Showing posts with label retreat. Show all posts
Showing posts with label retreat. Show all posts

Friday, October 26, 2012

Big changes in Arctic within years

Above interactive graphic illustrates the decline of the annual sea ice minimum volume in the Arctic over the years.

What trend can best be fitted to these data? Below, I've added a trendline that I believe best fits the data, but I encourage others to come up with better trends.


The trend points at 2014 as the year when Arctic sea ice will first reach zero volume for some time during that year. As discussed in the earlier post Getting the Picture, the Arctic Ocean looks set to be ice-free for a period of at least three months in 2015 (August, September and October), and for a period of at least 6 months from the year 2020 (June through to November).

Natural variability and strong feedbacks may speed things up further. Decline of sea ice in 2012 was such that we can expect a very low volume in December 2012, which could lead to inclusion of December in the period projected to be ice-free from 2020. That would make the ice-free period seven month long, i.e. well over half a year.

The image below shows the three areas where albedo change will be felt most in the Arctic, i.e. sea ice loss, decline of albedo in Greenland and more early and extensive retreat of snow and ice cover in other areas in the Arctic.


Related

- Getting the Picture
arctic-news.blogspot.com/2012/08/getting-the-picture.html

- Albedo change in the Arctic
arctic-news.blogspot.com/2012/07/albedo-change-in-arctic.html

- Greenland is melting at incredible rate
arctic-news.blogspot.com/2012/07/greenland-is-melting-at-incredible-rate.html

- Albedo change in the Arctic threatens to cause runaway global warming

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