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Fluctuating jet stream may be causing extreme weather events

By Karen Graham     Jan 13, 2018 in Environment
A new study suggests that the polar jet stream has been fluctuating more than normal as it passes over parts of the Northern Hemisphere, and that's affecting weather in Europe and North America.
According to research by a University of Arizona-led team, an increase in fluctuations in the North Atlantic jet stream since the 1960s coincide with more extreme weather events in Europe such as heat waves, droughts, wildfires, and flooding.
Researchers at the University of Arizona and the Swiss Federal Research Institute attempted to put recent Northern Hemisphere polar jet stream anomalies and their link to extreme weather events into a historical perspective by studying two tree-ring records from the British Isles and the northeastern Mediterranean. Tree samples from four species of trees in Europe, including Scots pine, dating back to 1725 were studied. These revealed what kind of weather Europe had each year.
Flood waters surround a car parked in Hamburg's Fish Market district as a storm hits many parts...
Flood waters surround a car parked in Hamburg's Fish Market district as a storm hits many parts of Germany and cenral Europe
Daniel Bockwoldt, dpa/AFP
The team wanted to find out how widely and how often the jet stream meanders. They reconstructed those regions' late summer weather going back almost 300 years - to 1725. "We find that the position of the North Atlantic Jet in summer has been a strong driver of climate extremes in Europe for the last 300 years," said Biologist Valerie Trouet.
The biggest thing the researchers found was the jet stream's meanderings have become far more extreme since the 1960s. "Since 1960 we get more years when the jet is in an extreme position," Trouet said, noting it is either the northernmost or southernmost position. Most surprisingly, Trouet says this pattern of more frequent, extreme shifts north and south has never been seen before in her 290-year record.
Banded cirrus clouds running perpendicular to the jet stream—a telltale feature photographed by an...
Banded cirrus clouds running perpendicular to the jet stream—a telltale feature photographed by an astronaut aboard Space Shuttle Discovery.
NASA
The North Atlantic Polar Jet Stream
Jet streams are fast flowing, narrow, meandering air currents in the Earth's atmosphere. On Earth, the main jet streams are located near the altitude of the tropopause and flow from West to East, with the strongest jet streams being the Polar Jets at 9–12 kilometers (30,000–39,000 feet) above sea level.
While there are both a Northern Polar and Southern Polar Jet, we will talk about the Northern Polar Jet Stream. The polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans.
When the North Atlantic Polar Jet is in the extreme northern position, the British Isles and western Europe have a summer heat wave while southeastern Europe has heavy rains and flooding, the researchers said. And conversely, when the jet stream is in the extreme southern position, the situation flips: Western Europe has heavy rains and flooding while southeastern Europe has extreme high temperatures, drought, and wildfires.
"Heat waves, droughts and floods affect people," Trouet said. "The heat waves and drought that are related to such jet stream extremes happen on top of already increasing temperatures and global warming - it's a double whammy."
Firefighters battling blazes in central Portugal are worried about the hotter weather forecast  whic...
Firefighters battling blazes in central Portugal are worried about the hotter weather forecast, which increases the risk that old fire sites will rekindle or new fires will break out in the coming days
PATRICIA DE MELO MOREIRA, AFP
Extreme summer weather events in the American Midwest are also associated with extreme northward or southward movements of the jet stream, the authors write. "We studied the summer position of the North Atlantic jet. What we're experiencing now in North America is part of the same jet stream system," Trouet said.
Trouet adds that the extreme cold and snow in the North American Northeast and extreme warmth and dryness in California and the American Southwest are all related to the position of the North Pacific Polar Jet.
There is a discussion in scientific circles over whether an increased variability of the jet stream is linked to man-made global warming and the faster warming of the Arctic compared to the tropics. Many researchers believe there is a connection.
More about Jet stream, Europe, extreme weather events, Climate change, Dendrochronology