The Arctic Oscillation is an atmospheric phenomenon that can govern weather and climate patterns across mid- and high-latitude areas not only in North America but across Europe and Asia in the Northern Hemisphere, and it is especially pronounced during winter. The AO is a measurement of the air pressure closer to the surface and can be in sync or couple with the polar vortex, which is measured higher up in the atmosphere. When the air pressure is low over the pole and higher over the middle latitudes, a strong counter-clockwise wind flow occurs around the pole. This causes a milder westerly flow of air across the United States and southern Canada, which will cut off and isolate the Arctic air to the north."When you have a strong polar vortex that remains in the polar region, it tends to keep frigid air pent up so that it is difficult for long-lasting outbreaks of frigid conditions to reach the middle latitudes, including portions of the Midwest and Northeast, " said AccuWeather lead long-range meteorologist Paul Pastelok.