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Tuesday, 25 September 2012

A Look at Arctic Sea Ice Loss and Extreme Weather Events

With Arctic sea ice reaching its lowest level in the satellite record after an astonishingly rapid summer melt, the question of whether disappearing sea ice might lead to more extreme winters in Europe and North America needs more scrutiny.
In an article on September 12, I reported on a 2012 paper by Jennifer Francis of Rutgers University and Stephen Vavrus of the University of Wisconsin, which showed that the loss of Arctic summer sea ice cover is adding enough heat to the ocean and atmosphere that it is helping to redirect the jet stream — the fast-moving high-altitude river of air that steers storm systems across the northern hemisphere.                    A NASA illustration showing the jet stream flowing across North America. 

Since March, the amount of Arctic sea ice that has disappeared is about equal to the areas of Texas and Canada, combined. This continues recent trends, ever since the previous record melt year of 2007. The six lowest Arctic sea ice extents in the satellite era have occurred during the past six years. As a result of such extensive sea ice melt, says the study, the jet stream, which has always been rather capricious, is behaving differently now. It has become slightly more elongated during fall and winter, with bigger troughs and ridges. A more wavy jet stream may have contributed to recent extreme winters that have featured historic blizzards along the East Coast of the U.S. as well as in Europe, the study found.

But from an email conversation with Francis, Vavrus, and several other atmospheric scientists this week, it became clear that there may be more questions than answers at this point, given the large amount of natural variability that affects winter weather patterns, and the very short observational record of how the atmosphere responded to extreme losses of sea ice (only five winters of records since 2007).
The underlying logic is sound: as sea ice melts, it exposes darker ocean water, which absorbs more of the sun’s heat, causing the water temperatures to increase. During the fall, the heat that was added to the oceans gets released into the atmosphere as sea ice reforms, and this added heat is bound to change weather patterns somehow (this is a process known as "Arctic Amplification"). The “how” part is what’s open to debate.
The study by Francis and Vavrus proposed that the added heat is altering jet stream patterns, but other mechanisms are certainly possible, as both researchers said via email. 
“It’s implausible that such a redistribution of heat in the Arctic system wouldn’t change weather patterns somehow, somewhere,” Vavrus said. But he cautioned that people should view their study as a “launching point” for more research, and not interpret it as having any predictive value for the upcoming winter, as some media outlets have portrayed it.
 One way that Arctic sea ice decline could result in extreme winter weather is by changing either the effects of theArctic Oscillation or the workings of the oscillation itself. The Arctic Oscillation is a climate index that describes the state of atmospheric circulation over the Arctic, and it has a so-called “positive” and “negative” phase. When the Arctic Oscillation is negative, cold Arctic air masses tend to plunge southward and into the U.S., spawning snowstorms and leading to cooler than average conditions. A positive Arctic Oscillation, by contrast, tends to mean warmer than average weather with less snow for the eastern U.S.

The Arctic Oscillation can vary considerably on short timescales, such as within a single month or from one month to the next,  and the causes of its variability aren’t entirely known. It was largely positive during the winter of 2007-8, which followed the previous sea ice record low, and then turned record negative during the winter of 2009-10, during which the infamous “Snowmageddon” blizzard occurred along the East Coast. It was then largely negative during the winter of 2010-11, and positive to weakly negative last winter, which was the mildest winter on record for the U.S.

The continued swings in the Arctic Oscillation can make it difficult for climate scientists to determine how sea ice loss is altering winter weather, since there is so much natural variability in the system in the first place. Other sources of climate variability, like El Niño and La Niña, also help set up prevailing winter weather patterns. Francis, of Rutgers University, said the natural climate cycles will help determine where some of the extremes will occur  
“Given this summer’s ice loss, and resulting enhanced absorption of solar heat into the Arctic Ocean, it’s hard to imagine that some interesting jet stream patterns won’t occur this fall and winter,” she said. “But where and when they appear are influenced by many factors...”
Judah Cohen, a climate forecaster atAtmospheric and Environmental Research in Massachusetts, who issues seasonal snowfall forecasts several months in advance for his company’s clients, said the relationship between sea ice loss and the Arctic Oscillation is a key area for future research to focus on. One study, published in Proceedings of the National Academy of Sciences in January, hinted at such a mechanism when it linked sea ice decline to apparent changes in the Arctic Oscillation during several winters preceded by large sea ice losses.
“I certainly believe that the loss in sea ice has a large impact on the energy budget, which can have a profound impact on the weather around the Northern Hemisphere,” Cohen said. “However, there are many moving parts to the climate system and Arctic sea ice is just one of many.”
Cohen has published research showing that above average fall snowfall in Siberia can lead to colder and snowier winters in the U.S. The sea ice loss-related increase in open water north of Siberia can contribute to increased fall snowfall in that region.
“It is not simple to explain how sea ice loss that peaks in August and September impacts the circulation at mid-latitudes [during] December through March. It is possible but it seems to me that it makes the most sense if sea ice first impacts another component of the climate system that then impacts the winter circulation,” Cohen said. 
So, the bottom line when it comes to this research is — it’s an active area of interest, but no firm conclusions have been reached other than that you simply cannot lose so much sea ice without altering the weather somewhere. That may not be actionable intelligence, but it’s a disturbing truth, once it sinks in.


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