Wednesday, June 3, 2015

What can we blame this winter’s weather on?

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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