Warm Arctic, Nasty North AmericaIn recent years, North America has been hammered by severe winter we

Warm Arctic, Nasty North AmericaIn recent years, North America has been hammered by severe winter weather. A few years ago, scientists at NOAA developed a “Winter weather misery index” called the Accumulated Winter Season Severity Index (AWSSI), which takes into account all the features of winter weather such as how much snow has fallen, the depth and extent of extreme cold snaps, and how long it has been since the last fall temperatures. Using recorded weather data from several dozen weather stations around the U.S., the scientists who created the index were able to characterize what a mild winter was at a certain location compared to an extreme winter, and the range in-between.Not every winter has been extreme – the 2016 to 2017 winter was particularly mild around the country, but winters such as 2014-2015 and 2017-2018 have pushed many cities up to the “Extreme” level on that scale. For example, in January of this year, more than ½ of the sites in the Eastern U.S. were experiencing extreme winters.There has been substantial discussion, including here at the Earth Story, about the cause of these extreme winters. It has not gone unnoticed by scientists that many of them, including noreasters like the one seen in this satellite photo from early March, were occurring at the same time as persistent warm periods hitting the Arctic. Once scientist at Rutgers University, Dr. Jennifer Francis, has published intriguing climate model results that suggest these states are linked; the warm conditions in the arctic allow weakening of the jet stream, as a consequence large bends form in the jet stream as major frontal boundaries move to the south, and these bends and frontal boundaries trigger severe weather.When scientists see two things that seem to be happening at the same time, like warm weather in the arctic and cold weather in the states, an important first step is to assess whether the events are really correlated or if it just seems to be happening because we’re paying attention. To test whether warm weather in the Arctic really was correlated with severe winters in the U.S., Dr. Francis and colleagues compared the AWSSI misery index to two measurements that record how warm it is in the Arctic. One measurement is the actual arctic temperature averaged over the ice sheets, and the other records the height of a boundary in the atmosphere over the Arctic – a measure of how much energy is stored in the lower part of the atmosphere.Sure enough, when the scientists tested for correlations between these indices, they found that there was a good correlation, particularly for the Eastern U.S. As the Arctic warmed, and particularly as air pressure increased over the Arctic, about 5 days later there was commonly a substantial increase in the AWSSI index, indicating a severe storm or severe wintry conditions. This correlation was the strongest in late winter, such as the time that noreasters like the one in this photo have lined up in 2018.In this study, Dr. Francis’s group didn’t directly address the cause of the severe weather or why these are linked. They have other work suggesting that the linkage comes through the jet stream, but they limited this study to testing whether these weather events actually correlate statistically, and they do.The Arctic is rapidly warming; the area is more than 2 °C warmer than it was only 3 or 4 decades ago. This warmth is hitting the whole area; for example, 2018 saw the lowest winter ice extent on record, breaking records set only a year or two ago, and the hottest Arctic winter temperatures ever recorded happened in February (https://tmblr.co/Zyv2Js2Vb3E_y). This work supports the idea in previous studies that as the arctic warms, one end result is severe winter weather over the United States. Scientists will still have work to do to understand the details of these interactions and how they will evolve as the arctic continues to shift, but as of right now, when it’s warm in the Arctic winter, it’s a mess down in North America.-JBBImage credit: NOAA/GOES-Easthttp://bit.ly/2DqOUf1Original paper:http://go.nature.com/2FzygQBReferences:http://bit.ly/2HwWOWGhttp://wxch.nl/2tMCkr6AWSSI Scale: http://bit.ly/2tKveTZhttp://bit.ly/1lpm7z7 -- source link

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