Climate Change and Arctic Oscillation (6)

Climate Change and Arctic Oscillation (6)

Once again the day job has interfered with the blogging life. Even I need to remember where I was.

This is a continuation of my compelling series on the Arctic Oscillation / North Atlantic Oscillation. Links to background material and previous entries are at the end.

In the third entry and fourth entry in the series, I introduced a simple description of atmospheric flows to help understand fluctuations of warm and cold air. I used the example of a fast-flowing current of water in the middle of a stream. If you drop leaves into both sides of the stream, then they do not cross the stream. Rather they are carried down the stream on the edges of the fast-flowing current. In the atmosphere, there are jets of fast-flowing air. If it is cold on the poleward side of the jet and warm on the equatorward side, then if the jet wobbles back and forth there are jumps between warm spells and cold snaps. To help understand the Arctic Oscillation think of it as the variation of an atmospheric jet stream at southern edge of vortex of air that circulates around the North Pole (see previous entry).

When we have discussions about climate change often we fall into something of a trap – how do we decide if what we are observing and experiencing is natural variability or man-made climate change? We all know that there have always been warm spells and cold snaps – droughts and floods. When I teach climate change, I introduce the Arctic Oscillation in the lecture on natural variability. Calling it natural variability is one of those places where language sets us up for argument, because it implies, perhaps, that there is unnatural variability. So it is perhaps better to call it “internal” variability, but that takes it a little more in the direction of arcane science speak. The idea is that there are recurring patterns of weather, hot and cold, wet and dry. Because a principle of science is that we can measure those things that we observe, we end up with definitions such as the Arctic Oscillation to describe atmospheric patterns.



Figure 1: 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


The top plot in Figure 1 shows the Arctic Oscillation Index. The "high index" of the Arctic Oscillation is defined as periods of below normal Arctic sea level pressure. The "low index" of the Arctic Oscillation is defined as periods of above normal Arctic sea level pressure. 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. The plot goes back to 1900, and establishes that the Arctic Oscillation has been occurring for a century. It is variability related to the rotation of the Earth and the heating and cooling that set up a circulating vortex around the pole. (Barriers in the Atmosphere and Wobbles in the Barriers)

The patterns of variability associated with the Arctic Oscillation occur at different levels of carbon dioxide. This is established in Figure 1, where there has been an increase of about 30% in carbon dioxide over the time span shown in the graph. Since the underlying pressure patterns are in someway related to heating and cooling in the atmosphere, it is reasonable to propose that the behavior of the Arctic Oscillation might change as carbon dioxide increases. Why? Because carbon dioxide changes the pattern of the Earth’s heating and cooling. Therefore, the Arctic Oscillation is a pattern of variability that occurs “naturally,” and its behavior might change as the climate changes. It’s not like there is one “natural climate” with the Arctic Oscillation and a separate “man-made warming climate” with the Arctic Oscillation behaving like it did in the “natural climate.”

Figure 2 in 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, say, this year’s Arctic Oscillation Index is correlated with last year’s. A number close to zero is a measure of being unrelated. If it were one, then it would be perfectly related. 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, you can convince yourself that the Arctic Oscillation stays stuck in one mode or another for several years. If you go back up to Figure 1, you might also conjecture that the relationship with temperature is a little stronger after 1960 as well, but it’s not an easy relationship.



Figure 2: 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 2 shows a model simulation with the NCAR Community Atmosphere Model. One of the strengths of using models to investigate the climate is the ability to prescribe exactly the amount of carbon dioxide in the atmosphere. 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. Hmm. Perhaps something has changed? The simulation does suggest that in the nature that the model represents, the Arctic Oscillation does naturally occur.

The next entry, and I predict the last of the series, will discuss the Arctic Oscillation and climate change more deeply.

r

Previous entries:

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.”