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Why is it so cold this winter?
Notice to Blue Water Outlook Subscribers
You might have noticed a slightly modified title to this BWO Newsletter. BWO subscribers have indicated time and time again the
importance of an understanding of the processes and causes of
current weather and how these processes will impact future
weather patterns—especially in the long term.
Most subscribers utilize BWO content in making informed decisions
regarding water management or farm or ranching operations.
As a result, I plan a greater focus on an insight and analysis of key
weather trends, especially relating to precipitation and soil moisture. Thus a slightly new title to this report—the BWO Insight and
Analysis Report.
The winter of 2013/2014 has been unusually harsh in a number of
ways. In this report, I provide detail into the frequently-mentioned
polar vortex, and insight into what is causing this interesting and
unusual winter.
As spring nears (optimistically-thinking), I will continue to provide a
wide variety of information that will help you make informed operational decisions.
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Understanding the Polar Vortex
Before this winter, many might not have heard the term—Polar
Vortex (PV). At this point in the current record-setting cold winter,
almost everyone is familiar with it.
This term is not new but has gained significant attention due to the
unusual winter weather this year. A Polar Vortex is “caused when
an area of low pressure sits at the rotation pole of a planet. This
causes air to spiral down from higher in the atmosphere, like water
going down a drain..” Universe Today
Polar vortices are located within the middle and upper troposphere
and the stratosphere. A PV is a persistent, large-scale cyclone that
can exist at one or both of the earth’s geographical poles. The core
of coldest air is usually associated, and confined, to the region
within the PV. The vortex is the strongest in winter and can disappear in summer.
The winter jet stream typically aligns to the south of
the PV confining arctic air to the north.
The PV can occasionally break down during an event known as a
sudden stratospheric warming (SSW). During a SSW event, the
vortex completely breaks down and an associated warming of 30-50
degrees Celsius over a few days can occur.
The PV is not always symmetrical in shape. It can, and often is,
elongated in shape—extending in a NW-SE alignment across the
Upper Midwest, Great Lakes, and then southeast into the Ohio Valley and parts of the eastern U.S.
Furthermore, “lobes” can also rotate around the primary core vortex which can be anchored well to the north. Each individual
lobe rotates southward reinforcing pockets of cold arctic air.
Occasionally, the PV will expand or re-position further south. In response, the jet stream will dip
which allows arctic air to spill south.
Sources: Washington Post (illustrations)
A Wobbly Polar Vortex
The polar vortex has been unusually “wobbly”
this season. The two images to the left indicate a
typical compact configuration compared to the
wavy configuration that has been observed this
season.
High pressure over the Arctic has tended to slow
down the jet stream and shape it into deep
folks thus allowing cold arctic air to sink into
the central and eastern U.S.
Climate scientists have noticed that the jet
stream has taken on a more wavy shape over
recent winters instead of the more typical
oval around the North Pole, leading to outbreaks of colder weather down in the midlatitudes and milder temperatures in the Arctic.
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Why So Cold?
While there has nearly constant discussion via media sources on the “Polar Vortex” or “Arctic Plunge” this winter, the simple fact is
that the position of the jet stream has been usually and persistently far south which has allowed pure arctic air to settle over
large portions of the U.S. While it is not at all unusual for arctic air to sink south over the winter months, what has made the winter of 2013/2014 exceptional is the intensity, and in some cases, persistence of the record-setting cold.
Thus, the question arises: why so cold? First, as is the case in many aspects of weather, a complete certainty of the causes of this
winter are likely not fully understood. However, there are several interesting aspects worth discussion.
BWO subscribers are likely somewhat knowledgeable about the Arctic (and North American) Oscillation (AO/NAO). The AO is an
atmospheric teleconnection. When the AO is negative, it is typical for colder-than-normal air to sink southward over the eastern
half of the nation. During a negative-phased AO event, the polar jet stream weakens and re-forms further south. Keep in mind that
the strong winter jet stream tends to define the southern extent of cold air. When the jet stream is strong, and well to the
north, it tends to confine arctic air within polar regions. When the polar jet stream weakens, cold air sinks southward.
The image to the right indicates that the
winter started off with a predominant positive AO, but transitioned into strong and
repeated negative-phase episodes since
the first of the year. These strong negative
-phase events are indicated by the red arrows.
The image to the left indicates the typical jet stream pattern so far this
winter. Areas shaded in blue and green indicate unusually low upper
atmosphere pressure while areas shaded in red indicate unusually high
pressure. The yellow line indicates the position of the jet stream.
While the eastern U.S. has experienced unusually cold weather, the
west—especially parts of western Canada, the Pacific Northwest, and
Alaska have been quite mild.
