https://insideclimatenews.org/news/27092017/polar-vortex-cold-snap-arctic-ice-loss-global-warming-climate-change
[links and images in on-line article]
Ice Loss and the Polar Vortex: How a Warming Arctic Fuels Cold Snaps
The loss of sea ice may be weakening the polar vortex, allowing cold
blasts to dip south from the Arctic, across North America, Europe and
Russia, a new study says.
By Bob Berwyn, InsideClimate News
Sep 28, 2017
When winter sets in, "polar vortex" becomes one of the most dreaded
phrases in the Northern Hemisphere. It's enough to send shivers even
before the first blast of bitter cold arrives.
New research shows that some northern regions have been getting hit with
these extreme cold spells more frequently over the past four decades,
even as the planet as a whole has warmed. While it may seem
counterintuitive, the scientists believe these bitter cold snaps are
connected to the warming of the Arctic and the effects that that warming
is having on the winds of the stratospheric polar vortex, high above the
Earth's surface.
Here's what scientists involved in the research think is happening: The
evidence is clear that the Arctic has been warming faster than the rest
of the planet. That warming is reducing the amount of Arctic sea ice,
allowing more heat to escape from the ocean. The scientists think that
the ocean energy that is being released is causing a weakening of the
polar vortex winds over the Arctic, which normally keep cold air
centered over the polar region. That weakening is then allowing cold
polar air to slip southward more often.
The polar vortex has always varied in strength, but the study found that
the weaker phases are lasting longer and coinciding with cold winters in
Northern Europe and Russia.
"The shift toward more persistent weaker states of the polar vortex lets
Arctic air spill out and threaten Russia and Europe with extreme cold,"
said the study's lead author, Marlene Kretschmer, a climate scientist
with the Potsdam Institute for Climate Impact Research. "The trend can
explain most of the cooling of Eurasian winters since 1990."
Some other scientists aren't as sure that melting sea ice affects the
polar vortex so strongly. They think other factors, like long-term
variations in sea surface temperatures like El Niño, and changes in the
tropics, might play bigger roles.
Primed for Longer Stretches of Extreme Cold
The research, published in the Bulletin of the American Meteorological
Society, helps explain one way that rapid and intense Arctic warming
affects climate extremes in the populated mid-latitudes of the Northern
hemisphere.
Kretschmer and her colleagues focused on the region from Scandinavia
through Siberia, where winter snow cover has increased and average
winter temperatures have dropped since 1990. Co-author Judah Cohen, a
climate researcher at MIT, said the results also provide new clues about
how the Arctic affects cold extremes in the U.S.
The study tracked changes in the polar vortex in the months of December
and January between 1979 and 2015. It concluded that the polar vortex is
primed for extreme cold outbreaks for longer stretches—from 5.3 days
during the first half of the study period to 14.1 days in the second
half. During the same time, average winter surface temperatures in
northern Eurasia declined.
"It's a piece of the whole puzzle which really helps us understand the
linkages between Arctic changes and mid-latitude circulation changes,"
said Dörthe Handorf, a climate researcher with the Alfred Wegener
Institute who was not involved in the study.
Previous studies have also concluded that the changes in the
stratosphere are important. "Without the stratospheric changes, we can't
explain why we see an increase in cold days over Eurasia," Handorf said.
A Step Toward More Accurate Forecasts
Along with helping explain how melting sea ice affects the atmosphere,
the new study is a step toward more accurate seasonal forecasts that can
help prepare communities for extreme conditions, Cohen said.
Models used in forecasting don't currently anticipate these changes in
the polar vortex, he said. Comparing polar vortex phases with
temperatures in the study area and data on sea ice extent can
potentially improve forecasts two to six weeks in advance, he said.
With that information, scientists soon may be able to say that, when the
sea ice forms very late in the Arctic Ocean north of Russia, people
living eastern Scandinavia and Siberia should prepare for harsh early
winter conditions.
The picture is not as clear for North America, said Jim Overland, an
oceanographer with the National Oceanic and Atmospheric Administration
(NOAA), who was not involved in the study. Natural year-to-year
variations in weather still masks the global warming signal to some
degree, he said.
"You can take one view or another, but the research helps make people
think about the effects and how to forecast them. What we know for sure
is, the Arctic is warming and losing ice and the forcing is there," he
said, referring to the potential effect of melting sea ice on weather
patterns. Pinpointing the impacts on areas where millions of people
live, he said, would pay off for those communities.