Category Archives: Weather

Are we in a snow drought?

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The central and southwestern portions of the United States have experienced drought for the past several years. This photograph was taken in mid-February, 2010. By that point in winter, these rows would normally have been fully covered in snow. Photograph credit: Kristy Johnson (http://www.flickr.com/photos/kayaker1204/4359744393/)

In spite of the massive blizzards that have slammed parts of the northeastern United States, much of the country is experiencing a pronounced lack of snow. And where there is snow, it is less than usual. It follows a very low snow year from the previous season, causing people to worry if low snowfall is the new normal.

A previous Icelights post, Arctic sea ice and U.S. weather, discussed possible causes behind the changes in winter weather, trying to see if there are correlations to larger climate changes. Researchers like Jennifer Francis of Rutgers University are exploring the possibility that declining sea ice in the Arctic is altering atmospheric temperatures and weakening the jet stream. This change may shift the Northern Hemisphere storm track, leaving some places unusually snowy, or leaving normally snowy locations dry. Can a few good blizzards help reverse the snow drought across parts of the United States? Or is this string of dry winters yet another symptom of climate change? Continue reading →

What caused last summer’s Greenland surface melt?

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Data image showing extreme Greenland surface melt in summer 2012

These satellite-derived maps show the extent of surface melt over Greenland’s ice sheet during the summer of 2012. On July 8, about 40 percent of the ice sheet had melted at or near the surface. By July 12, nearly 97 percent of the ice sheet surface had melted. High pressure ridges that persisted over Greenland created a lingering heat dome that caused the extensive melt. Credit: Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory

During the summer of 2012, scientists noticed something unusual in Greenland. On July 8, about 40 percent of the Greenland Ice Sheet surface showed signs of melting. But a mere four days later, 97 percent of the surface was melting, an extent that was unprecedented in the satellite record.

Just how unprecedented was this event? Does it mean that the Greenland Ice Sheet is possibly on the way out, like summer sea ice in the Arctic?

The Greenland block

To be clear, the Greenland Ice Sheet is still very much intact. It will take much more than a warm summer to melt ice that covers 660,000 square miles and averages 1.43 miles in thickness. And surface melt is quite shallow, including only the top several centimeters, according to Thomas Mote, a professor at the University of Georgia. Mote is one of many scientists studying Greenland’s ice, investigating what caused such a dramatic melt. Mote credits last summer’s extreme melt in large part to a weather pattern called the Greenland block. “Greenland blocking is a persistent high pressure ridging over Greenland,” Mote said. “That is when we will see warmer than normal conditions and more melt.” During a normal summer, about half of the ice sheet surface experiences melt. Last summer, this high-pressure block essentially parked over Greenland, creating a lasting dome of heat that rapidly melted nearly the entire ice sheet surface.

Melting in context

Photograph of researchers righting an instrument tower that is leaning due to melting ice

In 2004, researchers snowmobiled across part of the Greenland Ice Sheet to stabilize this weather station. The surrounding ice had slowly started to melt, causing the instruments to lean. Credit: John Maurer

Much of the media coverage cited a historical precedent for the melt, focusing on an ice core record from Summit Station, Greenland. This core indicated that such extreme melts are not unusual, and have occurred as frequently as every 150 years. But records from a single location, while invaluable, may oversimplify the history of such a vast ice sheet, and may not account for more recent changes in the broader environment surrounding Greenland.

“You’re talking about a single event at Summit that is quite remarkable but very short-lived, versus this sort of broad warming and increase in melt over much of Greenland,” Mote said. “The increased melt that we’ve seen in Greenland over the past several years is associated with the general warming across the Arctic.” Greenland experienced record warming in 1999, 2007, 2010, and in 2012, mirroring years in which the Arctic as a whole experienced warming, and often, record sea ice minimums.

Mote and others hope to place the extreme event into a larger environmental context, seeing how factors like air temperatures and winter accumulation might play a role. And in spite of the recent warming, Mote cites some positive evidence among the many questions that Arctic warming often raises. “There is some evidence recently that suggests that the Greenland Ice Sheet can recover more easily perhaps than we might have thought,” Mote said. “I think we’re still trying to get a sense of just how inter-related these different cryospheric measures are across the Arctic.”

Reference

Nghiem, S. V., D. K. Hall, T. L. Mote, M. Tedesco, M. R. Albert, K. Keegan, C. A. Shuman, N. E. DiGirolamo, and G. Neumann. 2012. The extreme melt across the Greenland ice sheet in 2012. Geophysical Research Letters, 39, L20502, doi:10.1029/2012GL053611.

The Arctic Oscillation, winter storms, and sea ice

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The Arctic Oscillation (AO) is a large-scale climate pattern that influences weather throughout the Northern Hemisphere. It alternates between a positive phase (left) and a negative phase (right). In its positive phase the AO tends to bring warmer weather to the middle latitudes, while in its negative phase, winter storms are more common in the Eastern United States and Europe. Credit: J. Wallace, University of Washington.

Last year, many scientists blamed the winter storms that blasted the Northeastern United States and Europe on the negative mode of a weather pattern called the Arctic Oscillation. This winter, the Arctic Oscillation started out in the opposite mode, which scientists connect to the warmer-than-average temperatures and unusually low snowfall over much of the U.S. The swings of the Arctic Oscillation also help control how sea ice moves in the Arctic Ocean, which is of great interest to climate scientists. Readers often write in to ask us about this powerful but mysterious climate phenomenon, and how it affects weather where they live. What is the Arctic Oscillation, and how does it affect Arctic sea ice and the rest of the Northern Hemisphere? Continue reading →

An Arctic hurricane?

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This satellite image from November 8 shows the hurricane-like storm that hit Western Alaska earlier this month. Credit: NASA Earth Observatory image created by Jesse Allen, using data obtained from the Land Atmosphere Near real-time Capability for EOS (LANCE).

On November 8 and 9, a strong storm hit the Western Alaska coast, bringing blizzard conditions, storm surge of up to 10 feet and wind gusts as fast as 93 miles per hour. Along the Western Alaskan coastline, towns and villages prepared for the worst. “Up here, cities are much more sparse, but a storm like this still impacts the people that live there,” said Kathleen Cole, an ice forecaster at the National Weather Service. Damage reports after the storm indicated extensive flooding, wind damage to buildings, as well as power outages, which led to many evacuations to higher ground and to shelters with generator power. Some reports referred to the storm as a “blizzicane,” or an Arctic hurricane. What was unusual about this storm—and was there any connection to changes in the Arctic climate? Continue reading →