Definitions and Some Background: Arctic Oscillation (1)

Definitions and Some Background: Arctic Oscillation (1)

Every now and then I take an unexpected blogging hiatus because the day job is overwhelming. That’s the last three weeks as the project that I have been working on the past couple of years came to its first major milestone – a workshop on the evaluation of model projections to improve their usability in planning. Plus it is canning season – any good chutney recipes?

During the run up to the workshop, thanks to my expertise in time management, I gave a seminar on the Arctic Oscillation for a National Park Service webinar series “Climate Change in America's National Parks - Post-Sandy Recovery Series I: Storms, Barrier Islands, and Implications for the Atlantic Coastline.” I’m going to spend a few entries going through some the ideas in the presentation. First, however, here is the link to my presentation. It was recorded, but I have not figured out how to post that yet. 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.

The reason I was asked to give this talk followed from my participation in a planning exercise for Isle Royale National Park. During that planning project the Arctic Oscillation emerged as a topic of special interest. I have written a number of blogs in the past that discussed the Arctic Oscillation, regionally often referred to as the North Atlantic Oscillation, and its role in variability of winter and spring temperatures. We hear about the Arctic Oscillation the most when winters in the eastern half of the United States are cold and snowy. People get excited and start writing that climate change is bogus. I have put just a few of the links to previous blogs at the end.

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. Going back to the original focus, the Arctic Oscillation, rather than the pressure differences at sub-polar latitudes being over the North Atlantic, they might be over some other place, like the North Pacific. Here is a schematic figure showing the North Atlantic Oscillation from educational material at Lamont-Doherty.



Figure 1: Positive Phase of the North Atlantic Oscillation. from LDEO



Figure 2: Negative Phase of the North Atlantic Oscillation. from LDEO


These changes in the weather pattern have large consequences on the weather in the U.S. When the North Atlantic Oscillation is in its positive phase, the winters in the Mid-Atlantic and Southeastern U.S. are moist and mild. When the North Atlantic Oscillation is in the negative phase, the winter in the same regions of the U.S. are cold and snowy. Though snowy, the actual amount of water that falls from the sky is less than average.

The discussion of the Arctic Oscillation often focuses on the winter and spring because in the U.S. the discussion of weather and climate often over emphasizes what is happening in the Interstate 95 corridor. (Isn’t it great that I-95 has its own website?). However, the Arctic Oscillation is the dominant mode of variability in the Northern Hemisphere middle latitudes, and this is true all of the year. When we say that something is the “dominant mode,” we mean that if we formally measure the variance and then try to describe the variance by recognizable patterns, then the single largest way to describe the variance is with the Arctic Oscillation.

Meteorologists describe the Arctic Oscillation as an atmospheric phenomenon as opposed to a phenomenon that might represent the coupling between the atmosphere and the ocean. The El Nino – La Nina oscillation involves both the atmosphere and ocean. Since the ocean is important, El Nino and La Nina are at least a little bit predictable. The Arctic Oscillation is notoriously difficult to predict.

The reason the Arctic Oscillation took on as much importance as it did in the Isle Royale National Park project was its impact on ecosystems. In the area around Lake Superior, when the Arctic Oscillation is in the positive phase it tends to be warm and dry. There is very little snow. When the Arctic Oscillation is in the negative phase, there are cold air outbreaks from Canada and the likelihood of large snowstorms is higher. If the atmosphere bounces back and forth between the positive and negative phase, then you can imagine a snowstorm followed by a thaw. This stands to change the ebb and flow of the annual water cycle with winter thaws and perhaps winter floods. There might be a lot of snow in the winter, but there is less snow on the ground going into spring. An example of an ecosystem impact is in the forest – if it is warmer and dryer in the spring at peak growth time, this is a major stress on the forest. Next blog a little more on the Arctic Oscillation and temperature.



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(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