Showing posts with label James Overland. Show all posts
Showing posts with label James Overland. Show all posts

Friday, October 26, 2012

Amplification of climate change in the Arctic

In contrast to multi-year old ice, first-year old ice—ice that formed only since the last melt season—is thinner, saltier, and much more prone to melt.


Over the years, the loss of sea ice has become especially manifest in the older ice, as illustrated by the image below.


Salt content and hardness play a part in multi-year ice’s resistance to melt, explains a recent NOAA article, but the main characteristic that allows the ice to survive the melt season is thickness.

Screenshots from: PIOMAS Arctic Sea Ice Thickness Simulation 1978-2011
The decline in thickness over the years goes a long way to explain the self-reinforcing character of sea ice decline in the Arctic.

As another recent NOAA article describes, there is “something extra” behind the record ice retreats of the past 6 years: each June, the prevailing winds shifted from their normal west-to-east direction and instead blew strongly from the south across the Bering and Chuchki Seas (left on the image below), over the North Pole, and out toward Fram Strait. (The length of the lines is qualitative: longer lines mean stronger winds.)

Average June wind vectors in 2007-2012 (orange) compared to 1981-2010 average (white) based on NCEP reanalysis data provided by Physical Sciences Division at NOAA ESRL. Map by Dan Pisut, NOAA Environmental Visualization Lab.

The image below shows the unusual air pressure patterns that gave rise to the wind shift. Air pressure across the Arctic in Junes from 2007-2012 was completely lopsided, with two pockets of higher-than-average pressure sprawled across the North American Arctic and Greenland. These areas of high pressure act like boulders in a river. They slow and disrupt the normal westerly flow of the wind, forcing it to make, large, meandering detours to the north or south.

Average geopotential height anomaly at 700 millibar pressure level in Junes from 2007-2012 compared to the long-term average (1981-2010) based on NCEP reanalysis data provided by PSD at NOAA ESRL. Orange colors are higher-than-average pressure; blue is lower-than-average pressure.     Map by Dan Pisut, NOAA Environmental Visualization Lab.
Arctic oceanographer and his NOAA colleagues think these “blocking highs” on the North American side of the Arctic created the unusually strong southerly flow that brought warm air into the central Arctic and over Greenland. The persistent southerly winds would help explain both the record low sea ice extent in summer 2012, as well as the island-wide melting of the surface of the Greenland Ice Sheet, which satellites detected in July 2012.

“This story started with us trying to figure out why the sea ice extents of the past 6 years or so have been so much lower than we would expect based on the long-term warming trend alone,” says Overland, “and we think this unusual circulation of the Arctic atmosphere is major part of it.”

Why, asks Overland, have these high pressure patterns have been forming so consistently each June for the past six years? The repeated appearance of these atmospheric features each June is so unusual that it’s the equivalent of a 1-in-a-1000 event. Can this be attributed to natural variability?

Instead, Overland’s hunch is that the cause is a change in the atmosphere that is itself connected to climate change in some way, possibly linked to record and near-record low June snow cover in the Canadian Arctic in recent years. “We don’t know that part of the story yet,” he says, “but this would certainly be the type of amplification of climate change [warming triggers changes that lead to more warming] we have been expecting to see in the Arctic.”

References

- Arctic Sea Ice Getting Thinner, Younger
http://www.climatewatch.noaa.gov/article/2012/arctic-sea-ice-getting-thinner-younger

- June wind shift a little something extra behind recent Arctic ice losses
http://www.climatewatch.noaa.gov/article/2012/june-wind-shift-a-little-something-extra-behind-recent-arctic-ice-losses

- Poles apart: A record-breaking summer and winter
http://nsidc.org/arcticseaicenews/2012/10/poles-apart-a-record-breaking-summer-and-winter/

- PIOMAS Arctic Sea Ice Thickness Simulation 1978-2011, published Sep 14, 2012 by ArctischePinguin
https://www.youtube.com/watch?v=G1TLzgSlGtQ

Related

- Arctic summer wind shift
http://arctic-news.blogspot.com/2012/10/arctic-summer-wind-shift.html

- The recent shift in early summer Arctic atmospheric circulation
http://www.agu.org/pubs/crossref/2012/2012GL053268.shtml

- Presentation by Dr. Jennifer Francis, Rutgers University
https://www.youtube.com/watch?v=RtRvcXUIyZg
http://marine.rutgers.edu/~francis/pres/Francis_Vavrus_2012GL051000_pub.pdf

Wednesday, October 10, 2012

Arctic summer wind shift

Arctic summer wind shift could affect sea ice loss and U.S./European weather

Changes in summer Arctic wind patterns contribute not only to an unprecedented loss of Arctic sea ice, but could also bring about shifts in North American and European weather, concludes the NOAA-led study The recent shift in early summer Arctic atmospheric circulation.

Image from the North Pole webcam shows (July 27, 2010) ponds created by the summer sea ice melt.  (Credit: NOAA)
A research team led by James Overland, Ph.D., of NOAA’s Pacific Marine Environmental Laboratory in Seattle, Washington, examined the wind patterns in the subarctic in the early summer between 2007 and 2012 as compared to the average for 1981 to 2010. They discovered that the previously normal west-to-east flowing upper-level winds have been replaced by a more north-south undulating, or wave-like pattern. This new wind pattern transports warmer air into the Arctic and pushes Arctic air farther south, and may influence the likelihood of persistent weather conditions in the mid-latitudes.

“Our research reveals a change in the summer Arctic wind pattern over the past six years. 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 Overland, an oceanographer who leads the laboratory’s Coastal and Arctic Research Division.

The shift provides additional evidence that changes in the Arctic are not only directly because of global warming, as shown by warmer air and sea temperatures, but are also part of an “Arctic amplification” through which multiple Arctic-specific physical processes interact to accelerate temperature change, ice variability, and ecological impacts.

The study was co-authored by scientists from Rutgers University in New Jersey, the University of Sheffield in the United Kingdom, and the Joint Institute for the Study of the Atmosphere and Ocean, a partnership of NOAA and the University of Washington.

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. These winds transfer additional heat from the south toward the North Pole and push sea ice across the Arctic and out into the Atlantic Ocean, contributing to record losses of summer sea ice. The 2012 Arctic summer sea ice minimum far surpassed 2007 as the lowest on record.

“Higher pressure over the North American continent and Greenland is driving these changes in the early summer wind patterns,” said Edward Hanna, Ph.D, of the University of Sheffield.

These shifts in winds not only affect weather patterns throughout the Arctic but are also thought to influence weather in Greenland, the United States, and western Europe. Understanding such links is an ongoing area of research, the scientists said. The effects of Arctic amplification will increase as more summer ice retreats over coming decades. Enhanced warming of the Arctic affects the jet stream by slowing its west-to-east winds and by promoting larger north-south meanders in the flow. Predicting those meanders and where the weather associated with them will be located in any given year, however, remains a challenge.

The researchers say that with more solar energy going into the Arctic Ocean because of lost ice, there is reason to expect more extreme weather events, such as heavy snowfall, heat waves, and flooding in North America and Europe but these will vary in location, intensity, and timescales.

“What we're seeing is stark evidence that the gradual temperature increase is not the important story related to climate change; it's the rapid regional changes and increased frequency of extreme weather that global warming is causing. As the Arctic warms at twice the global rate, we expect an increased probability of extreme weather events across the temperate latitudes of the northern hemisphere, where billions of people live,” said Jennifer Francis, Ph.D, of Rutgers.

Screenshot from above video

View related video, links to further videos and references below.