In-depth study of satellite data obtained at the onset of the 1997-98 El Niño event has given scientists more conclusive evidence that the unusual warming of waters along the equator and the west coast of the U.S. was linked to changes in wind patterns in the Pacific ocean.
Using measurements taken by the NASA Scatterometer (NSCAT) in early 1997, scientists confirmed that an unusual weakening of the trade winds preceded an increase in sea surface temperatures along the central and eastern equatorial Pacific, according to Dr. W. Timothy Liu, the NSCAT project scientist at JPL. "With NSCAT we are able to see the whole El Niño picture, and we now know that the unusually high ocean temperatures at the equator, along the North American coast and off of Baja, Mexico are all linked together because of the winds," Liu said.
Liu is presented his findings last week at the American Meteorological Societys Conference on Satellite Meteorology and Oceanography in Paris.
An El Niño condition is thought to be triggered when steady westward blowing trade winds weaken and even reverse direction. This change in the winds allows the 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.
"The collapse of the trade winds and the depression of the thermocline (cooler water) resulted in an increase of sea surface temperature and sea surface height in early 1997, and this interaction appears to be the typical mechanism of equatorial changes associated with an El Niño," Liu explained.
In addition to measuring the beginning of El Niño along the equator, NSCAT also revealed that, when the trade winds weakened, an unusual low-pressure system with cyclonic (counterclockwise) circulation moved toward the North American coast. NSCAT observed that winds branched off from the equator, bypassed Hawaii, and brought heat and moisture from the tropical ocean towards San Francisco via a route often called the "Pineapple Express."
"This moist and warm air from the south kept evaporation low and the ocean water warmer than normal near the North American coast," Liu suggested. "This change brought with it strong ecological changes, such as the tropical fish caught off the coast of Oregon and the deaths of sea lions on the Channel Islands in California."
An image of this "teleconnection" between the warming of equatorial water associated with El Niño and the warming of the North American coastal waters through wind patterns is available at http://www.jpl.nasa.gov/elnino .
NSCAT was a radar instrument that flew on Japans Advanced Earth Observing Satellite (ADEOS) until the satellite suffered an electrical failure and ceased functioning on June 30, 1997. JPL is currently building the SeaWinds scatterometer that will fly on the Quick Scatterometer (QuikScat) satellite set for launch in November 1998 will continue the important wind observations begun by NSCAT. SeaWinds will provide better coverage of the ocean than NSCAT and will improve our ability to observe the influence of El Niño from the equatorial water to regions closer to our home, Liu said.
The scatterometery program is managed by JPL for NASAs Earth Sciences Enterprise, Washington, D.C. JPL is managed for NASA by the California Institute of Technology.
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