Shrinking Arctic Sea Ice Means Scorching U.S. Summers

http://www.weather.com/news/science/environment/shrinking-arctic-sea-ice-means-scorching-us-summers-20131209

Thirty years of shrinking Arctic sea ice has boosted extreme summer weather, including heat waves and drought, in the United States and elsewhere, according to a study published Monday in the journal Nature Climate Change.
The new study — based on satellite tracking of sea ice, snow cover and weather trends since 1979 — links the Arctic's warming climate to shifting weather patterns in the Northern Hemisphere's midlatitudes.
"The results of our new study provide further support and evidence for rapid Arctic warming contributing to the observed increased frequency and intensity of heat waves," said study co-author Jennifer Francis, an atmospheric scientist at Rutgers University in New Jersey. [Images of Melt: Earth's Vanishing Ice]
Weakened Jet Stream
Changes in the Arctic can perturb midlatitude weather in such regions as the United States, Europe and China because temperature differences between the two zones drive the jet stream, the fast-moving river of air that circles the Northern Hemisphere, explained lead study author Qiuhong Tang, an atmospheric scientist at the Institute of Geographic Sciences and Natural Resources Research in Beijing.
"As the high latitudes warm faster than the midlatitudes because of amplifying effects of melting ice, the west-to-east jet-stream wind is weakened," Tang told LiveScience in an email interview. "Consequently, the atmospheric circulation change tends to favor more persistent weather systems and a higher likelihood of summer weather extremes."

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In the past 30 years, the amount of summer sea ice covering the Arctic Ocean shrank by 8 percent per decade. The total area of summer ice lost would cover 40 percent of the lower 48 U.S. states. The amount of high-latitude snow cover during June waned even more quickly, at almost 18 percent per decade. Ultimately, these two measures mean the Arctic is warmer when summer starts, because the open ocean and meltwater on ice absorb more of the sun's rays than ice does.
When the temperature difference between the Arctic and midlatitudes lessens, the jet stream starts to take swooping swings on its journey around the globe, like a river flowing over a flat plain, Francis said. The ridges and troughs in the jet stream create stagnating weather systems, such as high-pressure heat waves, that are stuck in the swoops. The Arctic sea ice effects were even blamed for Hurricane Sandy's swing toward the Mid-Atlantic Coast.