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 , 5:14 AM GMT on November 19, 2013
Change in the Weather: Climate Change and Arctic Oscillation (7)
This is the end-for-a-while of my series on the Arctic Oscillation / North Atlantic Oscillation. Links to background material and previous entries are below.
At the end of the previous blog I showed the following figure. The top panel shows the observed Arctic Oscillation Index from 1864 to 1960. The middle panel shows the observed Arctic Oscillation Index from 1864 to about 2000. The little number “r” in the panel is a measure of how well one year’s Arctic Oscillation Index is linked to or correlated with the previous year’s. A number close to zero is a measure of being unrelated. Prior to 1960, the observations were almost unrelated from year to year (r=-0.03). After 1960 there is a much stronger relation (r=0.4). Just looking at the graph after 1960, you can convince yourself that the Arctic Oscillation stays stuck in one mode or another for several years.
Figure 1: The top two plots in the figure show the observed Arctic Oscillation Index. The bottom plot shows a model simulation of the Arctic Oscillation Index. See text for more description. Thanks to Jim Hurrell
The bottom panel of Figure 1 shows a model simulation with the NCAR Community Atmosphere Model. In this model simulation the model’s carbon dioxide is held constant at levels prior to the industrial revolution, when man-made carbon dioxide was quite small. This simulation does not represent any particular year; it is 200 years which when taken together might look, statistically, like the atmosphere. An interesting feature of this simulation is that the Arctic Oscillation does look like the observations before 1960, but not after 1960. One possible suggestion of the reason why the model loses its ability represent the behavior of the Arctic oscillation is that carbon dioxide has increased enough to change the Arctic Oscillation.
I will come back to this below, but first a reminder of the other ideas I introduced in the middle part of the series. Most importantly, there is a stream of air that wants to flow around the North Pole. Likely in a world that has no mountains, no land and water sitting next to each other, then that air would actually circulate with the pole in the center. We live in a world with mountains and oceans and continents, which distort this stream of air. It’s a little like boulders in a creek, and water going around the boulders. The stream becomes wavy. There are other factors that also cause the air to be wavy, but I have introduced enough to make my points, and you can go back to the earlier blogs linked at the bottom for words and pictures. What causes the air to spin around the North Pole? The first thing to consider is the rotation of the Earth. The Earth’s atmosphere wants to line up with the rotation. Another important factor in determining the details of the air circulating around the North Pole is heating and cooling. The patterns of heating and cooling contribute to setting up high-pressure and low-pressure systems. Air flows from high to low pressure and as it flows towards low pressure it does its best to line up with the rotation of the Earth. This relation between high and low pressure and the Earth’s rotation is one of the most important features of the motion of the air in the atmosphere and the water in the ocean.
The way carbon dioxide changes the Earth’s climate is by changing the heating and cooling. A common comparison is to compare additional carbon dioxide to a a blanket which holds the Sun’s heat closer to the Earth’s surface. This blanket causes the Earth to heat up more at the pole than at the Equator. The poles are also special because the Sun goes down for the winter and it cools off. In fact, it gets very cold, and as discussed in the previous blogs, the stream of air that gets spun up isolates the pole enough to let the cooling really get going. With these changes to heating and cooling, if we add a lot of carbon dioxide to the atmosphere, then it is reasonable to expect that the Arctic Oscillation might change.
The studies prior to, say, 2008, suggested that the effect of carbon dioxide being added to the atmosphere would be to cause the Arctic Oscillation Index to become more positive. This would be the pattern of the Arctic Oscillation where the cold air is confined to the pole; that is, the less wavy pattern (scientific references: for example, Kuzmina et al. 2005 and the 2007 IPCC AR-4). The studies prior to 2008 support the idea that the additional carbon dioxide is a leading suspect in the changes after 1960 noted in Figure 1. That is, without carbon dioxide increasing in the simulation, the models cannot reproduce the statistical characteristics of the observations and with it increasing, they can.
