Judith Curry: Arctic Sea Ice And Extreme Weather
Is the dramatic decline of Arctic sea ice, spurred by manmade global warming, making the weather where we live more extreme? Several recent studies have made this claim. But a new study finds little evidence to support the idea that the plummeting Arctic sea ice has meaningfully changed our weather patterns.
Arctic ice loss and extreme weather
Last October, a paper was published entitled Arctic ice loss may be making North America weather more extreme. The original NOAA press release is [here]. The basic idea of the paper is summarized in an article by Jason Samenow entitled Study: Arctic ice loss may be making North America weather more extreme. Excerpts:
Before 2007, typical summer winds at the Arctic surface were more variable but tended to flow from the west. Since then, the summer winds were found to blow more consistently from the south, through the Bering Strait, across the North Pole, and out toward the Atlantic Ocean relative to the mean pattern in previous decades.
The shift coincided with the rapid decline of Arctic sea ice.
“This shift demonstrates a physical connection between reduced Arctic sea ice in the summer, loss of Greenland ice, and potentially, weather in North American and Europe,” said James Overland, of NOAA’s Pacific Marine Environmental Laboratory and study lead author.
The resulting meandering flow regime has often favored atmospheric blocking, where weather systems lock in place and get stuck. This means if it’s cold, warm or stormy, such conditions are likely to persist under these circumstances.
Some scientists find this over simplistic:
Take for example, Randall Dole, deputy director for research at NOAA’s Earth System Research Laboratory, who said, “it cannot be assumed that the changes will necessarily make weather worse, at least by some metrics.”
Judah Cohen, a climate forecaster at Atmospheric and Environmental Research in Massachusetts added: “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.”
This is a topic that I am pretty knowledgeable about (see this previous post on a paper from my research group.) Without going into detail, I certainly agree with the statement that the Overland et al. paper is over simplistic.
One of the paper’s coauthors, Jennifer Francis, has gotten quite a bit of mileage out of this paper. She has been involved with Al Gore’s Climate Reality Project, and was recently invited to give Congressional Testimony (discussed here). Francis has also been the chief proponent of a link between the Arctic sea ice decline and Hurricane Sandy:
Francis, a leading scientist of some of the research linking Arctic amplification and more extreme weather patterns in the mid-latitudes, said the unusual path of Superstorm Sandy may have been a consequence of the connection. The record loss of Arctic ice last fall favored a “block” or extreme slowing in the jet stream over Greenland that pushed Sandy towards the Jersey shore Francis contends.
Disputing the Overland et al. paper
This week, a new paper has been published:
Abstract. Previous studies have suggested that Arctic Amplification has 3 caused planetary-scale waves to elongate meridionally and slow-down, resulting in more frequent blocking patterns and extreme weather. Here, trends in the meridional extent of atmospheric waves over North America and the North Atlantic are investigated in three reanalyses, and it is demonstrated that previously reported positive trends are an artifact of the methodology. No significant decrease in planetary-scale wave phase speeds are found except in OND, but this trend is sensitive to the analysis parameters. Moreover, the frequency of blocking occurrence exhibits no significant increase in any season in any of the three reanalyses, further supporting the lack of trends in wave speed and meridional extent. This work highlights that observed trends in midlatitude weather patterns are complex and likely not simply understood in terms of Arctic Amplification alone.
Jason Samenow also reports on this paper, in a post entitled Arctic warming and our extreme weather: no clear link new study finds. Excerpts:
Is the dramatic decline of Arctic sea ice, spurred by manmade global warming, making the weather where we live more extreme? Several recent studies have made this claim.
But a new study finds little evidence to support the idea that the plummeting Arctic sea ice has meaningfully changed our weather patterns. The research, published today in Geophysical Research Letters, says links between declining Arctic sea ice and extreme weather are “an artifact of the methodology” and not real.
But the new research by Colorado State professor Elizabeth Barnes, which examined the waviness of the jet stream over the period 1980-2011, found no changes in its speed and no signs of increased blocking.
“We conclude that the mechanism put forth by previous studies … that amplified polar warming has lead to the increased occurrence of slow-moving weather patterns and blocking episodes, is unsupported by the observations,” Barnes writes.
Is Barnes’ paper the ‘last word’ on this topic? Heck no. But I find it to be an interesting and valuable contribution to the literature on this topic.