Tuesday, October 2, 2012

The Effects of Losing Arctic Sea Ice

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.

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.

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.

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.