Those pre-2008 studies, effectively, only considered increasing carbon dioxide. They did not represent the huge changes in the surface of the Arctic that have been observed. Notably, sea ice and snow cover have declined. These surface changes also cause changes in heating and cooling. The decline of sea-ice, for example, changes the surface of the Arctic Ocean from white to dark. This changes the surface from a reflector of energy to an absorber of energy. Sea ice is also a temperature insulator; hence, without the ice the ocean and atmosphere exchange heat more easily. There are many other changes as well, but all I want to do here is establish the plausibility that large changes at the surface are also likely to change the behavior of the Arctic Oscillation. Why? Changes in the patterns of heating and cooling, leading to changes in high and low pressure systems, which then with the influence of the Earth’s rotation, change the waviness of the stream of air around the Arctic.
There have been a series of papers in the past couple of years that suggest that the changes in sea ice and snow cover are having large effects on the weather in the U.S. If you look across these papers, then there is growing evidence that the meanders (or waviness) of the Arctic Oscillation are getting larger and that storms over the U.S. are moving more slowly. Here is a list of quotes from these papers.
From a paper I have previously discussed:
Francis and Vavrus (2012): Evidence linking Arctic amplification to extreme weather in mid-latitudes - “Slower progression of upper-level waves would cause associated weather patterns in mid-latitudes to be more persistent, which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves.”
Liu et al. (2012): Impact of declining Arctic sea ice on winter snowfall – “ … some resemblance to the negative phase of the winter Arctic oscillation. However, the atmospheric circulation change linked to the reduction of sea ice shows much broader meridional meanders in midlatitudes and clearly different interannual variability than the classical Arctic oscillation.”
Greene et al. (2012): Superstorm Sandy: A series of unfortunate events? - “However, there is increasing evidence that the loss of summertime Arctic sea ice due to greenhouse warming stacks the deck in favor of (1) larger amplitude meanders in the jet stream, (2) more frequent invasions of Arctic air masses into the middle latitudes, and (3) more frequent blocking events of the kind that steered Sandy to the west.”
There is some controversy about the work connecting the changes in the sea ice and snow cover to changes in the Arctic Oscillation and to changes in extreme weather in the U.S. (Barnes (2013): Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes, Francis response, and Freedman @ Climate Central ).
I think there is significant merit in the work that connects changes in the Arctic Oscillation to increases in carbon dioxide and related changes to the surface of the Earth. Part of my intuition comes from a career of working with atmosphere models. If a model is radiatively dominated, then the vortex over the pole is very strong. In this case, there is little waviness in the jet stream. This is analogous to the case of increasing carbon dioxide and the Arctic Oscillation becoming more common in its positive phase. If a model is less driven by radiative forcing, then it is easier for the waves that are initiated by the flow over the mountains to grow and distort the edge of the jet stream – more waviness. This is like the negative phase of the Arctic Oscillation. Though in the end it will require a careful calculation of the energy budget, the removal of sea ice from the surface of the Arctic Ocean allows more heat into the polar atmosphere, which means the radiative cooling will be less intense. Hence, the vortex will be weaker or the Arctic Oscillation will more commonly be in its negative phase. If there are changes in the Arctic Oscillation, which are realized as changes in the waviness and speed of the jet stream around the Arctic, then there will certainly be consequences to the weather in the U.S.
Potential changes in the character of the Arctic Oscillation are an important issue for those thinking about how to respond to climate change. The loss of sea ice is a large change, which will undoubtedly have important impacts in the Arctic. It is reasonable to expect large impacts on weather at lower latitudes, in the U.S., Europe and Asia. The change in the Arctic sea ice has happened very rapidly. This challenges the assumption often made in planning that climate change is a slow, incremental process. The weather of the here and now and/or the next fifty years, a common length of time for planning, is likely to be quite different from the past fifty years. Since we rely on our past experience to plan for the future, this is a direct challenge to our innate planning strategies. If we are cognizant of the possibility of significant changes to weather patterns on decadal lengths of time, then we can develop new planning strategies that will improve our resilience and make our adaptation decisions more effective.
Climate Change and the Arctic Oscillation
Wobbles in the Barriers
Barriers in the Atmosphere
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.”
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