Computer models suggest this pattern may hang on for weeks, continuing to limit the extent and severity of winter weather in the Lower 48.
The AO, in essence, is a reflection of how much cold air in the Arctic is able to penetrate into the mid-latitudes. When the index is strongly positive, frigid air remains parked near the Arctic Circle as it has for much of the winter. When it’s negative, the cold is unleashed, resulting in punishing Arctic blasts into the Lower 48 and often major snowstorms, even reaching into the Southern states.
On Monday, the National Weather Service’s Climate Prediction Center reported that the AO topped its previous record from Feb. 26, 1990.
“We’re seeing extreme conditions, but we’re seeing cold air staying where we expect it rather than meandering out of the Arctic,” said Andrea Lang, associate professor of atmospheric sciences at the State University of New York at Albany.
The strongly positive AO signifies abnormally low pressure over the Arctic and unusually high pressure over the northern Atlantic and Pacific oceans. This resulting pressure difference strengthens the winds in the jet stream, the highway of air located about 30,000 feet up in the atmosphere, which separates cold air to the north and warm air to the south. Over the weekend, the jet stream over the Atlantic reached speeds of over 260 mph, turbocharging commercial aircraft across the Atlantic in record time.
When the jet stream is this strong, it tends to travel more in a straight line in the northern latitudes rather than meandering south, and this helps keep Arctic air bottled up.
A strongly positive AO also means the polar vortex, the zone of frigid air in the high latitudes, is stable, further limiting opportunities for cold air to charge south.
Lang said the stability of the vortex may be linked to the state of a large overturning circulation in the tropics known as the Madden-Julian Oscillation. This phenomenon has connections to the northern latitudes.
Because the AO has been strongly positive on most days since early December, intrusions of Arctic air into the mid-latitudes have been infrequent, weak and fleeting. Europe posted its warmest January on record, and the contiguous United States has logged its warmest winter on record, based on temperatures in December (sixth warmest on record) and January (fifth warmest on record).
An index of the “accumulated winter season severity” over the Lower 48 shows most locations experiencing mild conditions and, in some cases, record warmth.
While the mid-latitudes have endured little winter weather as a result of this AO pattern, brutally cold conditions have frequented the northern latitudes.
Alaska had its 13th coldest January on record, its most bone-chilling since 2012. The average temperature in Fairbanks during January (minus-21 degrees) was 13.5 degrees colder than average. And more bitterly cold weather is in the forecast.
Computer model projections over the next two weeks continue to reflect a strongly positive AO pattern, with frigid air lodged over northern latitudes and abnormally mild conditions over much of the Lower 48 states and Europe.
The state of the AO is the opposite of its condition in the winter of 2009-2010, when it was strongly negative. That winter, the AO ranked as the most negative on record in both December and February and was the sixth most negative in January. Washington, D.C., received 56.1 inches of snow that winter, its snowiest winter on record. The snowstorms that year included the blizzard known as Snowmageddon,
Projections of the AO show it remaining strongly positive into early March and then trending toward more of a weakly positive phase by late March. If these projections are correct, they would offer little promise for sustained severe winter weather in the mid-latitudes, including Washington.
However, confidence in these projections decreases with time, and a positive AO does not preclude the possibility of large, disruptive winter weather events, even if they are short-lived.
Lang said the Madden-Julian Oscillation could shift “toward a favorable phase” to disrupt the polar vortex between now and mid-March, leading to a bit more wintry weather.
The strongly positive AO this winter fits into a pattern of the past several winters, which have also seen this generally mild pattern prevail. “We haven’t had a truly negative AO winter since 2012-2013,” said Judah Cohen, a meteorologist at Atmospheric and Environmental Research, in an interview.
Cohen said the AO had been trending negative from the late 1980s through early 2010s, and there was some thought that climate change was behind the trend. Some hypothesized a “warm Arctic, cold continents” pattern might become the new winter norm. But now, as the AO trend has reversed, the link between the AO and climate change seems less clear.
“The models are now more muddled [on how climate warming will influence the AO] and not consistent with each other, and there is a large spread as to what the outcome might be,” Cohen said.
“I think the message is we really don’t know.”