NAO: Driving Climate Across the Atlantic
How NAO Does Its Thing
For the French, the winter of 1879 was terrible. In fact, it was cold enough to make a 25-year-old fledgling researcher at France’s Central Bureau of Meteorology wonder why. So Léon-Phillipe Teisserenc de Bort decided to investigate common threads between it and other anomalous winters in Europe’s recent memory. And indeed he found a shared link, which he coined “ centres d’action”—large regions of air pressure hovering over distinct spots on the globe. Today these centers, located above Iceland and the Azores Islands off the coast of Portugal, are still where “ d’action” is that drives the North Atlantic Oscillation, or NAO.
The NAO can be defined most simply as the relationship between the center of low atmospheric pressure over Iceland and the center of high pressure above the Azores. We’ve discovered in the century since Teisserenc de Bort that the centers tend to vary in intensity relative to one another. They do this year-round, but between December and March their swayings seesaw dramatically. Then, when the winter Icelandic low is very low, the Azores high prefers to be very high. That leaves a large pressure difference between the two centers—what we call a positive-phase NAO. Conversely, when the winter low is greater than usual, the high tends to be lower than usual. The pressure difference is slim, thus, a negative-phase NAO. The phases modulate major air currents and storm tracks over the Atlantic, which ultimately propel large-scale climate patterns over much of the Northern Hemisphere.
Climate Rollercoaster
Each NAO phase spins its particular brand
of atmospheric tumult, affecting temperature, precipitation, cloudiness, and
windiness in different regions—sometimes to drastic ends. Since the phases
can fluctuate over the course of weeks, months, and years, researchers like
to track them with a rollicking plot of average pressure difference over
time called the NAO index.
Jim Hurrell, an atmospheric scientist, is one such researcher. He’s a director at the National Center for Atmospheric Research in Boulder, CO, and the December-March index he compiles is used worldwide by those keeping tabs on the NAO’s erratic behavior.
Hurrell asserts that although its sign can flip-flop on a weekly basis, the NAO can’t be blamed for weekly weather per se: one smarting winter storm, for example, or a rainy spell. “Rather, the NAO conditions the atmosphere so that it’s much more likely you’re going to have such events,” he explains. A key distinction here is the difference between weather and climate. Average the weather in one area for decades and you’ve got a natural regional climate, which is what NAO ultimately influences. (Climate can denote a collection of weather from a time period as short as a season or as long as a century. The industry standard is 30 years.) “Really, the NAO is something that appears when you look at longer time scales,” says Hurrell.
