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 , 4:49 AM GMT on September 10, 2013
Behavioral Science: Arctic Oscillation (2)
The day job continues to overwhelm – so it’s been a while. 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.
What is the Arctic Oscillation? Here from the CPC Climate Glossary is the start of 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.” I think the definition is a little easier to explain if I focus on the North Atlantic Oscillation and, again from the glossary, “The North Atlantic Oscillation is often considered to be a regional manifestation of the Arctic Oscillation.” In the negative phase of the North Atlantic Oscillation there is higher than average pressure over the pole and lower than average pressure over the North Atlantic, for example, over Iceland. In the positive phase of the North Atlantic Oscillation there is lower than average pressure over the pole and higher than average pressure over the North Atlantic.
To give some measure of the Arctic Oscillation meteorologists developed an index. This is a common method to give some sense of behavior. In the Arctic Oscillation the index is the difference in pressure between some middle and some higher latitude regions or weather station. Historically, indices were often developed prior to our modern observing system, and sailors, explorers and scientists noticed that when the weather was, say, wet in one region it was reliably dry in another. As we get more observations and develop a more complete understanding of processes responsible for weather patterns, new definitions of indices are proposed. This paper by Kuzmina et al. 2005 includes an analysis of how some climate model results are related to some different definitions of the North Atlantic Oscillation.
The first figure shows North America temperature differences for a set of winters in the late 1970s and a set of recent winters. What is done here is to take the average temperature for a particular winter, and from that subtract a 30-year average from 1961-1990. Thirty-year averages are used to define “climate” by the standard definition of climate as average weather. Blues mean it is colder than average and reds mean it is warmer than average. Look first at 1978 and 1979. These were very cold winters, and 1979 was the coldest observed North American winter. The spatial extent of the cold included much of the U.S. and Canada. Indeed, in the 1970s some scientists and the press started talking about the coming ice age. I was a student then, and I remember both it being very cold in Tallahassee, Florida and seeing talks about cycles of climate. In class here in 2013, I ask students to think about how today’s claims of warming are different from this episode in the 1970s. 2006 and 2007 are notably warm. 2010 and 2011 received a lot of press for being cold, but compared to the 1970s they don’t seem so extreme.
Figure 1: North America temperature differences for a set of winters in the late 1970s and a set of recent winters. What is done here is to take the average temperature for a particular winter, and from that subtract a 30-year average from 1961-1990. Thanks to Jim Hurrell
Figure 2: The top plot in the figure shows the Arctic Oscillation Index. The bottom plot shows the December, January and February average temperature for North America. Thanks to Jim Hurrell
In Figure 1 at the very highest latitudes plotted, it is always warmer than the thirty-year average. This statement is least convincing in 2008. Otherwise in the recent record, the temperatures in northern Canada and Greenland are flirting with being 10 degrees F above average.
The maps with the cold temperatures over the U.S. are the times of the negative phase of the Arctic Oscillation and the times of the warmer temperatures are the positive phase. This is shown better in Figure 2. The top plot in the figure shows the Arctic Oscillation Index. The bottom plot shows the December, January and February average temperature for North America. In the years since 1970 there is a very strong correlation between the phase of the Arctic Oscillation and whether or not it is colder than average. There is not an easy relationship between the strength of the Arctic Oscillation Index and how much colder. This will be discussed more fully in a future entry.
In the earlier part of the record, prior to 1960, the relation between warm and cold and the Arctic Oscillation is not as simple as it has been in the more recent years. Not only that, but the magnitude of the Arctic Oscillation index does not reach either the positive or negative extremes recently observed. Further examination, for example in Jim Hurrell’s classic 1995 paper, shows that in about 1960 there is a change in the statistical behavior of North Atlantic / Arctic Oscillation.
There are many interesting features in these two figures. From a climate perspective, February 1985 was the last month when we observed global-average temperatures that were below the twentieth century average. When we arrive at February 2015 we will have had thirty years above average – an entire definition of “climate.” The warming has been especially large at the highest northern hemispheric latitudes. This means that there is a distinct trend in the North American averages temperature of the past twenty years. The warm parts at higher latitude are much warmer and the cool parts at middle latitudes are also warmer. If we stay on this trend, soon the average might not be below the 1961-1990 average used in Figure 2. There are also many other interesting climate and climate change features that will be discussed in the next, not too far off, blog.
(I will look for new likes on old blogs!)
Confounding Variability: A short blog from the early times.
Bumps and Wiggles (8)Ocean, Atmosphere, Ice, and Land
La Nina and Missouri River Flooding
Jeff Masters Extreme Arctic Oscillation
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