Topex/Poseidon data

The U.S.-French ocean topography satellite, known as the TOPEX/Poseidon mission and scheduled for launch in mid-July, will be able to study global weather conditions like the El Nio that brought heavy rains to Southern California this month.

The weather abnormality, caused by an anomalous warming of the surface water throughout the central equatorial Pacific Ocean, occurs about every four to seven years, usually beginning in December.

The El Nio that we are having warms the Pacific Ocean along a region of about 240 kilometers (150 miles) north and south of the equator, said Dr. David Halpern, an oceanographer and senior research scientist at JPL. El Nios typically warm the equatorial waters by about 2 to 4 degrees Centigrade (4 to 7 degrees Fahrenheit).

Sometimes it produces the heavy rains, but not always, he said. The last time an El Nio occurred, in 1987, it did not influence our rains in Southern California.

While El Nios do not always bring storm fronts to Southern California, Halpern said, they are almost always associated with seasons of intense rainfall.

During a normal winter, storm patterns impacting Southern California originate from two geographic areas.

Most of the clouds in the Pacific develop in the western tropical Pacific, where the water is generally quite warm, Halpern said. They follow a path -- known as Pineapple Alley -- from the Hawaiian Islands into Washington and Oregon.

The other source of storms in Southern California comes from the Gulf of Alaska and heads southward, usually veering toward the Pacific Northwest.

During an El Nio, these storm patterns along Pineapple Alley and from the Alaskan gulf are changed, moving about 1,100 to 1,600 kilometers (700 to 1,000 miles) south so that they now fall over Southern California rather than in Oregon and Washington, said Halpern, the first to observe the El Nio ocean current along the equator.

Winds are the driving force behind ocean weather, he said. The ocean warming in the equatorial Pacific west of the date line is maintained by westward-blowing trade winds. Every several years, the strength of these winds diminishes and sometimes reverses direction.

This creates a massive flow of warm water into the central and eastern equatorial Pacific, where the surface waters are normally much cooler than west of the date line, he said.

Changes in sea-surface temperature in the Pacific equatorial waters occur at irregular intervals and in conjunction with the southern oscillation -- the seesawing fluctuation of atmospheric pressure between the eastern and the western South Pacific.

Because this phenomenon occurs in one of the most sparsely populated regions of the world where very few observations are made, it was not linked to global weather patterns, including the United States, until quite recently.

Oceanographers and meteorologists have traditionally relied on National Oceanic and Atmospheric Administration (NOAA) satellites to learn more about these ocean-atmospheric interactions in the Pacific Ocean.

Methods for measuring global surface winds -- a very important dimension of sea-surface temperature fluctuations associated with El Nio s -- were introduced by an instrument called a scatterometer on NASA-JPL's 1978 Seasat experimental satellite. The instrument -- which used radar to measure ocean winds -- was the granddaddy of a generation of scatterometers under development at JPL.

The U.S.-French TOPEX/Poseidon mission will provide another dimension to studies of ocean circulation.

In order to better predict El Nios, we have to understand something about how the temperature is changed within the interior of the ocean, Halpern said. The TOPEX altimeter will allow us to measure the sea-surface height in the equatorial Pacific, where only a few islands exist to record sea level.

TOPEX will be able to estimate how much the water warms from the surface to about 100 meters (328 feet) below the surface of the ocean. By measuring the rise of the equatorial sea surface, scientists will know how much warm water has been introduced by eastward-flowing currents in the upper ocean.

Halpern, a recent visiting professor at Caltech, will be able to improve predictions of the El Nio currents using TOPEX data. The surface oceanographic data from satellite radar altimeters, scatterometers and radiometers will be assimilated into time-dependent, three-dimensional ocean general circulation models with advanced supercomputers to produce realistic descriptions of the subsurface current and temperature conditions.

Halpern is chairman of the Pacific Ocean Panel of the Committee on Climate Changes and the Ocean (CCCO) and a member of the National Academy of Science's Advisory Panel on TOGA (Tropical Ocean Global Atmosphere). His research is conducted for the Climate and Hydrologic System branch of NASA's Office of Space Science and Applications. The branch is directed by Dr. Robert Schiffer.

The U.S. portion of the TOPEX/Poseidon mission is managed by JPL's Charles Yamarone, project manager, and Dr. Lee-Lueng Fu, project scientist, for NASA's Office of Space Science and Applications.

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