SALT LAKE CITY — Last winter’s curvy jet stream pattern brought mild temperatures to western North America and harsh cold to the East. A University of Utah-led study shows that pattern became more pronounced 4,000 years ago, and suggests it may worsen as Earth’s climate warms.
“If this trend continues, it could contribute to more extreme winter weather events in North America, as experienced this year with warm conditions in California and Alaska and intrusion of cold Arctic air across the eastern USA,” said geochemist Gabe Bowen, senior author of the study.
“A sinuous or curvy winter jet stream means unusual warmth in the West, drought conditions in part of the West, and abnormally cold winters in the East and Southeast,” said Bowen, an associate professor of geology and geophysics at the University of Utah.
“We saw a good example of extreme wintertime climate that largely fit that pattern this past winter,” although in the typical pattern California often is wetter.
Forecasting the climate
It is not new for scientists to forecast that the current warming of Earth’s climate due to carbon dioxide, methane and other “greenhouse” gases already has led to increased weather extremes and will continue to do so.
The new study shows the jet stream pattern that brings North American wintertime weather extremes is millennia old — “a long-standing and persistent pattern of climate variability,” Bowen said.
Yet it also suggests global warming may enhance the pattern so there will be more frequent or more severe winter weather extremes or both.
“This is one more reason why we may have more winter extremes in North America, as well as something of a model for what those extremes may look like,” Bowen said.
Human-caused climate change is reducing equator-to-pole temperature differences; the atmosphere is warming more at the poles than at the equator.
Changes in intensity
Based on what happened in past millennia, that could make a curvy jet stream even more frequent and-or intense than it is now, he said.
Bowen and his co-authors analyzed previously published data on oxygen isotope ratios in lake sediment cores and cave deposits from sites in the eastern and western United States and Canada. Those isotopes were deposited in ancient rainfall and incorporated into calcium carbonate.
They reveal jet stream directions during the past 8,000 years, a geological time known as middle and late stages of the Holocene Epoch.
Next, the researchers did computer modeling or simulations of jet stream patterns — both curvy and more direct west to east — to show how changes in those patterns can explain changes in the isotope ratios left by rainfall in the old lake and cave deposits.
They found that the jet stream pattern — known technically as the Pacific North American teleconnection — shifted to a generally more “positive phase” — meaning a curvy jet stream — over a 500-year period starting about 4,000 years ago.
In addition to this millennial-scale change in jet stream patterns, they also noted a cycle in which increases in the sun’s intensity every 200 years make the jet stream flatter.
Bowen conducted the study with Zhongfang Liu of Tianjin Normal University in China, Kei Yoshimura of the University of Tokyo, Nikolaus Buenning of the University of Southern California, Camille Risi of the French National Center for Scientific Research, Jeffrey Welker of the University of Alaska at Anchorage, and Fasong Yuan of Cleveland State University.
The study was funded by the National Science Foundation, National Natural Science Foundation of China, Japan Society for the Promotion of Science and a joint program by the society and Japan’s Ministry of Education, Culture, Sports, Science and Technology: the Program for Risk Information on Climate Change.