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Arctic Oscillation (AO)
This is an index of pressure patterns over the Arctic Ocean, usually centered near the north pole. During the warm phase, the upper level pressure is lower than the surrounding region, and thus tend to cut off cold air from crossing the pole. This in turn can reduce the amount of cold air across northern Canada and can have an impact on reducing the heights over Greenland and thus negating the developing of the cold phase of the NAO, ie less high latitude blocking. The cold phase on the other hand has the opposite effect, the pressure pattern is higher than across surrounding areas which can aid in what's called cross-polar flow. This is when bitterly cold arctic air over Siberia can cross the pole and flood into North America. Also the cold phase of the AO tend to help raise heights over Greenland as well thus aiding in the development of the cold phase of the NAO and high latitude blocking.
The above image is courtesy
University of Washington
http://tao.atmos.washington.edu/analyses0500/ao_definition.html