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Meteorologist Jim Woodmencey |
Almost everyone probably agrees that we had a weird winter, not just
here in Jackson Hole, but across the entire United States. Characterized by
warmer and drier than normal conditions in the Western United States and
Alaska, contrasted by unusually cold and record breaking snow in the Eastern
United States.
Even though the Winter of 2014-15
is behind us, people are still asking, “Why was our winter so weird? What
caused it to be so warm and dry?” More to the point, what can we blame that on?
In this week’s column I will
attempt to explain the phenomena that affected our winter’s weather and why
weather patterns were so disrupted across the U.S.
Name Blame
Anytime we experience unusual
weather we like to blame it on something. It makes it easier if we can blame it
on something with a catchy nickname, like “El Nino” or “La Nina”. We couldn’t
do that this year, as the sea-surface temperatures in the Equatorial Pacific remained
neutral until very late in the winter season.
If it wasn’t El Nino, then what
was it? Besides El Nino, there are a number of other ocean and atmospheric interactions
that can alter the “normal” weather patterns.
One possible explanation is the
Pacific Decadal Oscillation or PDO, which has to do with sea-surface
temperatures in the northern part of the Pacific Ocean.
The PDO usually runs on a 20 to
30 year cycle of either warmer or colder than normal. Back in the 1990’s and
early 2000’s it was in a warm phase. Then, there was a marked trend of cooling
temps in the north Pacific between 2007 and 2013. By early 2014 the ocean was
warming again, reaching a peak in December and January 2014-15, and temps were
still above normal by a couple degrees through March 2015.
I thought maybe we could blame
this past winter’s weather pattern disruption on the PDO.
Then I came across a study done
by the Atmospheric Science Department at the University of Washington. They
described a slightly different phenomenon, which one of their climate scientists,
Nick Bond, called, “The Blob”.
“The Blob” sounds more like science
fiction than it does science, however, the more technical name for it is the
North Pacific Mode (NPM). The NPM is similar, but a separate phenomena to the
PDO. In short, the NPM (aka the Blob) is a large area of very warm ocean temperatures
that extend up along the West Coast and across the Gulf of Alaska in a
horseshoe pattern. That warm water also extends down into the sea 300 feet
deep.
According to their recent paper published by the American Geophysical
Union, the University of Washington scientists
found that “The Blob’s” origins are
related to, “a
persistent high-pressure ridge that caused a calmer ocean during the past two
winters, so less heat was lost to cold air above. The warmer temperatures we
see now aren’t due to more heating, but less winter cooling.”
In other words, the ocean never cooled down this past fall and winter,
like it usually does, and high-pressure stayed put, creating a blocking pattern
in the Pacific that would not go away.
Blocking Highs
When big, tall, warm ridges of
high-pressure form in the atmosphere they tend to persist, and that is exactly
what occurred this winter. That big ridge essentially cut the western U.S. off from
the flow of Pacific moisture and left us in a warmer and drier weather regime,
with only a few brief storm periods.
Those big ridges in the
atmosphere are like a standing wave in a river, usually caused by a big boulder
that disrupts the normal flow. Just downstream of that big, tall wave is a big
eddy, or trough. That is what the eastern U.S. was under most of last winter, a
large, cold trough of Low-pressure. And what did we name that weather monster?
The “Polar Vortex”. Not a new concept, just a catchier name.
This standing wave weather
pattern –ridge to the west and trough to the east- was hard to break, and
persisted for much of January, February, and March 2015. Coincidentally, when the “Blob” was at its
peak.
The next time somebody asks you
why this past winter was so weird; you can tell them it was “The Blob”. Let’s
just hope the Pacific Ocean continues to cool and we don’t have to endure
watching “The Blob 2” next winter.
Posy by Jim Woodmencey
(Note: This post is re-printed from the original article that appeared in the Jackson Hole News & Guide's Mountain Weather Column in April 2015. )
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