A primary reason for the persistent and extreme pattern this winter
may be an interaction of the lower atmosphere (troposphere) with
the middle atmosphere (stratosphere). An event called a sudden
stratospheric warming (SSW) occurs when there is rapid intense warming of the lower half of the stratosphere over a period of a few days.
Early January Arctic Outbreak
A major SSW event tends to slow down or reverse westerly winds and can persist for weeks. SSW’s can promote a negative NAO and an subsequent outbreak of
unusually cold weather over the central and eastern
U.S.
When a strong SSW event occurs, the polar jet, which
has been keeping the arctic air confined north, weakens
and re-establishes a position further south (similar to the
image above) thus allowing an outbreak of unusually
cold temperatures.
But the question remains: Why the increased frequency
of SSW events this winter? The answer likely is complex,
but perhaps a combination, or alignment between larger-scale teleconnections and changes in solar activity.
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Solar Cycle 24
The BWO winter outlook discussed the possibility that the unusual solar cycle 24 could impact winter weather. Studies by
climate scientists identified a relationship between low solar
activity and active U.S. winter weather.
Studies have found that winter temperature and precipitation
patterns associated with low solar activity can resemble the
negative phases of the North Atlantic Oscillation (NOA) resulting in unusually cold weather. During years of low solar
activity, unusually cold air forms over the tropics in the stratosphere, which is balanced by a more easterly flow of air over
mid latitudes allowing cold air to sink southward at low levels
of the atmosphere.
While the recent extreme cold temperatures set many records, from one
perspective, the cold was not nearly as bad as over the past four decades.
Another way to access the cold wave was the duration of cold outbreaks.
Taking a look at data for a number of U.S. cities, January’s deep cold was
briefer than past cold waves. NOAA scientists have found that many cold
spells during the 1970s and the 1980s were longer in duration.
There is some speculation that the duration of cold waves could be getting
shorter in the U.S.
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There have been four distinct snow tracks so far this
winter season, however two of these have not persisted.
These four bands are indicated on the image to the left/
top. The red arrow marks the most persistent track,
across the Upper Midwest and Great Lakes.
A secondary active track extends across portions of the
Ohio Valley into the Northeast U.S. as indicated by the
white arrow.
There were several other active areas earlier this season
as indicated by the yellow arrows. Both of these tracks
have dissipated and have not extended into the middle
of the winter season.
While the Upper Midwest typically experiences snowy
winters, this season has been unusually active.
The image to the left/bottom indicates anomalies of
snow cover with areas in green experiencing snowfall
above seasonal normal while areas in red have received
less snow than normal.
Heading into early February, several new, and perhaps
significant, snow tracks are likely to develop over parts
of the Midwest.
The image to the right plots the percentage of the contiguous U.S.
that was covered by snow this season. You can see that snow
cover peaked early in December 2013, and despite some upticks
from time to time, overall snow cover has been slowly decreasing ever since.
Close to 37 percent of the contiguous was covered by snow
on the first day of 2014. This was very close to the average
(red line) of 41 percent.
Last year at this time, nearly 70 percent of the U.S. was
snow covered — the most snow the U.S. had seen on January 1 in the prior ten years.
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Observed 2013 Precipitation
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Percent of Normal 2013 Precipitation
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An analysis of global temperatures by NASA
scientists shows that 2013 was the seventh
warmest year since 1880 (tied with 2006 and
2009). Nine of the 10 warmest years on record
all have occurred since 2000, with 2010 and
2005 ranking as the warmest. Scientists at
NASA’s Goddard Institute for Space Studies
(GISS) reported that 2013 continued the longterm trend of rising air temperatures over the
land and sea surface.
The top map left depicts global temperature
anomalies in 2013. It does not show absolute
temperatures, but instead shows how much
warmer or cooler the Earth was compared to an
averaged base period from 1951 to 1980.
The global average temperature for 2013 was
14.6° Celsius (58.3° Fahrenheit), which is 0.6°C
(1.1°F) warmer than the mid-20th century baseline. The average global temperature has risen
about 0.8°C (1.4°F) since 1880.
Weather patterns and other natural cycles
cause fluctuations in average temperatures
from year to year. This is especially the case on
regional and local levels. For instance, while the
globe experienced notably warm temperatures
in 2013, the continental United States had its
42nd warmest year.
It has been 38 years since the recording of a
year with cooler than average temperatures.
This above chart plots global surface temperature anomalies from 1880 through 2013. The thin red line shows the annual temperature anomaly, while the thicker red line shows the five-year running average, a number that helps smooth out extreme fluctuations
upward or downward.
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