A
dramatic loss of Arctic sea ice occurred this year resulting in a
record-shattering minimum in mid-September.
The implications were touched upon in a previous post. However, it is recognized that the loss of
Arctic sea ice is changing the ecology, geology, and meteorology of the region.
I
will focus on the meteorology in this post.
The Arctic is warming at a rate that is twice that of the mid-latitudes in
a phenomenon known as arctic amplification.
This faster warming is reducing the temperature gradient between the
high and mid-latitudes which in turn is changing the dynamics of the jet
stream.
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| Image Credit: NOAA. |
The
jet stream is a river of high velocity winds in the atmosphere. It meanders generally from west to east
around the globe. The speed of the wind
in the jet stream is usually stronger in the winter than in the summer, because
the north-south temperature gradient is much stronger in winter.
A
cross-section of the atmosphere shows how the lowest layer of the atmosphere,
the troposphere, changes from the equator to the pole. There are three circulation cells in the
northern hemisphere and the jet streams are at the upper boundaries of the
cells. The jet stream helps to drive
storm systems across the country so that knowledge of its future helps forecast
future weather.
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| A cross-section of the troposphere from the Equator to the North Pole. The jet streams are at the intersection of the upper boundaries of the different circulation cells. Image Credit: NOAA. |
Normally
the polar jet rides across the northern part of the U.S. while the subtropical
jet rides across the southern U.S. or farther south. However, the change in the Arctic environment
is changing this dynamic. As the sea ice
recedes the temperature gradient is reduced.
This weakens the jet stream and leads to a buckling of the jet
stream. A more north-south movement of
the jet stream slows down weather systems and can cause them to become
stationary in what is referred to as a blocking pattern.
 |
| Normal positions for the jet streams. Image Credit: NASA. |
The
flow becoming meridional (north-south) rather than zonal (east-west) results in
a very slow weather pattern. This can
lead to extreme weather such as flooding, droughts, cold waves, and heat
waves. NASA has illustrated this in the
video below.
This
resulting change in the weather pattern is a result of the changing climate and
has great implications for our weather. A
dramatic example of this has played out over the U.S. over the past 5
years. Jennifer Francis and Stephen Vavrus
published a paper in the journal Geophysical Research Letters in February
entitled “Evidence linking Arctic amplification to extreme weather in mid-latitudes.” Stu Ostro first brought this idea of blocking
patterns to my attention some time ago; however the paper by Dr. Francis takes
this a step farther.
It
is not just changing mid-latitude weather.
The retreat of the sea ice is also creating a thermal boundary between
the ice and open ocean. Storms are
taking advantage of the difference and this is resulting in stronger storm
systems in the Arctic. This is breaking
up the ice faster and leading to far more coastal erosion. Little thought has been directed as to how
storm systems will behave once the ice vanishes in the summer. We may soon see in reality.
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| This mosaic of NASA/MODIS satellite images from August 5 shows a massive low-pressure center spinning across the central Arctic Ocean on August 5, 2012. Canada and Alaska are located to the left, with Europe and Russia to the right. Click on the image for a larger view. (Image courtesy NASA Earth Observatory.) |
Predictions
of the ultimate collapse of Arctic sea ice vary, but it is only a question of
when. However, the rapid decline has
complicated seasonal forecasts.
Forecasting for the winter is not just about looking at various climate
cycles like ENSO. The situation in the
Arctic must be considered as well and it cannot be forecast beyond two weeks at
this point. Thus forecasts for the upcoming
winter will have to carry the caveat of what might happen in high-latitudes.
