Yes the Northern
hemisphere has been somewhat warmer over the past twenty years, sea ice has continued to steadily decline by
an annual delta H and the jet stream has become much more responsive and
interesting.
Our problem is not
understanding how they are all linked.
Having the temperature changes act to draw the jet streams north might
explain the increase in variability through natural compression.
The bigger problem is
that it is all subtle and any isolated factor is never the whole story.
Extreme Weather Tied To
Unusual Jet Stream
By Seth Borenstein
Wednesday, June 26, 2013
Washington — Lately, the
jet stream isn’t playing by the rules. Scientists say that big river of air
high above Earth that dictates much of the weather for the Northern Hemisphere
has been unusually erratic the past few years.
They blame it for
everything from snowstorms in May to the path of Superstorm Sandy.
And last week, it was
responsible for downpours that led to historic floods in Alberta, Canada, as
well as record-breaking heat in parts of Alaska, experts say. The town of
McGrath, Alaska, hit 94. Just a few weeks earlier, the same spot was 15
degrees.
The current heat wave in
the Northeast is also linked.
“While it’s not unusual
to have a heat wave in the east in June, it is part of the anomalous jet stream
pattern that was responsible for the flooding in Alberta,” Rutgers University
climate scientist Jennifer Francis said yesterday in an email.
The jet stream usually
rushes rapidly from west to east in a mostly straight direction. But lately
it’s been wobbling and weaving like a drunken driver, wreaking havoc as it
goes. The more the jet stream undulates north and south, the more changeable
and extreme the weather.
It’s a relatively new
phenomenon that scientists are still trying to understand. Some say it’s
related to global warming; others say it’s not.
In May, there was
upside-down weather: Early California wildfires fueled by heat contrasted with
more than a foot of snow in Minnesota. Seattle was the hottest spot in the
nation one day, and Maine and Edmonton, Canada, were warmer than Miami and
Phoenix.
Consider these unusual
occurrences over the past few years:
∎ The winter of 2011-12 seemed to disappear,
with little snow and record warmth in March. That was followed by the winter of
2012-13 when nor’easters seemed to queue up to strike the same coastal areas
repeatedly.
∎ Superstorm Sandy took an odd left turn in
October from the Atlantic straight into New Jersey, something that happens once
every 700 years or so.
∎ One 12-month period had a record number of
tornadoes. That was followed by 12 months that set a record for lack of
tornadoes.
And here is what federal
weather officials call a “spring paradox”: The U.S. had both an unusually large
area of snow cover in March and April and a near-record low area of snow cover
in May. The entire Northern Hemisphere had record snow coverage area in
December but the third lowest snow extent for May.
“I’ve been doing
meteorology for 30 years and the jet stream the last three years has done stuff
I’ve never seen,” said Jeff Masters, meteorology director at the private
service Weather Underground. “The fact that the jet stream is unusual could be
an indicator of something. I’m not saying we know what it is.”
Rutgers’ Francis is in
the camp that thinks climate change is probably playing a role in this.
“It’s been just a crazy
fall and winter and spring all along, following a very abnormal sea ice
condition in the Arctic,” Francis said, noting that last year set a record low
for summer sea ice in the Arctic. “It’s possible what we’re seeing in this
unusual weather is all connected.”
Other scientists don’t
make the sea ice and global warming connections that Francis does. They see
random weather or long-term cycles at work. And even more scientists are taking
a wait-and-see approach about this latest theory. It’s far from a scientific
consensus, but it is something that is being studied more often and getting a
lot of scientific buzz.
“There are some viable
hypotheses,” Stanford University climate scientist Noah Diffenbaugh said.
“We’re going to need more evidence to fully test those hypotheses.”
The jet stream, or more
precisely the polar jet stream, is the one that affects the Northern
Hemisphere. It dips down from Alaska, across the United States or Canada, then
across the Atlantic and over Europe and “has everything to do with the weather
we experience,” Francis said.
It all starts with the
difference between cold temperatures in the Arctic and warmer temperatures in
the mid-latitudes, she explained. The bigger the temperature difference, the
stronger the jet stream, the faster it moves and the straighter it flows. But
as the northern polar regions warm two to three times faster than the rest of
the world, augmented by unprecedented melting of Arctic sea ice and loss in
snow cover, the temperature difference shrinks. Then the jet stream slows and
undulates more.
The jet stream is about
14 percent slower in the fall now than in the 1990s, according to a recent
study by Francis. And when it slows, it moves north-south instead of east-west,
bringing more unusual weather, creating blocking patterns and cutoff lows that
are associated with weird weather, the Rutgers scientist said.
Mike Halpert, the deputy
director of NOAA’s Climate Prediction Center, said that recently the jet stream
seems to create weather patterns that get stuck, making dry spells into
droughts and hot days into heat waves.
Take the past two
winters. They were as different as can be, but both had unusual jet stream
activity. Normally, the jet stream plunges southwest from western Washington
state, sloping across to Alabama. Then it curves slightly out to sea around the
Outer Banks, a swoop that’s generally straight without dramatic bends.
During the mostly snowless
winter of 2011-12 and the record warm March 2012, the jet stream instead formed
a giant upside-down U, curving dramatically in the opposite direction. That
trapped warm air over much of the Eastern U.S. A year later the jet stream was
again unusual, this time with a sharp U-turn north. This trapped colder and
snowier weather in places like Chicago and caused nor’easters in New England,
Francis said.
But for true extremes,
nothing beats tornadoes.
In 2011, the United
States was hit over and over by killer twisters. From June 2010 to May 2011 the
U.S. had a record number of substantial tornadoes, totaling 1,050. Then just a
year later came a record tornado drought. From May 2012 to April 2013 there
were only 217 tornadoes — 30 fewer than the old record, said Harold Brooks, a
meteorologist at the National Severe Storms Laboratory. Brooks said both
examples were related to unusual jet stream patterns.
Last fall, a dip in the
jet stream over the United States and northward bulge of high pressure combined
to pull Superstorm Sandy almost due west into New Jersey, Francis said. That
track is so rare and nearly unprecedented that computer models indicate it
would happen only once every 714 years, according to a new study by NASA and
Columbia University scientists.
“Everyone would agree
that we are in a pattern” of extremes, NOAA research meteorologist Martin
Hoerling said. “We don’t know how long it will stay in this pattern.”
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