative to the expensive and tedious business of measuring sea surface temperatures directly at many locations throughout the Pacific Ocean.

Two researchers collecting data from a buoy
Figure 8.24.2 Researchers collect data from one of the 70 buoys in the Tropical Atmosphere Ocean (TAO) array. TAO buoys are equipped with sea surface temperature monitoring instruments. Hourly observations are stored in instrument memory and must be retrieved by operators (Tropical Atmosphere Ocean Project, n.d.).

From 1992 through 2005, the TOPEX/POSEIDON radar altimeter measured heights of the ocean surface with centimeter accuracy. The sensor transmitted and received longwave energy at 6 km intervals along ground tracks spaced 315 km apart. The satellite that carried the sensor completed a polar orbit every 112 minutes at an altitude of 1,335 km, passing over the same point every 10 days. Sea level deviations (differences between the geoid and measured sea level) were determined from measurements of the height of the ocean surface relative to the sensor, which is calculated from the time difference between the transmission and return of signals reflected from the ocean surface. Sea surface temperature can be inferred from deviations of sea surface heights relative to long-term mean values.

World map showing sea surface elevations coded by color
Figure 8.24.3 Deviations of sea surface elevations from long-term averages, measured by the TOPEX-POSEIDON radar altimeter. Sea level deviations are used as a surrogate measure of the warming of the eastern equatorial Pacific Ocean that indicates an El Niño event. The S-shaped curves trace the path of the satellite's polar orbit as the Earth rotates beneath it (NASA, Jet Propulsion Laboratory, 2006).
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