Weather 101: Arctic Oscillation (Positive Phase)
The Arctic Oscillation plays an important role in winter weather in the Northern Hemisphere.
It has two "phases": negative and positive. We will discuss the positive phase. In reality, negative phase is opposite in setup and effects.
In the positive phase, higher pressure at mid latitudes drives ocean storms farther north. "Middle latitudes" refers to the general region around the world that includes much of the United States. See "Middle" in this figure.
In the positive phase of the Arctic Oscillation, relatively low pressure resides around the north pole, as shown in the following image. Notice the blue jet stream (arrows). It is held tightly in place by higher atmospheric pressure over the middle latitudes. Think of the higher pressure acting as a brick wall, not allowing the jet stream to dip southward.
Because the Jet Stream is held relatively far north during the winter months, the Arctic air is bottled up from Canada northward. This means that most of the continental United States escapes Arctic blasts of air. Thus, during the positive phase years, the United States has relatively warm winters.
In addition, since the jet stream (blue arrows in the second figure from the top) is held north of the continental United States, and since we can think of the jet stream as the "steering wheel for storms," most storms stay well north. This means a dearth of snow for the Midwest and for ski resorts in the Rockies.
At the same time the jet stream is flowing right through Alaska. During positive phase years, Alaska is usually buried in snow.
Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s, however, the oscillation has tended to stay in the positive phase, causing lower than normal arctic air pressure and higher than normal temperatures in much of the United States.