Wednesday, December 23, 2015

What Happened to the Polar Vortex?

Visualization of wind speed and direction in the lower stratosphere, looking from the top of the Northern Hemisphere down. The polar vortex is highlighted within the black rectangle, with its circulation extending south above the U.S. (Credit: Click to Enlarge.
Over the past two years, the term polar vortex became synonymous with the bitter outbreaks of Arctic air that sent teeth chattering from Boston to Atlanta (after all, polar is right there in the name).

The polar vortex is a feature of the atmosphere defined as the fast moving current of air encircling the Arctic that forms during the cold months because of the increased temperature difference between the dark, frigid Arctic and the warm tropics.  There are actually two vortices, one in the layer of the upper atmosphere known as the stratosphere and one in the lower section, where our weather happens, called the troposphere.  These two features interact with each other and can affect the meteorological goings-on outside of the Arctic.

The polar vortex also interacts with other features and fluxes in the atmosphere, which causes it to vary in strength over time.

When the polar vortex is strong, as it is now, it keeps arctic air fenced in.  That is part of what is currently keeping the weather so mild in the eastern U.S..  But when the vortex is perturbed or weakened, the jet of air becomes more wobbly and can set up southward excursions of frigid air — what we saw plenty of times over the last couple winters.  The polar vortex is particularly susceptible to such weakening when it reaches its peak in mid-winter, Judah Cohen, an atmospheric scientist with the private firm Atmospheric and Environmental Research (AER), said in an email.

And that is what Cohen is forecasting will take place in January.

Cohen expects some pulses of energy working their way from the lower to upper atmosphere to perturb and weaken the polar vortex over the next few weeks.  That weakening would favor a dip in temperatures over the eastern U.S. and potentially an uptick in snowstorms, Cohen wrote in an AER blog post.

Some research suggests that the rapid warming of the Arctic, which is happening at about twice the pace of the planet as a whole, could be impacting the polar vortex.  Cohen and others think that declining sea ice and increased snow cover in Siberia are two manifestations of this amplification that are exerting a weakening influence on the polar vortex, which could be fueling more outbreaks of Arctic air.

Cohen thinks that the pulses of energy he expects to perturb the vortex over the next few weeks are related to particular areas of low sea ice and high snow cover present this fall and winter.

Read more at What Happened to the Polar Vortex?

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