The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East.
Application for the Great Lakes Region:
The AO is a description of wintertime atmospheric conditions over the poles and mid-latitudes (including Great Lakes region).
Negative Phase (Low pressure over Great Lakes) = wet and cold conditions
Positive Phase (High pressure over Great Lakes) = dry and warm (relative to winter temperatures) conditions