The Arctic Oscillation is a climate pattern that influences winter weather in the Northern Hemisphere. It is defined by the pressure difference between air at mid-latitudes (around 45 degrees North, about the latitude of Montreal, Canada or Bordeaux, France) and air over the Arctic. A low-pressure air mass usually dominates the Arctic, and while higher pressure air sits over the mid-latitudes. This pressure difference generates winds that confine extremely cold air to the Arctic. Sometimes, the pressure systems weaken, decreasing the pressure difference between the Arctic and midlatitudes and allowing chilly Arctic air to slide south while warmer air creeps north. A weaker-than-normal Arctic Oscillation is said to be negative. When the pressure systems are strong, the Arctic Oscillation is positive.
Meteorologists and climatologists who study the Arctic pay attention to the Arctic Oscillation, because its phase has an important effect on weather in northern locations. The positive phase of the Arctic Oscillation brings ocean storms farther north, making the weather wetter in Alaska, Scotland, and Scandinavia and drier in the western United States and the Mediterranean. The positive phase also keeps weather warmer than normal in the eastern United States, but makes Greenland colder than normal.
In the negative phase of the Arctic Oscillation the patterns are reversed. A strongly negative phase of the Arctic Oscillation brings warm weather to high latitudes, and cold, stormy weather to the more temperate regions where people live. Over most of the past century, the Arctic Oscillation alternated between its positive and negative phase. For a period during the 1970s to mid-1990s, the Arctic Oscillation tended to stay in its positive phase. However, since then it has again alternated between positive and negative, with a record negative phase in the winter of 2009-2010.
The Arctic Oscillation and Wisconsin
The Arctic Oscillation (AO) index describes the relative intensity of a semipermanent low-pressure center over the North Pole. A band of upper-level winds circulates around this center, forming a vortex. When the AO index is positive and the vortex intense, the winds tighten around the North Pole, locking cold air in place. This can result in a milder winter for Wisconsin. The weaker polar vortex of the negative phase allows cold air to plunge into Wisconsin and other parts of the midwestern United States.
In early 2013 a cold snap that extended into March was attributed to a negative phase of the AO. Of course, last winter a weak Polar Vortex added some cold to our weather. For 2014-2015, it appears that the AO is trending toward a positive phase. The National Weather Service used this factor, among others, in predicting a warmer and drier season.
The Arctic Oscillation in the Northern Hemisphere
As noted above, the AO will act with other major weather systems such as the Polar Vortex and El Nino/La Nina. Here is the influence of the AO as an isolated phenomenon:
|Region||AO Positive Phase||AO Negative Phase|
|Eastern U.S.||Above average temperatures||Colder winters and an increase in nor’easters (coastal storms) for New England states|
|Western U.S.||Warm, dry conditions||Cooler weather|
|Canada & Greenland||Below average temperatures||Warmer in Western Greenland and Eastern Canada|
|Northern Europe & Asia||Warmer and more precipitation, most notably in Scotland and Scandinavia||More frigid temperatures|
|The Mediterranean||Drought conditions||Increased frequency of storms|
The Arctic Oscillation is a result of natural variations within Earth’s atmosphere. Unlike other climate patterns which occur every so many years, the AO is always present and switches between its phases seemingly at random (science hasn’t yet discovered a pattern and there may not be one). For this reason, its episodes remain difficult to predict more than a few weeks in advance.