A new 1,100-year-long history of the El Niño-La Niña climate cycle that dominates seasonal weather patterns now and then promises to sharpen the skills of computer models trying to simulate the impacts of global warming.
Climate specialists at the University of Hawaii-Manoa used an archive of tree ring measurements in the Southwestern United States to extend an El Niño historical record previously limited by Pacific Ocean coral data to only a few hundred years.
“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,” lead scientist Jinbao Li said in a statement released by the university. The study is published in the current issue of the journal Nature Climate Change.
El Niño music is a complex symphony of movements between the equatorial ocean and the atmosphere. Shifts in ocean surface waters are often driven by wind bursts. As warm waters migrate eastward toward the shores of the Americas, the towering cumulus clouds that fuel many of Earth’s storms shift along with the warmth into the central Pacific — bending the jet streams and changing storm tracks. This warm El Niño condition brings wetter winters to the Southwest, and wider growth rings on the trees in the mountains, and cold La Niña waters mean dry winters and thinner tree rings.
While climate models have achieved a fairly accurate forecasting of the comings and goings of El Niño and La Niña months in advance — a source of great help to improving the accuracy of seasonal outlooks — they have been unable to agree on the impact of global warming on the cycles.
Looking across the centuries of their new El Niño history, Li and colleagues noticed certain patterns of variability that reflect large-scale swings between warm and cool phases in the tropical Pacific that range from 50 to 90 years. In phases when the average tropical temperature is warm, El Niño and La Niña are more intense. The team says their research supports “the emerging idea that there is a positive feedback” between warmer tropical waters and stronger El Niño cycles.
“This observation alone implies that the continued increase in eastern-central tropical Pacific (sea surface temperatures) in a future warmer climate will lead to enhanced (El Niño) variability and more extreme climate conditions around the globe,” they write. Still, El Niño music is complicated by “a host of ocean-atmospheric feedbacks” and more work needs to be done.
Writing separately, tree-ring specialist Matthew Therrell of Southern Illinois University said the 1,100-year history “provides new and highly valuable insights into the timing, duration and magnitude of one of the most important large-scale circulation features on Earth.”
IMAGE: Bristlecone pines such as this tree, more than 1,000 years old, in the Great Basin National Park, Nevada, contributed to the tree-ring record on El Niño. Credit: Courtesy University of Hawaii-Manoa, International Pacific Research Center
GRAPH: Shows El Niño amplitude derived from North American tree rings (blue) and instrumental measurements (red). The green curve represents the long-term trend in El Niño strength. (Individual El Niño events occur typically at intervals of 2-7 years.) Periods of strong El Niño activity are indicated by amplitudes above 1.0. Superimposed on a general rising trend, cycles of strong activity occurred about every 50-90 years. Credit: Courtesy University of Hawaii-Manoa, International Pacific Research Center.