An international team of climate scientists from the University of Hawai‘i at Mānoa have found that tree ring data, specifically from the US Southwest, agree well with the 150-year instrumental sea surface temperature records in the tropical Pacific that we already have of El Niño events.
The tree rings also match well with existing reconstructions of the El Niño-Southern Oscillation and correlate highly with data gathered from living corals and coals that lived hundreds of years ago, in the region around Palmyra in the central Pacific.
“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,” explained Li. “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.”
To quote from the University of Hawai‘i at Mānoa press release, “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.”
As a result, the tree ring data shows us that the intensity of El Niño has been highly variable over the past millennium, with decades of strong El Niño events and decades of little activity at all. According to the data recorded from the tree-rings, the weakest El Niño event took place during the Medieval Climate Anomaly in the 11th century, while the strongest event took place during the 18th century.
These highs and lows in El Niño activity are directly related to long-term changes in the climate of the Pacific Ocean.
Cores taken from lake sediments in the Galapagos, northern Yucatan, and the Pacific Northwest show that the climate of the eastern-central tropical Pacific Ocean swings between warm and cool phases, each lasting from 50 to 90 years.
Looking back over the data from the tree rings and the lake sediments, scientists saw that during the warm phases, El Niño and La Niña events were more intense than usual, whereas during the cooler phases, they deviated little from the long-term average.
“Since El Niño causes climate extremes around the world, it is important to know how it will change with global warming,” says Xie. “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.”