A new study has found that the wavy jet stream pattern that tends to bring warm winter weather to the U.S. West and cold weather to the East was set in place 4,000 years ago.

The research, published in the journal Nature Communications, also suggests that climate change may help keep the wavy pattern in place.

"It's possible the kinds of changes we are seeing with increased jet stream sinuosity might continue into the future as a result of anthropogenic CO2 emissions, although it's not a perfect comparison," said Gabriel Bowen, a geochemist at the University of Utah and an author on the study.

When the jet stream is very wavy, it tends to bring warm air up from the tropics into the U.S. West. It then kinks up toward the Arctic and brings cold air down to the Great Plains and the East, Bowen said.

Some researchers have suggested that climate change, which has resulted in a rapidly warming Arctic, is leading to jet stream kinks that keep extreme weather in place, although that hypothesis is still being debated (ClimateWire, April 3).

When the jet stream got loopy
In order to track the jet stream's behavior thousands of years ago, Bowen and his fellow researchers looked at oxygen isotope ratios from lake cores and cave formations in the eastern and western United States. A high ratio of oxygen-18 to oxygen-16 indicates wetter weather, a low one indicates drier.

"Changes in the ratios of oxygen-18 to oxygen-16 propagate through that system and are recorded in what we are looking at," Bowen said.

In winter, which is when jet stream influence on weather is the most pronounced, a wavy jet stream would generally lead to dry, cold weather in the East and warm, wet weather in the West.

When the researchers analyzed the oxygen isotope ratios over the past 8,000 years, they found that the jet stream became wavier—with colder Eastern winter weather and warmer Western winter weather -- on about a 4,000-year cycle.

They also found a smaller cycle of waviness about every 200 years, Bowen said, although the margin of error in their records makes that finding less robust.

Bowen suggested that some of what drives the 4,000-year cycle is changes in the Earth's temperature gradient. This is why climate change, which is warming up the Arctic and changing the temperature gradient, may keep the jet stream wavy.

Lesleigh Anderson, a research geologist with the U.S. Geological Survey whose work focuses on historical climate records, said that papers like Bowen's are important because they give a historical context for how the jet stream is behaving now.

"The paper is a nice highlight of the state of the research on what do we know about how the atmosphere behaved in the past," Anderson said.

One of history's many puzzles
The researchers also chose a good assortment of locations to show the changes they are documenting, which make their findings relevant to the entire country, Anderson said. She also praised the researchers for using models to verify their findings.

"To have this model component combined with multiple data records is really a sound approach. And I like to see that as a scientist," Anderson said.

As to why these shifts in the jet stream happen, Anderson said that is still an open question. A lot of researchers are focusing their research in the tropical Pacific, where phenomena like El Niño and the Pacific Decadal Oscillation also appear to shift in large-scale time cycles, she said.

One explanation is the change in solar insolation, or the amount of the sun's energy that hits the Earth. In winter, that quantity has slowly decreased over the past 10,000 years or so, and, in summer, it had increased.

But those changes do not adequately explain sudden shifts like the one seen in Bowen's paper.

"The fact that in this paper you see things kind of cruising along, and then, at 4,000 years, a lot of the records change ... we just don't understand the system well enough to know why," Anderson said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500