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Monday, April 2, 2012

Jeff Masters: Arctic Sea ice loss tied to unusual jet stream patterns

Arctic Sea ice loss tied to unusual jet stream patterns

by Jeff Masters, Ph.D., WunderBlog, April 2, 2012

Earth has seen some highly unusual weather patterns over the past three years, and three new studies published this year point to Arctic sea loss as a potential important driver of some of these strange weather patterns. The record loss of sea ice the Arctic in recent years may be increasing winter cold surges and snowfall in Europe and North America, says a study by a research team led by Georgia Institute of Technology scientists Jiping Liu and Judith Curry. The paper, titled "Impact of declining Arctic sea ice on winter snowfall," was published on February 27, 2012, in the online early edition of the journal Proceedings of the National Academy of Sciences. 

"Our study demonstrates that the decrease in Arctic sea ice area is linked to changes in the winter Northern Hemisphere atmospheric circulation," said Judith Curry, chair of the School of Earth and Atmospheric Sciences at Georgia Tech, in a press release. "The circulation changes result in more frequent episodes of atmospheric blocking patterns, which lead to increased cold surges and snow over large parts of the northern continents."

Figure 1. Arctic sea ice in September 2007 reached its lowest extent on record, approximately 40% lower than when satellite records began in 1979. Sea ice loss in 2011 was virtually tied with the ice loss in 2007, despite weather conditions that were not as unusual in the Arctic. Image credit: University of Illinois Cryosphere Today

Figure 2. The extent of Arctic sea ice loss in the summer July-August-September period in 2007 was about 1.4 million square miles (3.6 million square kilometers) greater than in 1980, according to the University of Illinois Cryosphere Today. For comparison, the lost ice coverage (orange colors) was equal to an area about 44% of the size of the contiguous U.S., or 71% of the non-Russian portion of Europe. 

Summertime Arctic sea ice loss: 40% since 1980
The Arctic has seen a stunning amount of sea ice loss in recent years, due to melting and unfavorable winds that have pushed large amounts of ice out of the region. Forty percent of the sea ice was missing in September 2007 compared to September 1980. This is an area equivalent to about 44% of the contiguous U.S., or 71% of the non-Russian portion of Europe. Such a large area of open water is bound to cause significant impacts on weather patterns, due to the huge amount of heat and moisture that escapes from the exposed ocean into the atmosphere over a multi-month period following the summer melt. The Georgia Tech study found that Arctic sea ice loss had caused a 20-60% weakening of the west-to-east belt of winds circling the pole in recent years, producing broader meanders in the jet stream that allowed it to get "stuck" in place 20-60% more often. When the jet stream gets stuck in place for a long period of time, we say a "blocking pattern" has set up. Since the jet stream marks the boundary between cold, Arctic air to the north, and warmer subtropical air to the south, areas on both sides of the jet are subjected to extended periods of unusually warm or cold weather during a blocking episode. 

Such a blocking pattern began on January 26, 2012, and lasted until February 11, bringing and exceptionally cold and snowy conditions to much of Europe, which lay on the cold side of an elongated loop of the jet stream that got stuck in place. Conversely, most of North America and northern Siberia saw unusually warm temperatures during this period, since they were on the warm side of the jet stream. 

Lead author Jiping Liu, a senior research scientist in the School of Earth and Atmospheric Sciences at Georgia Tech, added, "We think the recent snowy winters could be caused by the retreating Arctic ice altering atmospheric circulation patterns by weakening westerly winds, increasing the amplitude of the jet stream and increasing the amount of moisture in the atmosphere. These pattern changes enhance blocking patterns that favor more frequent movement of cold air masses to middle and lower latitudes, leading to increased heavy snowfall in Europe and the Northeast and Midwest regions of the United States." The paper concludes: "If Arctic sea ice continues as anticipated by climate modeling results, we speculate that episodes of the aforementioned circulation change will become more frequent, along with more persistent snowstorms over northern continents during winter." 

