I'm a professor at U Michigan and lead a course on climate change problem solving. These articles often come from and contribute to the course.
By: Dr. Ricky Rood , 12:50 AM GMT on October 03, 2013
Barriers in the Atmosphere: Arctic Oscillation (3)
I want to continue with the Arctic Oscillation / North Atlantic Oscillation. First, however, here is the link to my August presentation. Also here is a link to the GLISAclimate.org project workspace where I collected together the materials I used in the presentation - Arctic Oscillation: Climate variability in the Great Lakes.
Here are the previous entries in the series:
Definitions and Some Background
This blog is mostly a setup for the next one. (And yes I did notice that the IPCC AR-5 report was released, but I don’t have anything different to say about it than many of my more able colleagues. I’ll get to it.)
In the talk that I linked to above, I used a couple of diagrams that the audience told me worked very well. I am going to try them out in this blog. In the previous blogs I used the CPC Climate Glossary to give the definition of the Arctic Oscillation. “The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases.” This definition does not really do much for me. It’s one of those definitions that I imagine if I ask 10 atmospheric scientists to tell me what it means, I will get 12 answers. Therefore, I will draw a picture.
Figure 1: Adapted from Jim Hurrell – This picture is a schematic representation of the positive and negative phases of the Arctic Oscillation. In the positive phase the pressure is low at the pole and high at middle latitudes. This is the positive phase because if you calculate the difference between middle and high latitudes it is large. In the negative phase the pressure is not as low at the pole and not as high at middle latitudes. This is the negative phase because if you calculate the difference between middle and high latitudes it is small. The refrigerator suggests that this is like opening and closing the refrigerator door (see Behavior).
This figure helps me with the definition. I want to focus on the low pressure at high latitudes, which in this figure is drawn idealistically at the pole. In reality, it is likely to wander off the pole, a fact that will be important in the next blog. When the pressure is low at the pole, then there is a stronger vortex of air circulating around the pole. When the pressure at the pole is not as low, then there is a weaker vortex. In both cases, strong or weak vortex, the air generally moves from west to east.
For clarity, vorticity is a parameter that describes rotation in a fluid. A vortex is a feature in a fluid dominated by vorticity – that is it is rotationally dominated. Tornadoes and hurricanes are weather features that we often call vortices; there is an obvious circulation of air in these features. In the Earth’s atmosphere at middle and high latitudes rotation is an important characteristic of the flow, due to the rotation of the Earth. The reason air moves in the west to east direction for both the weak and strong vortex cases of Figure 1 is that the rotation of the Earth is important to the flow.
In Figure 2 I have set up an even more idealized figure. I also provide this link to a Powerpoint animation, that I am not smart enough to incorporate into the blog. In the animation I have five slides that clarify the point that I make in Figure 2.
Figure 2: A vortex and a ball. In the center of the figure is low pressure, meant to be an analogue to the vortex over the pole in Figure 1. Parcels of air move around the low pressure system. If it takes the same amount of time for a parcel farther away from the low pressure center to go around the vortex as a parcel nearer the center, then the parcel farther away has to go faster because the distance it has to go is longer. That is why I drew that arrow, saying that air moves “faster” at the outside edge of the vortex.
To set my point a little more, imagine you are on a bridge overlooking a running stream. If you drop a stick in the water near the edge where the water is moving slowly, then if the stick drifts towards the more rapidly flowing water, it is carried downstream at the edge of the fast moving water. It does not cross the core of fast moving water – this jet of water. In fact the jet is something of a barrier that keeps material from crossing the stream. Material is transported downstream.
Back to Figure 2: Imagine that you want to roll a ball into the center of a vortex. As the ball gets to the edge it gets caught up in the flow and pulled around the edge. It does not roll into the center. Look at the this link to a Powerpoint animation to get a better idea of what’s going on.
Now go back to Figure 1. The vortex in Figure 1 is also a barrier. The southern edge of vortex is a jet stream. Air on the two sides of the vortex often has different characteristics. Intuitively, there is colder air on the poleward side. If you look at trace gases, like ozone, they are different across the edge of the vortex. The takeaway idea is that the edge of the vortex is a barrier. It’s not a perfect barrier, but air on one side is largely separated from the air on the other side. In the next blog, I will describe the difference between the strong and the weak case and its relevance to weather, climate and, perhaps, climate change.
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