The Arctic Oscillation (AO) has fallen off the cliff in the last week, as incredible high latitude blocking ridges have developed both north of Alaska and over Greenland. The AO fell below -5.0 to -5.632 on March 20th, 2013 — making it by far the lowest reading after March 15th in the history of the AO’s recorded values. It also remains, historically, the lowest AO after March 6th.
The Arctic Oscillation, or AO, refers to the generally opposing pattern between pressure in the northern-middle latitudes and the Arctic. It is a climate pattern, that is characterized by counterclockwise winds which circulate around the Arctic near 55°N. The phase and degree of amplitude of the Arctic Oscillation can be a very useful forecasting tool, especially in the winter season. During a positive phase AO, a belt of strong winds circulating around the pole often acts to keep cold air confined to our north. But a negative AO opens the proverbial flood gates for the arctic air to penetrate much farther south.
It should come as no surprise, then, that this strongly negative AO phase has been accompanied by the presence of unseasonably cold air. The forecast, holds the potential for cold air through the next 7 days with chances for snow. The GFS ensemble and most medium range global models forecast the AO to rise over the next 7 days. Although it is typical for the AO to rise and fall rather consistently, the dramatically low values are being accompanied by rather high return values to normal, and in some cases a positive phase of the AO. Regardless, It remains to be seen if the wintry weather threats will work out Still, one thing is for sure: we are in uncharted territory as far as the Arctic Oscillation goes, for this late in the season.
For more on the Arctic Oscilation check out this article, which details the different phases and amplitudes.