New study shows shifts in far-reaching weather pattern
By Summit Voice
SUMMIT COUNTY — The recently ended 2009-2010 El Niño was the strongest on record — but was at its most intense in the central Pacific rather than in the eastern reaches of the vast ocean, NASA and National Oceanic and Atmospheric Administration researchers said in a study released this week.
The scientists are calling it a new type of El Niño that has been evolving and growing progressively stronger since the early 1980s. This past winter’s El Niño was nearly double the intensity of earlier episodes for which there are measurable data.
The results don’t show that the changing pattern is related to global warming. More research is needed to investigate those links.
“Our study concludes the long-term warming trend seen in the central Pacific is primarily due to more intense El Niños, rather than a general rise of background temperatures,” said Tong Lee, of NASA’s Jet Propulsion Laboratory, Pasadena, California.
The scientists said the stronger El Niños help explain a steady rise in central Pacific sea surface temperatures observed over the past few decades in previous studies.
Lee and fellow researcher Michael McPhaden, of NOAA’s Pacific Marine Environmental Laboratory, Seattle, observed a rise in sea surface temperatures during El Niño years. But no significant temperature increases were seen in years when ocean conditions were neutral, or when El Niño’s cool water counterpart, La Niña, was present.
The scientists compared NOAA satellite observations of sea surface temperature with blended with directly measured ocean temperature data. The strength of each El Niño was gauged by how much its sea surface temperatures deviated from the average.
“These results suggest climate change may already be affecting El Niño by shifting the center of action from the eastern to the central Pacific,” said McPhaden. “El Niño’s impact on global weather patterns is different if ocean warming occurs primarily in the central Pacific, instead of the eastern Pacific.
“If the trend we observe continues,” McPhaden added, “it could throw a monkey wrench into long-range weather forecasting, which is largely based on our understanding of El Niños from the latter half of the 20th century.”
El Niño, Spanish for “the little boy,” is the oceanic component of a climate pattern called the El Niño-Southern Oscillation, which appears in the tropical Pacific Ocean on average every three to five years. As the most dominant year-to-year fluctuating pattern in Earth’s climate system, El Niños have a powerful impact on the ocean and atmosphere, as well as important socioeconomic consequences. They can influence global weather patterns and the occurrence and frequency of hurricanes, droughts and floods; and can even raise or lower global temperatures by as much as 0.2 degrees Celsius (0.4 degrees Fahrenheit).
During a “classic” El Niño episode, the normally strong easterly trade winds in the tropical eastern Pacific weaken. That weakening suppresses the normal upward movement of cold subsurface waters and allows warm surface water from the central Pacific to shift toward the Americas. In these situations, unusually warm surface water occupies much of the tropical Pacific, with the maximum ocean warming remaining in the eastern-equatorial Pacific.
Since the early 1990s, however, scientists have noted a new type of El Niño that has been occurring with greater frequency. Known variously as “central-Pacific El Niño,” “warm-pool El Niño,” “dateline El Niño” or “El Niño Modoki” (Japanese for “similar but different”), the maximum ocean warming from such El Niños is found in the central-equatorial, rather than eastern, Pacific. Such central Pacific El Niño events were observed in 1991-92, 1994-95, 2002-03, 2004-05 and 2009-10. A recent study found many climate models predict such events will become much more frequent under projected global warming scenarios.
Lee said further research is needed to evaluate the impacts of these increasingly intense El Niños and determine why these changes are occurring. “It is important to know if the increasing intensity and frequency of these central Pacific El Niños are due to natural variations in climate or to climate change caused by human-produced greenhouse gas emissions,” he said.
Results of the study were published recently in Geophysical Research Letters.
For more information on El Niño, visit: http://sealevel.jpl.nasa.gov/.
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