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International Project Improves Satellite Measurements

11 April 2006

Method better for estimating El Niño-La Niña, tracking oil spills

Washington -- Researchers in France and the United States have improved a technique for obtaining satellite radar images in areas where calm or covered seas usually prevent accurate recordings.

The research was sponsored by NASA, the European Space Agency and the Centre National d'Etudes Spatiales (CNES), the French space agency.

The new method can provide more accurate ocean-surface readings, improve estimates of El Niño and La Niña events, and provide better observation and tracking of oil spills and other films on the ocean surface, according to an April 11 joint press release from the American Geophysical Union and the University of New Hampshire.

El Niño is a warming of the ocean surface off the western coast of South America that occurs every four years to 12 years when no upwelling of cold, nutrient-rich water occurs. La Niña is the periodic cooling of ocean waters in the east-central equatorial Pacific that affects the typical alignment of weather patterns.

The satellite altimetry (height measuring) method now used to measure height in ocean waves generally cannot provide data when the sea surface is calm, or "slick," because of oil spills, phytoplankton blooms or lack of wind. About 6 percent of all altimetry readings are affected by such data blackouts.

Altimetry works by sending 1,800 separate radar pulses per second down toward Earth from a height of 800 kilometers and recording how long their echoes take to bounce back.

The technique can measure sea height down to 1 centimeter, said Jean Tournadre of the French Research Institute for Exploitation of the Sea, who headed the research team.

The new system is similar to rainfall analysis methods, artificially increasing the number of altimetry signals sent to the ocean during calm conditions.

"Every year,” said study author Douglas Vandemark, “our altimeters make something like 10 million ocean measurements, so being able to recover at least a portion of that [lost data] gives us better information on sea surface height."

Vandermark, a research professor at the University of New Hampshire's Institute for the Study of Earth, Oceans, and Space, said the “percentage is much higher for calmer equatorial regions such as the Pacific, which tells us about the bulge of water that controls El Niño."

Picture the Pacific Ocean as a bathtub, he said. A bulge in the western end – created by warm water and winds – can set off an El Niño event by creating a wave that travels very quickly along the equator from Asia toward the Americas, eventually inducing large water temperature changes along the coasts of South and North America. When the bulge makes its inevitable and slower return back, that is a La Niña event.

The improvement comes from a modeling technique developed by scientists that should enhance and expand the number of altimeter measurements that NASA can collect – using the Jason-1 satellite – from the equatorial Pacific Ocean where El Niño events originate. Jason-1, launched in 2001, is a joint mission of NASA and CNES.

The new research, with what Vandemark calls "the problematic subset of echoes from very smooth ocean areas," suggests that altimeters now may be able to estimate the spatial extent of these smooth areas.

Having more accurate equatorial Pacific Ocean measurements, he said, "provides agencies like NASA and NOAA [the National Oceanic and Atmospheric Administration] more measurements for El Niño prediction and tracking."

A press release on the research is available on the University of New Hampshire Web site.

(Distributed by the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)