For Alaskans who have basked in record warmth, Atlantans who abandoned cars during a January snowstorm, or Californians enduring drought, this winter's extremes have been nothing if not memorable.
Drought or unusual warmth is in sync with the effects that climate scientists expect from global warming. But what about wintertime invasions of Arctic air into the U.S. Deep South or into China, where, a new study indicates, record cold events became more frequent over the past 10 to 20 years?
For some climate scientists, January's extremes and the atmospheric patterns that nurtured and sustained them are fresh bits of information to apply to these intriguing questions: Has global warming's effect on the Arctic set the stage for persistent weather patterns that lead to extremes? If so, is the decline in Arctic sea ice the stage manager for the wintry events?
Nearly two years ago, two researchers identified atmospheric features that appear to tie a warming Arctic to mid-latitude weather extremes. Since then, when persistent weather patterns have brought drought or heat waves or repeated invasions of cold air to usually mild locations in winter, these links to the Arctic have become a go-to explanation among many commentators and policymakers.
Researchers working on the issue, however, caution that while they are uncovering intriguing hints of this tie-in, it is far from ironclad.
Nor is it merely an arcane debating point. If the proposition holds up, in principle it could help improve seasonal forecasts for temperature and precipitation.
"That would have huge economic implications," says Cecilia Bitz, a climate scientist at the University of Washington in Seattle who specializes in the role that Arctic sea ice plays in the climate system.
The state of this science today is comparable to that of tropical cyclones and global warming following hurricane Katrina in 2005, suggests Jennifer Francis, an atmospheric scientist at Rutgers University in New Brunswick, N.J. For those phenomena, dueling studies appeared, and evidence for an existing fingerprint of global warming on patterns of tropical cyclone activity was equivocal.
"We are in the same situation," says Dr. Francis, who, along with colleague Stephen Vavrus at the University of Wisconsin-Madison, published the hypothesis about Arctic links to weather extremes in the journal Geophysical Research Letters in March 2012.
The duo starts by noting that warming in the Arctic has occurred at two to three times the pace of warming for the rest of the Northern Hemisphere, reducing the temperature difference between the Arctic and mid-latitudes. As the contrast shrinks, the jet stream slows.
The jet stream is a river of high-altitude winds that steers and spawns storms and in effect serves as an atmospheric boundary for cold Arctic air. A slower jet stream produces longer meanders, the pair says. This would bring cold air farther south and warm air farther north than otherwise would be the case.
At the same time, the two say, circulation patterns that block the migration of these meanders appear more frequently and persist. This would cause specific weather patterns to either creep along or dally for days to weeks.
Other researchers have connected a warming Arctic to weather extremes in specific regions. At a two-day workshop on the topic at the University of Maryland in College Park last September, climate scientist James Screen of the University of Exeter in England pointed to studies that link shrinking sea ice to cold winters in North America and Eurasia, wet summers in Europe, extreme rainfall in the Mediterranean, and the behavior of the East Asian monsoon.
But Dr. Screen also cautioned against claiming too much for the connections -- at least at this point. An apparent link doesn't say much about cause and effect, he and others noted.
The case for the link between the shrinking extent of Arctic sea ice and cold winters in Eurasia was the strongest, he offered. Even here, however, the influence is barely detectable and often not statistically significant.
The caution that he and others express centers in no small part on the radically different effects that Francis and Dr. Vavrus see compared with a decade's worth of modeling studies.
These studies suggest that a warming Arctic will draw the jet stream's average track north. Blocking patterns will decrease. Moreover, the models indicate no "robust" decrease in the jet stream's speed, notes Elizabeth Barnes, a climate scientist at Colorado State University in Fort Collins who focuses on the jet stream's behavior and the factors affecting it.
To be sure, the models could be wrong, she acknowledges. But when different teams with different models converge on the same answer, that inspires more confidence in the result.
The scientists' caution also hinges on the relatively short time that the Arctic has undergone rapid warming, Francis adds, noting that it has occurred only in the past 10 or 15 years. Model projections can help, but "it's going to take maybe another decade" before any signs of an Arctic influence can be confidently detected, she says.
Francis and colleagues also have looked specifically at sea ice loss and winter weather extremes, finding a link.
In effect, the ever-shrinking amount of sea ice in the fall makes more moisture available for snow. Increased snowfall over the North American or Eurasian Arctic earlier in the fall-winter season cools the air more quickly than a later snowfall would.
This more southerly mass of cold air affects pressure patterns in ways that boost the likelihood of blocking patterns and cold snaps.
This idea appears to draw some support from modeling work that was published last August and conducted by Yannick Peings and Gudrun Magnusdottir of the University of California, Irvine.
When they plugged the average Arctic sea ice decline between 2007 and 2012 into the model, it yielded cold conditions at mid-latitudes, mainly over Asia. The model also produced some increase in the depth of the jet stream's meanders, although the statistical significance was small. All this tended to take place in February.
But by 2090, according to their model projection, the Arctic warmed so much that wintertime cold extremes at mid-latitudes remained only as frequent as they were in 2010.
Further work by Dr. Magnusdottir and colleagues has added another wrinkle. Long-term swings in Atlantic sea-surface temperatures, known as the Atlantic multidecadal oscillation, appear to have the same effect on the jet stream's meanders and blocking patterns that Arctic warming and sea ice are purported to have.
When the AMO enters its warm phase – its condition since the 1990s – the jet stream tends to weaken and buckle. Blocking patterns increase, and colder temperatures prevail at mid-latitudes.
"This also supports the colder winters of recent years," Magnusdottir says, adding that the results seem robust, since they show up in real-world data as well as in computer simulations.
Others looking for predicted circulation patterns have a hard time finding anything. In a study published last month in Geophysical Research Letters, Dr. Barnes of Colorado State and colleagues used three independent approaches to identifying blocking patterns in climate data. They reported finding trends in isolated regions and at specific periods, but no trend was significant and common to all three approaches.
"Blocking is so variable from year to year that there's no evidence that anything out of the ordinary is occurring," Barnes says. Nor has she found trends in changes to the jet stream's meanders or travel time around the hemisphere.
Does this mean that Francis and Vavrus are wrong? "Not at all," Barnes says.
The evidence may be masked for now by the "noise" of natural variability. And other factors affect the jet stream. Perhaps "you have a tug of war going on" between the influence of sea ice and something else, Barnes says.
For her part, Francis acknowledges that for now, several of the links tying Arctic warming to patterns that foster bouts of extreme weather are still weak. On the other hand, no one is finding that the meanders in the jet stream are getting any smaller, she adds.
"Everybody's seeing a hint that this is happening," she says, "but we can't say it's definitely happening yet."