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Friday, December 13, 2013

Melting Arctic sea ice could be altering jet stream

The jet stream, here caught wiggling. NASA/GSFC
by Scott K. Johnson, ars technica, December 11, 2013

The rapidly warming Arctic isn’t noteworthy only for its own sake. Changes there affect the rest of the planet in a number of ways. Recently, there has been a lot of interest in whether the dwindling Arctic summer sea ice could be weirding the weather in the mid-latitudes.
There have been a number of recent summer extremes—Russia’s hellish summer in 2010, the drought in the US last summer, a very wet 2011 in Korea and Japan, plus a streak of soggy summers in the UK. There have been suggestions that lower summer sea ice in the Arctic could be gumming up the jet stream and contributing to these events, but some climate scientists aren’t so sure. A new study in Nature Climate Change brings more evidence to the table in support of the idea.
Qiuhong Tang and Xuejun Zhang, of the Chinese Academy of Sciences, and Jennifer Francis, of Rutgers, decided to look for patterns of atmospheric change correlated with the loss of Arctic summer sea ice and the decline of early summer snow cover. Using reanalyses, which generate global datasets based on all the available measurements, they examined how the lower, middle, and upper troposphere responded to variations in sea ice and snow cover from 1979 (the start of the satellite era) to 2012.
They found modest correlations with the behavior of high-level winds and the differences in atmospheric pressure that drive them, more so for sea ice than snow cover. Over most regions, the average position of the jet stream moved a little northward when summer sea ice was smaller, while the opposite was true for the western edges of continents. The high-level, west-to-east winds of the jet stream also slowed a bit.
Those two factors are consistent with the hypothesized link between sea ice and weather extremes. When the jet stream slows, it gets wigglier, with ponderous meanders extending north and south. Because the temperature difference across the jet stream is so large, these slow-moving excursions can lead to temperature extremes. The early loss of snow cover can exacerbate this, as it means soils can dry out earlier in the summer. Not only does that make a region susceptible to drought, but low soil moisture allows temperatures to rise higher.
Another recent paper published in Environmental Research Letters focused on Northern Europe, using different techniques. There, an unusual run of six wet summers left people wondering if Arctic sea ice loss could have contributed.
Looking through the data, University of Exeter researcher James Screen saw that wet conditions are associated with the jet stream coming south from its average position. Conversely, it’s drier when it stays far to the north. Screen ran two climate model simulations: one in which Arctic sea ice was present at its 1979 extent and one at its diminished 2009 extent. Each simulation was repeated for a century’s worth of summers to calculate the average position of the jet stream over Europe.
Consistent with the study by Tang, Zhang, and Francis, the lower sea ice extent in the model was associated with the jet stream moving a little southward over Europe as part of its amplified “wiggliness.” That brought more precipitation to Northern Europe in the model simulations.
However, Screen emphasizes that these things vary quite a lot from year to year on their own, and the simulated sea ice impact was only a slight shift. “This means that whilst low sea ice coverage increases the risk of wet summers, other factors can easily negate this influence and lead to dry summers during depleted ice conditions or wet summers during extensive ice conditions,” he writes.
The details of how declining Arctic sea ice affects atmosphere circulation and weather patterns is very much a hot topic for research and debate, particularly when discussing potential contributions to specific, extreme weather events. In an article accompanying the Tang, Zhang, and Francis study in Nature Climate Change, NOAA researcher James Overland notes, “Skeptics remain unconvinced that Arctic/mid-latitude linkages are proven, and this work will do little to change their viewpoint.” He’s not talking about the self-professed "climate skeptics" who reject most of the conclusions of climate science here, but rather researchers in the field who have weighed in on this hypothesis.
“As most changes in the frequency of mid-latitude extreme events have occurred only in the past decade,” Overland writes, “there is insufficient data to formally resolve the debate on whether these events are purely random or if their occurrence is enhanced by Arctic cryosphere changes.” Work like this, however, is informing that debate.
Nature Climate Change, 2013. DOI: 10.1038/NCLIMATE206510.1038/NCLIMATE2079Open access at Environmental Research Letters, 2013. DOI: 10.1088/1748-9326/8/4/044015 (About DOIs).

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