Supply Of Winter Arctic Air Is Drying Up

This image from Weather Bell depicts what is known as the
winter "cold pool" basically outlined in yellow. 
Scientists say the cold pool has been shrinking over
time due to climate change, leaving less frigid air 
to spill south in the winter. 

 There are plenty of reasons why we in Vermont just completed the warmest meteorological winter on record.  

Dumb luck. El Nino. A persistent weather pattern. Almost certainly climate change played a role.

One intriguing way climate change might have played a role is that the supply of frigid Arctic air up by the North Pole has been diminishing in recent years and decades. 

Almost every winter, there is at least one or two occasions - sometimes several or many - in which the weather pattern draws big packets of that frigid Arctic air southward into the United States. 

There's usually big pools of Arctic air way up north to supply repeated bone-chilling outbreaks. 

Historically, some of the worst of these outbreaks have given us in Vermont several days in a row of temperatures in the teens to 30s below zero.

You might have noticed our frigid spells have gotten shorter and less intense in recent decades. This winter, it hardly got cold at all. 

As the Washington Post tells us, that wintertime cold pool up north has been diminishing in recent decades  due very likely to climate change. 

Reports WaPo:

"There cold air supply in the Northern Hemisphere is being evaluated using temperature dat from out 5,000 feet high in the atmosphere. For about a decade, Jonathan Martin, a professor of meteorology at the University of Wisconsin, has analyzed the size of the cold pool at this level - or the area of the hemisphere covered by temperatures at or below 23 degrees. 

This winter's cold pool will finish the winter as the second smallest on record, Martin said."

Of course, using the size of the cold pool as areas 23 degrees or colder is pretty generous. That includes northern New England most winters. Including much of what we loosely call the winter we just had. 

All of the top 10 smallest winter cold pools have happened since the winter of 1997-98. Six of the top 10 smallest cold pools have occurred since 2013-14.

This look at Arctic air 5,000 feet above the ground makes sense. 

People who are skeptical of climate change will often say the "heat island effect" of urbanization where ground temperatures are measured are responsible for the observed rise in temperatures. 

Scientists are aware of the heat island effect and adjust data using peer-reviewed research methods. But Martin's research gets around that issue by measuring air temperature that can't be influenced by warmth generated by cities and commerce. 

The Washington Post also says that Martin's research shows that the shrinking cold pool is not influenced by El Nino.

El Nino is a periodic warming of the eastern Pacific Ocean.  El Nino was in full force this winter, and was at least partly blamed for the warm winter over most of North America. 

A shrunken cold pool does not mean everybody escapes frigid weather.  Jet stream patterns can ensure some places remain in the deep freeze even with a relative lack of cold air in the northern hemisphere. 

The smallest cold pool on record was in 2014-15.  That winter, much of the eastern United States was quite cold due to a persistent southward dip in the jet stream.  Here in Vermont, February 2015 was the third coldest on record, an exception to the mostly warm winters we've seen over the past two decades. 

Even with those exceptions, you can see the waning effects of cold air blasts, at least in the United States. For instance, nearly 8,000 warm weather records were set this winter, but only around 2,300 cold weather records, says the Washington Post.