|Arctic Oscillation Positive/Negative phases.|
Positive Phase on the left, Negative Phase on the right.
When the Arctic Oscillation is in a positive phase, a vortex in the upper regions of the Northern Hemisphere is strong and locks in cold air to keep it up north. This keeps the cold air up north as stated, and does not let the US get colder air.
When the Arctic Oscillation is in a negative phase, the vortex weakens, and the cold air locked in releases itself to more southerly regions of the North Hemisphere, typically affecting North America.
The AO can be a key player in winters when determining cold weather, and is an index that should be watched closely.