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Declining arctic snow cover is bad news for polar vortex

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As many in North America are learning, the health of the arctic affects weather in warmer, southern parts of the continent. Mark Fischetti does a great job explaining the “polar vortex” and how the belt that essentially keeps cold air tucked away up in the arctic is loosening, resulting in the historic and extreme cold snap we’ve all been tweeting about:

The polar vortex is a prevailing wind pattern that circles the Arctic, flowing from west to east all the way around the Earth. It normally keeps extremely cold air bottled up toward the North Pole. Occasionally, though, the vortex weakens, allowing the cold air to pour down across Canada into the U.S., or down into other regions such Eastern Europe. In addition to bringing cold, the air mass can push the jet stream—the band of wind that typically flows from the Pacific Ocean across the U.S.—much further south as well. If the jet stream puts up a fight, the moisture it carries can fall out as heavy snow, which atmospheric scientists say is the circumstance that caused the February 2010 “snowmageddon” storm that shut down Washington, D.C.

But we’ve seen that the jet stream isn’t putting up as much of a fight lately. The reason, is that arctic sea ice is melting. Another factor is the role of snow in reflecting sunlight back to the atmosphere and strengthens the jet stream. As with sea ice, snow cover has been declining. NASA’s Earth Observatory illustrates how snow cover is changing relative to historical averages:

In the image above, blue regions indicate more snow fall than historical records while brown regions indicate less snow cover:

In spring 2013, snow covered less area than the historical mean, with a new record low set in May in Eurasia, according to NOAA’s Arctic Report Card. The images above show snow day anomalies in April, May, and June; that is, they depict the percentage of days that snow cover was above or below the long-term average (1981-2010). Fewer snow days are represented in brown, while more snow days are shown in blue. The images are based on the weekly NOAA snow cover data records.

April had more days with snow cover than average because melting started late in northwestern Europe. However, once the melting started, it occurred rapidly, as shown in the May and June images. Eurasia reached a record low in May, and the snow cover extent in North America was at its fourth lowest in June. This lack of snow occurred because snow melted quickly, not because less snow fell during the winter.

But perhaps more striking is a look at the overall trend. A look at snow cover over several decades shows a warming trend (again from NASA where blue means higher than average snow cover; brown means less):

Trends in both arctic sea ice and snow are worrisome from a climate perspective, especially if the trends continue.

Top image courtesy NOAA Climate.gov based on data from the Rutgers Snow Lab; Bottom image by Robert Simmon, using data from the Rutgers University Global Snow Lab.

The views expressed are those of the author and are not necessarily those of Scientific American.

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