Arctic Oscillation (AO)

The Arctic Oscillation (AO) is a climate index of the state of the atmospheric circulation over the Arctic . It consists of a negative phase, featuring below average geopotential heights, which are also referred to as negative geopotential height anomalies, and a positive phase in which the opposite is true.  In the negative phase, the polar low pressure system (also known as the polar vortex) over the Arctic     is weaker, which results in weaker upper level winds (the westerlies). The result of the weaker westerlies is that cold, Arctic air is able to push farther south into the U.S.,  while the storm track also remains farther south. The opposite is true when the AO is positive: the polar circulation is stronger which forces cold air and storms to remain farther north. The Arctic Oscillation often shares phase with the North Atlantic Oscillation (NAO) (discussed below), and its phases directly correlate with the phases of  the NAO concerning implications on weather  across the U.S.

http://www.nc-climate.ncsu.edu/climate/patterns/NAO.html