New sea surface height measurements taken by the ocean-observing TOPEX/Poseidon satellite show the equatorial Pacific in a state of flux with the warm, high sea level El Nino-spawned waters in retreat and areas of colder, low sea level waters on the increase.

"Sea level is a measure of the heat stored in the ocean. In the last month or so, the tropical Pacific has been switching from warm to cold. Lower sea level indicates less heat, hence a colder ocean," said Dr. Lee-Lueng Fu, the project scientist for the U.S.-French TOPEX/Poseidon mission at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. "It appears now the central equatorial Pacific Ocean will stay colder than normal for some time to come because sea level is about seven inches below normal, creating a deficit in the heat supply to the surface waters. It is not clear yet, however, if this current cooling trend will eventually evolve into a long-lasting La Nina situation."

An El Nino condition begins when steady westward blowing trade winds weaken and even reverse direction. This change in the winds allows a large mass of warm water that is normally located near Australia to move eastward along the equator until it reaches the coast of South America.

This displaced pool of unusually warm water affects evaporation, where rain clouds form -- and, in turn, alters the typical atmospheric jet stream patterns around the world. The change in the wind strength and direction also impacts global weather patterns. The climatic event has been given the name El Nino, a Spanish term for "the Christ child," because the warm current first appeared off the coast of South America around Christmas.

The 1997-98 El Nino has been the strongest ever recorded. This phenomenon was responsible for record rainfall in California, heavy flooding in Peru, drought and wildfires in Indonesia, tornadoes in the southeast United States and loss of life and property damage worldwide. TOPEX/Poseidon's sea surface height measurements have provided scientists with their first detailed view of how El Nino's warm pool behaves because the satellite measures the changing sea surface height with unprecedented precision.

A "La Nina" (Spanish for "little girl") is essentially the opposite of an El Nino, where the trade winds are stronger than normal and the cold water that normally exists along the coast of South America extends to the central equatorial Pacific. A La Nina situation also changes global weather patterns and is associated with less moisture in the air, resulting in less rain along the coasts of North and South America. TOPEX/Poseidon will be able to track a potentially developing La Nina with the same accuracy.

"It may be too soon to say 'good-bye' El Nino and 'hello' La Nina, because the effects of El Nino will remain in the climate system for a long time," said Dr. Bill Patzert, a research oceanographer at JPL. "However, if the Pacific is transitioning to a La Nina, we'd expect to see clear, strong indication of it by late summer or early fall -- in approximately August or September -- just like we did last year with El Nino. The strongest impacts of a potential La Nina wouldn't be felt in the U.S. until next winter." A La Nina does not automatically follow an El Nino, Patzert added.

Developed by NASA and the Centre National d'Etudes Spatiales (CNES), the TOPEX/Poseidon satellite 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 ten days, scientists produce a complete map of global ocean topography, the barely perceptible hills and valleys found on the sea surface.

Ocean temperatures affect ocean topography, which is why the TOPEX/Poseidon radar altimeter is able to monitor the changing El Nino and La Nina conditions. With detailed knowledge of ocean topography, scientists can then calculate the speed and direction of worldwide ocean currents.

The new satellite image from June is available at URL:

JPL, a division of the California Institute of Technology, manages the TOPEX/Poseidon mission for NASA's Earth Science Enterprise, Washington, DC.

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