Topex/Poseidon finds global sea level change during El Nino

     The 1997-98 El Niño event may have been a major contributor in the average global sea level rising about eight-tenths of an inch before it returned to normal levels, according to scientists studying TOPEX/Poseidon satellite measurements of sea surface height.

     "This is the first time we have been able to identify that El Niño may cause a change in average global sea level," said Dr. R. Steven Nerem, a TOPEX/Poseidon science team member at the Center for Space Research at the University of Texas at Austin. "Understanding these short-term variations is important for understanding and detecting long-term variations caused by climate change."

     "TOPEX/Poseidon measures average global sea level at ten-day intervals with a precision of 0.16 inches, so detecting the 0.8-inch change associated with the El Niño was relatively easy," Nerem said. "However, these results tell us that detecting sea level variations caused by climate change will be more difficult because such changes are significantly smaller than the variations we have observed during the El Niño."

     Key to understanding the changes in the ocean are the global maps made by TOPEX/Poseidon. The sea level rise was not confined to the tropical Pacific, but also was observed in the Indian Ocean and the southern Pacific. Nerem's team then calculated the average global sea level.

     "These six years of satellite data are a good start, but we really need a decade or more of continuous measurements before we can accurately detect any climate-induced change," said Dr. Lee-Lueng Fu, the TOPEX/Poseidon project scientist at NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. "We need sustained observation records to understand the variations in the ocean."

     Global mean sea level change on seasonal and inter-annual time scales is a measure of the changing heat content of the ocean. The 0.8-inch rise during the El Niño implies that on average the global ocean may be gaining heat.

     "Average global sea level began rising in late March 1997, peaked at 0.8 inches above normal in early November 1997, and then began falling back to normal by the end of July 1998. Sea surface temperature began rising in late October 1996, peaked at 0.7 degrees Fahrenheit in late December 1997, and fell back to 0.2 degrees Fahrenheit at present," according to Nerem.

     Developed by NASA and the French Centre National d'Etudes Spatiales (CNES), the TOPEX/Poseidon satellite, launched in August 1992, uses an altimeter to bounce radar signals off the ocean's surface to get precise measurements of the distance between the satellite and the sea surface. These data are combined with measurements from other instruments that pinpoint the satellite's exact location in space. Every 10 days, scientists produce a complete map of global ocean topography, the barely perceptible hills and valleys found on the sea surface. A follow-on mission to TOPEX/Poseidon, Jason-1, is scheduled for launch in 2000.

         JPL manages the TOPEX/Poseidon mission for NASA's Earth Science Enterprise, Washington, D.C. The Earth Science enterprise will combine measurements like those from TOPEX/Poseidon with other information about the land, sea, air, and life on Earth to develop a greater understanding and predictive capability of the global environmental system.