This winter, weather reports have been full of extremes. California, for instance, has had the driest year in 119 years of recordkeeping, with theSierra snowpack less than a third of normal.
The January cold snap across the eastern two-thirds of the United States sent temperatures plummeting 20 to 40 degrees Fahrenheit (11 to 22 degrees Celsius) below average. Across the Atlantic, England and Wales experienced one of the wettest periods in at least 248 years.
Drought. Frigid cold. Devastating rain and floods. On the surface, they don’t seem to have much in common beyond the unpleasantness of bad weather. But some researchers argue that these recent events can be tied to a single weather pattern—one that may be caused by climate change.
Jennifer Francis, an atmospheric scientist at Rutgers University in New Jersey, suggests that more persistent weather patterns—for example, cold snaps, drought, and flooding—are connected to Arctic amplification, the accelerated rise in temperature in the far north, which has warmed up faster than anywhere else on the planet. As the Arctic warms, the difference in temperature between it and lower latitudes weakens the westerly winds of the polar jet stream. Instead of blowing strong and straight from west to east, the jet stream now takes a wavier path around the Northern Hemisphere.
When this happens, “weather patterns tend to change more slowly,” says Francis. In 2012 she and Stephen Vavrus published an influential paper arguing that a slower, wavier polar jet might lead to prolonged extreme weather, such as drought, flooding, cold spells, and heat waves.
“This winter is a great example of that,” Francis says. “A huge northward swing in the jet stream made it very warm in Alaska. It blocked Pacific storms from coming into California and contributed to dry conditions. And then on its way south over the Eastern two-thirds of the United States, it brought cold Arctic air to hang out for a while.”
In the Atlantic, the polar jet’s wavier path brought it farther south and closer than usual to the subtropical jet, which flows at higher altitudes and carries a lot of moisture. “When this happens,” says Francis, “big storms usually result.”
Great Britain bore the brunt, with heavy rains, strong winds, and high waves which led to extensive flooding. In a report issued this month, the UK Met Office found that “these extreme weather events … were linked to a persistent pattern of perturbations to the jet stream over the Pacific Ocean and North America.” Those same perturbations also brought warm weather to the Winter Olympics at Sochi.
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