This winter the Arctic Oscillation made quite an impression in the Northern Hemisphere, displacing the jet stream southwards and blowing cold Arctic air into the US and Europe, which caused an incredibly severe winter that 25 years ago was no big deal. Because all the cold air had been displaced temperatures in the Arctic regions reached new heights, with anomalies as high as 8.4 degrees C being recorded.
So, I wondered, what effect is the Arctic Oscillation having on the sea ice? What phase of the AO favours melting and what phase favours thickening? And how? Does it impact temperature? Winds? Ocean currents? Is the AO responsible for the observed decline in Arctic sea ice extent? A lot of questions that some people who have been following the Arctic closely probably know the answers to, but I'm sure a lot of people do not. And so I've decided to see what I could find and post it here, to spare others the search.
The Arctic Oscillation explained
As always the National Snow and Ice Data Center has a good explanation:
The Arctic Oscillation refers to opposing atmospheric pressure patterns in northern middle and high latitudes.
The oscillation exhibits a "negative phase" with relatively high pressure over the polar region and low pressure at midlatitudes (about 45 degrees North), and a "positive phase" in which the pattern is reversed. In the positive phase, higher pressure at midlatitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska, Scotland and Scandinavia, as well as drier conditions to the western United States and the Mediterranean. In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leaves Greenland and Newfoundland colder than usual. Weather patterns in the negative phase are in general "opposite" to those of the positive phase, as illustrated below.
figure courtesy of J. Wallace, University of Washington
The influence on the Arctic
I found a first piece of information at the NASA Earth Observatory:
During the “positive” phase of the Arctic Oscillation, winds intensify, which increases the size of leads in the ice pack. The thin, young ice that forms in these leads is more likely to melt in the summer. The strong winds also tend to flush ice out of the Arctic through the Fram Strait. During “negative” phases of the oscillation, winds are weaker. Multiyear ice is less likely to be swept out of the Arctic basin and into the warmer waters of the Atlantic.
To complement this information, I found some great National Geographic pictures at the JISAO (University of Washington) website that explain what is happening during the 'warm' phase and the 'cold' phase of the AO (click on the pics for a bigger version):
AO past and present
I found a short historic reconstruction of the AO on the wonderful Jeff Masters' Wunderground blog (the whole page is recommended reading):
When one looks at the wintertime pattern of the Arctic Oscillation (AO)
over the past 100 years, a mostly random pattern of positive and
negative AO modes is apparent (see figure below). However, one anomalous period
is very striking: a string of seven consecutive years with a positive
AO, including two years (1989 and 1990) with the highest AO index ever
observed. During this period, strong westerly winds rapidly flushed more
than 80% of the oldest, thickest sea ice out of the Arctic Ocean,
leaving most of the Arctic covered with ice less than three years old. Younger ice is much thinner, and melts much more readily.
Rigor and Wallace (2004) estimate that at least half of the loss of sea
ice in the Arctic since 1979 is due to these six years of strange
weather with very low surface pressure over the Arctic.
Like I've said at the start of the blog post a very strong negative AO was witnessed this past winter. The effect has been a reduction in the flow of sea ice out of the Arctic by
affecting the winds that can export the ice to warmer waters, where it
melts. It helped retain some the second- and
third-year ice through the winter, and rebuilded some of the
older, multiyear ice that had been lost over the past few years.
Whether the ice has thinned so much that it has become immune to the effects of the Arctic Oscillation remains to be seen:
"The more recent years have shown indications of a recovery in the Arctic Oscillation towards more neutral conditions, but we've still seen decay in sea ice," Serreze says. He wonders if the ice has thinned to a point where it has reached a threshold; a situation where thin ice and warming waters reinforce each other, regardless of pressure patterns like the Arctic Oscillation.
And what about this year? Currently the AO is positive, and as we have just learned, this means weather conditions that are favourable to sea ice melt:
From now on I'll be linking to this graph more often in my SIE updates. I hope people enjoyed reading this information and learned some more about the Arctic Oscillation.