Some Geophysical Background Notes Related to Climate and Weather - The Changing Arctic Environment: The Arctic Messenger - page 378

The Arctic Oscillation (AO):
This is closely related to the NAO and describes two contrasting atmospheric circulation
patterns over the Arctic. When the AO index is positive (AO + ), surface pressure is low in
the polar region. A strong jet stream supports a strong polar vortex and cold Arctic air is
trapped in the polar region. When the AO index is negative (AO − ), atmospheric pressure is
higher over the polar region and the west to east winds and the polar vortex are weakened.
This allows cold Arctic air to break out in winter into North America and Europe.
A word of caution: It is important to remember, however, that AO/NAO are just in-
dices. The AO is just a measure of the strength of the polar vortex, so it does not really make
sense to say it causes the strength of the vortex to change. Similarly, the NAO is an index of
the latitudinal position of the storm track in the North Atlantic.