Washington Post - Chris Mooney
More and more, we are learning that climate change can lead to some
pretty strange and counterintuitive effects, especially when it comes to
the wintertime.
For instance, scientists have
pointed out for a number of years
that warmer seas, and a wetter atmosphere, can actually fuel more
snowfall in massive nor’easters affecting the U.S. East Coast.
More controversial still is an idea called “
Warm Arctic, Cold Continents.”
This is the notion that as the Arctic warms up faster than the middle
latitudes, it may sometimes cause a displacement of the region’s still
quite frigid air to places that aren’t so used to it. In other words,
even as the planet warms, masses of cold air could also become more
mobile and deliver quite a shock at times when outbreaks occur in more
southerly latitudes.
In both November and December of 2016, for instance, temperatures at the
North Pole surged tens of degrees above normal while at the same time a
huge mass of abnormally cold air descended over Siberia. Capital
Weather Gang
reported that in November, during one of the excursions, Siberian temperatures were “up to 60 degrees below normal.”
Here’s what the configuration looked like last December:
 |
| Image obtained using Climate Reanalyzer, Climate Change Institute, University of Maine. |
Now, a
new study
in the Bulletin of the American Meteorological Society makes the case
that in January and February — later in the winter than those events —
another, perhaps related change is occurring. This one involves the
notorious “
stratospheric polar vortex,” a
loop of extremely cold and fast-flowing air, high in the atmosphere,
that tightly encircles the Arctic in the freezing dark of polar winter.
This vortex can sometimes develop outward bulges, allowing for a more
southerly invasion of air.
The study, led by
Marlene Kretschmer
of the Potsdam Institute for Climate Impact Research in Germany, sought
to find patterns in the stratospheric polar vortex over the past 37
years, categorizing its behavior into seven states, ranging from a tight
loop around the Arctic to “a weak distorted vortex.” And it determined
that the stronger and more defined vortex has been occurring less
frequently, while distorted states have been growing more common — a
change linked to colder temperatures over Eurasia.
“This study provides quite some evidence that the cooling trend over
Eurasia was at least partly affected by the weakening of the
stratospheric polar vortex,” said Kretschmer.
She conducted the study with five colleagues from universities in Germany, the Netherlands and the United States.
The “polar vortex” is both a popularly known and deeply confused concept — the problem is that there are
two of them,
which sometimes interact. The stratospheric polar vortex is far higher
in the atmosphere and forms a much tighter loop. Then there is a lower
“tropospheric” version that more directly affects the weather we all
experience.
Kretschmer provided this diagram to show how the two are situated and can interact:
The work also suggests there’s a role played by the loss of Arctic sea
ice, a phenomenon linked to climate change. When floating sea ice melts
north of the Eurasian continent, that can lead to a greater flux of heat
from the ocean to the atmosphere as an icy cap on that warmth is
removed. In turn, that can lead to a cascade of atmospheric effects that
ultimately weakens the stratospheric vortex, high above.
“It matches with this hypothesis that the Arctic does have an effect
and that climate change, leading to a decrease in sea ice, has an effect
on large scale circulation, in this case the stratosphere,” Kretschmer
said.
Previous research
by Kretschmer has found a link between low levels of sea ice in the
Kara and Barents Seas, north of Russia, and broader atmospheric
patterns.
Those living in the United States will instantly wonder
how all of this applies to the extreme “polar vortex” event of the
winter of 2014. But in fact, the new study finds stronger evidence of a
“Warm Arctic, Cold Continents” pattern over Eurasia than it does over
North America. Kretschmer said she believes more research is needed on
how stratospheric disruptions in winter could affect North America, too.
This
whole line of inquiry remains relatively novel in climate research,
however, and the chains of causation are nothing if not complicated.
Climate
scientist Kevin Trenberth of the National Center for Atmospheric
Research, for one, remains cautious about the work. In a comment on the
new study for The Post, Trenberth suggested that the picture is more
complex and that Arctic changes aren’t the only thing going on — citing
major trends in the Pacific and Atlantic oceans as well.
The new study presents “a number of quantities that are related to
one another, but one can not say they are causal, as claimed,” Trenberth
commented by email. “On the contrary, there is good evidence of other
influences that play a major causal role. Thus the Arctic amplification
goes along with and is consistent with profound changes in the
stratospheric polar vortex in January and February, even as profound
influences come into the region from lower latitudes.”
He’s not the only skeptic. A
study published last year in
the journal Geophysical Research Letters regarding the “Warm Arctic,
Cold Continents” hypothesis rejected the idea that continental cooling
was linked to the loss of Arctic sea ice.
“Whereas the
directionality toward warming Arctic surface temperatures is well
understood to be linked strongly with accelerating sea ice loss, there
is neither an established theory nor strong experimental evidence that
midlatitude temperature trends having opposite directionality results as
a dynamical response,” found the authors, a team of researchers with
the University of Colorado in Boulder and the National Oceanic and
Atmospheric Administration.
As all of this suggests, even as some
scientists suggest that the dramatic changes to the Arctic are
reverberating in the latitudes where many of us live, others continue to
point out that our weather also has well established and more
traditional drivers, like the Pacific Ocean. And those are also
changing. It’s a complicated picture with a lot of moving pieces — but
the fastest-moving one, the Arctic, seems more than capable of
delivering some surprises.
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