From: Andy Soos, ENN
Published May 10, 2011 07:27 AM

El Nino Tree Ring Story

El Niño and its partner La Niña, the warm and cold phases in the eastern half of the tropical Pacific, play havoc with climate worldwide. Predicting El Niño events more than several months ahead is now routine, but predicting how it will change in a warming world has been hampered by the short historical record. El Niño/La Niña-Southern Oscillation is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years. It is characterized by variations in the temperature of the surface of the tropical eastern Pacific Ocean—warming or cooling known as El Niño and La Niña respectively—and air surface pressure in the tropical western Pacific—the Southern Oscillation. record. An international team of climate scientists from the University of Hawaii at Mānoa recently found that annually resolved tree-ring records from North America, particularly from the U.S. Southwest, give a continuous representation of the intensity of El Niño events over the past 1,100 years and can be used to improve El Niño predictions.

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Tree rings in the U.S. Southwest, the team found, agree well with the 150-year instrumental sea surface temperature records in the tropical Pacific. During El Niño, the unusually warm surface temperatures in the eastern Pacific lead to changes in the atmospheric circulation, causing unusually wetter winters in the U.S. Southwest, and thus wider tree rings; unusually cold eastern Pacific temperatures during La Niña lead to drought and narrower rings. The tree-ring records, furthermore, match well existing reconstructions of the El Niño-Southern Oscillation and correlate highly, for instance, with d18O isotope concentrations of both living corals and corals that lived hundreds of years ago around Palmyra in the central Pacific.

The tree rings reveal that the intensity of El Niño has been highly variable, with decades of strong El Niño events and decades of little activity. The weakest El Niño activity happened during the Medieval Climate Anomaly in the 11th century, whereas the strongest activity has been since the 18th century.   This period was a time of warm climate in the North Atlantic region, that may also have been related to other climate events around the world during that time, including in China,  new Zealand, and other countries lasting from about AD 950—1250. It was followed by a cooler period in the North Atlantic termed the Little Ice Age.

"Our work revealed that the towering trees on the mountain slopes of the U.S. Southwest and the colorful corals in the tropical Pacific both listen to the music of El Niño, which shows its signature in their yearly growth rings.  The coral records, however, are brief, whereas the tree-ring records from North America supply us with a continuous El Niño record reaching back 1,100 years" explained Li (one of the authors). 

These different periods of El Niño activity are related to long-term changes in Pacific climate. Cores taken from lake sediments in the Galapagos, northern Yucatan, and the Pacific Northwest reveal that the eastern—central tropical Pacific climate swings between warm and cool phases, each lasting from 50 to 90 years. During warm phases, El Niño and La Niña events were more intense than usual. During cool phases, they deviated little from the long-term average as, for instance, during the Medieval Climate Anomaly when the eastern tropical Pacific was cool.

"Since El Niño causes climate extremes around the world, it is important to know how it will change with global warming.  Current models diverge in their projections of its future behavior, with some showing an increase in amplitude, some no change, and some even a decrease. Our tree-ring data offer key observational benchmarks for evaluating and perfecting climate models and their predictions of the El Niño-Southern Oscillation under global warming." says Xie (another author),

For further information: http://www.hawaii.edu/news/article.php?aId=4413

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