What Happened To Our Winter?

The mild winter of 2012 has many people asking, “Where’s the snow?” These two snow cover maps show the difference between snow extent on March 3, 2011, and March 5, 2012. The maps were compiled from data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard NASA’s Terra satellite.

Snow Cover as of March 3, 2011

Snow Cover as of March 5, 2012

In 2012, the snow cover is very spotty compared to 2011. In 2011, the Great Lakes were clearly defined by surrounding snow, and snow blanketed the Rocky Mountains, Nevada’s Basin and Range, and the Sierra Nevada all the way into southern California.  In 2012, areas that are usually snow covered are bare, including parts of Montana, Wyoming and South Dakota. The Snake River Plain in southern Idaho is clearly visible. This low-lying valley is the track of the hotspot that is now under Yellowstone National Park. In the east, the Great Lakes area, southern Ontario and the East Coast of the United States have much less snow cover than they did at this time last year.

2012’s relatively light snowfall is the result of two atmospheric processes, according to climatologist Bill Patzert of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. One is the La Niña conditions in the Pacific, which result in less moist air crossing the continental United States. The other is a strong Arctic Oscillation that keeps cold arctic air around the North Pole and away from more southern latitudes. First of all,” he explains, “we are experiencing a La Niña pattern of sea surface temperatures in the Pacific Ocean. This pushes the jet stream and the cold arctic air northward.”  “On top of that, this year’s Arctic Oscillation has been stronger.”

The Arctic Oscillation is a see-sawing pressure difference between the Arctic and lower latitudes.  When the pressure difference is high, a whirlpool of air forms around the North Pole. Last year, the whirlpool motion was weaker, allowing cold air to escape from the polar regions and head southward to the US.  “This year the whirlpool has been more forceful, corralling the cold air and keeping it nearer the pole. That has reinforced the La Niña impact.”

While the corralling action of the Arctic Oscillation has kept snow away from parts of the contiguous United States, it has brought extra snow to places inside the whirlpool.  “The strong positive AO has kept the Jet Stream north,” says Patzert. “Snow-delivering storm tracks  pounded Alaska.”

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