New study traces historic changes in North American weather patterns
By Bob Berwyn
FRISCO — A new University of Utah-led study suggests that this past winter’s persistent weather pattern across North America is linked with changes in the jet stream that may become even more pronounced as the 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.
“Our study is really focused on reconstruction of past climate states and past climate conditions. The historic record of pressure fields … trended toward more of a sinuous, curvy jet stream pattern,” Bowen said, explaining that the research is based on an analysis of isotopes in rainfall deposits across North America.
By studying those depositions, the scientists were able to determine where the moisture originated.
“Using data from many sites helped us to create a spatial fingerprint … We do think we’re seeing are some changes in the mean position of the jet streams. The crux of what we’re seeing is more of this western ridge and eastern trough,” Bowen said.
The study was published online April 16 by the journal Nature Communications.
“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,” Bowen said in a press release announcing the study findings. “We saw a good example of extreme wintertime climate that largely fit that pattern this past winter.
The research by Bowen and colleagues shows the jet stream pattern that brings North American wintertime weather extremes is millennia old, dating back to a global climate shift about 4,000 years ago.
“It was a well-documented time of transition in climate worldwide,” he said, explaining that the best explanation for the change is related to subtle changes of earth’s orbit.
“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. 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.
Jennifer Francis, a research professor at Rutgers University’s Institute of Marine and Coastal Research, was not involved with this study, but has been tracking possible shifts in jet stream patterns by other methods. According to Francis, Bowen’s work helps reinforce the idea that a warming world will change the jet stream, with potentially significant effects on northern hemisphere weather.
“This study, as well as another that was published this week seem to point to the same basic conclusion … that a warming world is likely to produce the type of amplified jet-stream pattern that was extremely persistent this winter,” said Francis, who has also bee studying the link between global warming and changes in the jet stream.
“In particular, both studies conclude that the tendency for strong ridging off the west coast of the US (contributing to warm, dry conditions there) along with a southward jet extension in the east (associated with abnormally cold conditions) will likely increase as global warming intensifies,” Francis said via email.
“Both studies also point to above-normal sea-surface temperatures in the eastern tropical Pacific as a source of wave energy that excites this pattern and contributes to strong ridging in the northwest Pacific,” she said.
After tracing the oxygen isotopes in rainfall deposits, Bowen and his colleagues modeled 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 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.
Historically, the Pacific North American teleconnection, or PNA, “is a pattern of climate variability” with positive and negative phases, Bowen said.
“In periods of positive PNA, the jet stream is very sinuous. As it comes in from Hawaii and the Pacific, it tends to rocket up past British Columbia to the Yukon and Alaska, and then it plunges down over the Canadian plains and into the eastern United States. The main effect in terms of weather is that we tend to have cold winter weather throughout most of the eastern U.S. You have a freight car of arctic air that pushes down there.”
When the jet stream is curvy, “the West tends to have mild, relatively warm winters, and Pacific storms tend to occur farther north. So in Northern California, the Pacific Northwest and parts of western interior, it tends to be relatively dry, but tends to be quite wet and unusually warm in northwest Canada and Alaska.”
The jet stream pattern – whether curvy or flat – has its greatest effects in winter and less impact on summer weather, Bowen says. The curvy pattern is enhanced by another climate phenomenon, the El Nino-Southern Oscillation, which sends a pool of warm water eastward to the eastern Pacific and affects climate worldwide.
More study needed
Extrapolating paleoclimate records must be done very carefully, warned Dr. Kevin E. Trenberth, with the Climate Analysis Section at the National Center for Atmospheric Research.
“I have doubts about many aspects of this study,” said Trenberth. “There is a natural tendency for a wave pattern to occur across North America with the west opposite to the east, and it is because of the Rockies,” Trenberth said. “The westerly winds encounter the Rockies and it causes them to change. The outcome is either they deviate to the north or they deviate to the south, but a wave is set up so that the westerlies avoid the Rockies,” Trenberth said via email. “This is a natural process and arises from the forcings the Rockies put on the atmosphere if it tries to go straight across.”
Other studies show no increase in jet stream waviness, he said, while acknowledging that the Pacific Decadal Oscillation (a large-scale shift in sea surface temperature patterns) has resulted in discernible pattern changes across North America.
In a press release, the University of Utah explained the recent study:
Over the millennia, oxygen in ancient rain water was incorporated into calcium carbonate deposited in cave and lake sediments. The ratio of rare, heavy oxygen-18 to the common isotope oxygen-16 in the calcium carbonate tells geochemists whether clouds that carried the rain were moving generally north or south during a given time.
Previous research determined the dates and oxygen isotope ratios for sediments in the new study, allowing Bowen and colleagues to use the ratios to tell if the jet stream was curvy or flat at various times during the past 8,000 years.
Bowen said air flowing over the Pacific picks up water from the ocean. As a curvy jet stream carries clouds north toward Alaska, the air cools and some of the water falls out as rain, with greater proportions of heavier oxygen-18 falling, thus raising the oxygen-18-to-16 ratio in rain and certain sediments in western North America. Then the jet stream curves south over the middle of the continent, and the water vapor, already depleted in oxygen-18, falls in the East as rain with lower oxygen-18-to-16 ratios.
When the jet stream is flat and moving east-to-west, oxygen-18 in rain is still elevated in the West and depleted in the East, but the difference is much less than when the jet stream is curvy.
By examining oxygen isotope ratios in lake and cave sediments in the West and East, Bowen and colleagues showed that a flatter jet stream pattern prevailed from about 8,000 to 4,000 years ago in North America, but then, over only 500 years, the pattern shifted so that curvy jet streams became more frequent or severe or both. The method can’t distinguish frequency from severity.
Simulations of climate and oxygen isotope changes in the Middle Holocene and today resemble, respectively, today’s flat and curvy jet stream patterns, supporting the switch toward increasing jet stream sinuosity 4,000 years ago.
Why did the trend start then?
“It was a when seasonality becomes weaker,” Bowen says. The Northern Hemisphere was closer to the sun during the summer 8,000 years ago than it was 4,000 years ago or is now due to a 20,000-year cycle in Earth’s orbit. He envisions a tipping point 4,000 years ago when weakening summer sunlight reduced the equator-to-pole temperature difference and, along with an intensifying El Nino climate pattern, pushed the jet stream toward greater curviness.
The new study is based mainly on isotope ratios at Buckeye Creek Cave, W. Va.; Lake Grinell, N.J.; Oregon Caves National Monument; and Lake Jellybean, Yukon.
Additional data supporting increasing curviness of the jet stream over recent millennia came from seven other sites: Crawford Lake, Ontario; Castor Lake, Wash.; Little Salt Spring, Fla.; Estancia Lake, N.M.; Crevice Lake, Mont.; and Dog and Felker lakes, British Columbia. Some sites provided oxygen isotope data; others showed changes in weather patterns based on tree ring growth or spring deposits.
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.