Cold Weather in Denver: Climate Change and Arctic Oscillation (8)

Cold Weather in Denver: Climate Change and Arctic Oscillation (8)

I’ve been living with this cold weather in Colorado this week. If you look around at the Wunderground personal weather station sites, we’ve seen a lot of about -10 F at nights. It’s been causing a lot of grief for homeless people, animals and pipes. There have been a few record lows set. The whole Arctic air mass is starting to move east, which means it will get a lot more press. According to Jeff Master’s blog 80% of the country will be below average.

I thought I had finished my series of blogs on the Arctic Oscillation a couple of weeks ago, but this cold air out break takes me back. It that series I wrote about cold air in the Arctic that is isolated because of barriers caused by streams of rapidly moving air that flows around polar latitudes. I described wobbles in the streams that caused cold air to move south and warm air to move north. Here is one of the figures that I used.



Figure 1: This figure is from the point of view of someone looking down from above at the North Pole (NP). This represents a weak, wavy, wobbly vortex displaced from the pole. The vortex encloses cold air, represented as blue. The line surrounding the cold air is the jet stream or the edge of the vortex. (definition of vortex)

Figure 1 shows an idealized schematic of the North Pole as viewed from above. This is the weak vortex case, when there is a large wobble. In this case, the point X is cold and the point Y is warm. In a case of a stronger, more circular vortex, then the case would be reversed, with point X warm and point Y cold.

Here is a figure from the European Center for Medium-Range Weather Forecasts (ECMWF), that I have marked up a bit. The colors are the temperatures at the 850 hecto-Pascal surface, which is about 1.5 kilometers above the surface. The 850 hecto-Pascal temperatures are a good indicator of where it is hot and cold at the surface.


Figure 2: This figure is from the point of view of someone looking down from above at the North Pole (NP). The contour lines on the figure are the height of the 500 hecto-Pascal surface, which is between 5 and 6 kilometers above the surface of the Earth. The colors are the temperatures at the 850 hecto-Pascal surface, which is about 1.5 kilometers above the surface. The 850 hecto-Pascal temperatures are a good indicator of where it is hot and cold at the surface. Figure from the European Center for Medium-Range Weather Forecasts (ECMWF)

I drew a blue arrow showing that the cold air at the pole has wobbled off of the pole and it is pushed towards Colorado. To the west there is warm air, red arrow, pushing up towards Alaska. So while it has been cold in Colorado, it has been quite warm in much of Alaska. Though a less prominent signal, there has also been warm air moving up the East Coast of the U.S. The Alaska – Colorado contrast is a nice real-world example of what I showed in Figure 1. For completeness with my example, the big, black dashed line is the jet stream of air flowing around the pole.

There were several points in my series on the Arctic Oscillation. The first important point is that even in a world that is getting warmer, the polar latitudes become isolated as the Sun goes down for the winter and jet stream intensifies. In this isolation it gets cold, because there is no heating from the Sun and the polar latitudes have a barrier between themselves and the warmer lower latitudes. The second important point is this wobble, the pushing of air off of the pole in some direction. In this case the coldest air is over Greenland, Canada and the U.S. If there is sufficient wobble to push the air far to the south or if it gets pushed to some place it did not get pushed before, then it is even likely to have record cold. These points are all work together and are not correctly viewed as independent events. (I was recently annoyed by the parenthetical dismissal of global warming in this otherwise nice prediction of early strong lake effect snow in Michigan. The statement was essentially pockets of cold Arctic air should not exist.)

I will finish with the Arctic Oscillation. The Arctic Oscillation Index from the Climate Prediction Center is shown in Figure 3. The discussion in my Arctic Oscillation series focused on the positive and negative phases of the Arctic Oscillation Index. Much of the attention was on the eastern U.S. The negative phase was when it is likely to be very cold in the eastern U.S.



Figure 3: Arctic Oscillation Index for early August 2013 until December 7, 2013 from the Climate Prediction Center

In this measure of the Arctic Oscillation Index, the most recent times have been weakly positive, tending towards negative. (Perhaps suggesting movement of the cold air towards the U.S. east coast?) Perhaps more important Figures 2 and 3 together show that large undulations with warm air pushing far northward and cold air displaced off the pole can occur in other parts of the world when the index is weak. As pointed out many other times over the years of this blog, what goes on in the U.S. is not good instantaneous editorial content for climate change.

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Previous entries:

Climate Change and the Arctic Oscillation 2

Climate Change and the Arctic Oscillation 1

Wobbles in the Barriers

Barriers in the Atmosphere

Behavior

Definitions and Some Background

August Arctic Oscillation presentation

CPC Climate Glossary “The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases.”