Terry’s Take: The Arctic Oscillation and its role in winter weather
The Arctic Oscillation (AO) is known to be a reliable predictor of climate conditions, especially temperatures during the winter. Essentially, the AO has 3 phases which are defined by winds circulating counterclockwise around the Arctic at around 55°N latitude. There is a negative, positive, and neutral phase.
When the AO is in its positive phase, a ring of strong winds circulating around the North Pole acts to confine colder air across polar regions. This belt of winds becomes weaker and more distorted in the negative phase of the AO, which allows an easier southward penetration of colder, arctic air masses and increased storminess into the mid-latitude, including the central and eastern U.S.
I’ve inserted a graphic which shows winter temperature anomalies for the 3 phases. Notice the blues associated with the negative phase representing cold temps over the central and eastern United States. The major cold outbreaks in this part of the country are tied to the negative AO. The North Atlantic Oscillation (NAO), is a close cousin and can work in unison to facilitate cold and winter storms when it too is in a negative phase.
On the flip side, the positive phase results in a warmer and less snowy pattern for the central U.S. and much of the area west of the Appalachians. That means, if you like your winters cold and snowy, you do not want the positive phase of the AO and NAO.
Included is a graphic showing a comparison of some of the winters in the late 70s to more recent ones. Also included is a graphic showing showing the winter phases of the AO to winter temperatures. You can clearly see the stronger the negative phase the colder the winters. Many of you recall the legendary winter of 78/79, very cold and snowy. The AO that year was extremely negative for a prolonged period of time. After that, many of the winters in the 80s, 90s, and mid 2000s were heavily skewed towards a positive AO. Not much cold and very little snow. The index trended negative again in 2008-2011.
As winter approaches, these phases will become especially important and telling as to what type of winter we experience. You will hear plenty about the phases of the AO, NAO, PNA, and a few more signals in the coming months.