Figure 3. Waiting for the warm-up after a rare snowfall in Italy during the February 2012 European cold blast. Image credit: wunderphotographer cathykiro.

Two other studies link Arctic sea ice loss to atmospheric circulation changes
"The question is not whether sea ice loss is affecting the large-scale atmospheric's how can it not?" That was the take-home message from Dr. Jennifer Francis of Rutgers University, in her talk "Evidence Linking Arctic Amplification to Extreme Weather in Mid-latitudes," presented at December's American Geophysical Union meeting in San Francisco. Dr. Francis presented new research that has just been published in the journal Geophysical Research Letters, which shows that Arctic sea ice loss may significantly affect the upper-level atmospheric circulation, slowing its winds and increasing its tendency to make contorted high-amplitude loops. High-amplitude loops in the upper level wind pattern (and associated jet stream) increases the probability of persistent weather patterns in the Northern Hemisphere, potentially leading to extreme weather due to longer-duration cold spells, snow events, heat waves, flooding events, and drought conditions. Dr. Francis describes her work in a March 5, 2012, post on the Yale environment360 web site.

"Even if the current weather situation may seem to speak against it, the probability of cold winters with much snow in Central Europe rises when the Arctic is covered by less sea ice in summer." That was the opening sentence of a January 26, 2012, press release by a group of European scientists, led by Ralf Jaiser of the Alfred Wegener Institute in Germany. The words proved prescient, because that day marked the beginning of a brutal two-week cold air outbreak over Central and Eastern Europe that killed 823 people and did over $660 million in damage, according to preliminary estimates by insurance broker Aon Benfield. 

Dr. Jaiser's team, using modeling studies, showed that Arctic sea ice loss weakens upper-level winds over the Arctic in winter, allowing an increased chance of cold air surges over Europe.

Figure 4. Digging out in Maryland after "Snowmageddon" on February 4, 2010. Image credit: wunderphotographer chills.

Why was the winter of 2011-2012 so warm in the U.S.?
The winter of 2011-2012 in North America was unusually warm--the fourth warmest on record. The cold air spilling out of the Arctic during the winter was confined to Europe, unlike that previous two winters, which were unusually cold and snowy in the Eastern U.S. Obviously, loss of Arctic sea ice is not having the same impact each winter; such factors as El Niño/La Niña, the phase of the 11-year sunspot cycle, and the amount of snow cover in Siberia also have strong influences on the winter weather pattern that sets up. Cold air is less likely to spill out of the Arctic during a solar maximum, as we are now headed towards, so this factor may tend to reduce the odds of getting big cold blasts in the U.S. during the coming two winters. However, cold air may be more likely to spill out of the Arctic in winter due to the decades-long pattern of warming and cooling of Atlantic Ocean waters known as the Atlantic Multidecadal Oscillation (AMO). A 2012 study by NASA scientists found that the warm phase of the AMO (like we have been in since 1995) causes more instances of atmospheric blocking, where the jet stream gets "stuck" in place, leading to long periods of extreme weather. It will be interesting to see how all these factors play out in the coming years. 

If these three newly-published studies are correct, the U.S. should see more winters like 2010-2011 and 2009-2010 in coming decades, as Arctic sea ice continues to melt and affect global atmospheric circulation patterns more strongly.

Francis, J. A., and S. J. Vavrus. (2012). Evidence linking Arctic amplification to extreme weather in mid-latitudes, Geophysical Research Letters, 21 February, 2012, in press.

Jaiser, R., K. Dethloff, D. Handorf, A. Rinke, J. Cohen. (2012). Impact of sea ice cover changes on the Northern Hemisphere atmospheric winter circulation, Tellus A 2012, 64, 11595, DOI: 10.3402/tellusa.v64i0.11595

Liu et al. (2012). Impact of declining Arctic sea ice on winter snowfall, Proc. Natl. Academy of Sciences, Published online before print February 27, 2012, doi: 10.1073/pnas.1114910109

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