If you think this winter has been weird, blame the North Atlantic Oscillation.
If you find the term confusing, think of it as a crossing guard. The NAO lets cold fronts cross the Atlantic. If it is positive, it lets cold fronts whiz through the Midwest and Eastern states and dash out to sea. If it is negative, it stops them. They linger on the shore, freezing everything. If it stops them long enough, several cold fronts pile up and it gets really frigid.
When scientists discuss the North Atlantic Oscillation, they are talking about air pressure. When the low-pressure area around Iceland is similar to the high-pressure around the Azores Islands in the mid-Atlantic, they say it is negative. In their words, negative NAOs cause blocking. It blocks cold fronts and eventually steers them north. Cold front after cold front enters North America and lingers. The temperatures drop and the frozen air plunges south.
Last year’s Negative NAO blocked cold fronts and the winter was cold. SOURCE: NOAA
That is what happened last year.
When the two air pressures are very different, the NAO is positive. The cold Arctic air remains trapped to the north. That is what happened throughout most of November, all of December and early January.
This year’s Positive NAO kept early winter warm and is letting storms zip out to sea. SOURCE:NOAA
The low pressure around Iceland began to weaken in January and, by January 14, the NAO was neutral. Cold fronts could finally drop south of the Arctic area. However, nothing blocked them, so they could quickly sail out to sea.
Since the middle of January, the NAO has been positive or near neutral. When it becomes near neutral, Arctic air shoots south and then east across the Atlantic. Then warm air from the Atlantic heats everything up again.
Of course, the crossing guard NAO is not the only thing shaping weather. La Niña is making the West and the South dry. The Arctic Oscillation lets cold polar air escape. (Europe is buried in frigid weather from Siberia. They are calling the storm “The Beast from the East.”) It is a complicated and crazy mess.
However, if you have enjoyed not getting as cold as last winter, you can thank the brave little cross-guard. The NAO has kept the nasty weather from loitering and being a nuisance.
Evelyn Browning Garriss, historical climatologist, is a longtime writer for The Old Farmer's Almanac. She is also editor of The Browning World Climate Bulletin  and has advised farmers, businesses, and investors worldwide on upcoming climate events and their economic and social impact for the past 21 years.