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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 centerswhat 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 regionssometimes 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.

A Turbulent Tale

So how does the NAO do its thing? Hurrell points to the particular positioning of the low and the high pressure centers. The high happens to be directly south of the low. Through a sequence of events, this placement is responsible for steering the westerly winds that send distinct weather patterns to different regions during a positive or negative NAO.

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The first step is the fact that in the Northern Hemisphere, air tends to move counter-clockwise around a low-pressure center like that over Iceland. Conversely, air moves clockwise around the Azores high. Picture this air flow as two giant atmospheric gears whirring over the Atlantic, spinning their wheels in figure-8 formation. “The flow around these two centers meets at about 40Â-45Â north latitude,” says Hurrell. The giant jet stream of wind that naturally flows west to east in the midlatitudes gets channeled straight through the meeting point by these cranking cogs of current.

In a positive NAO phase, the high and low are particularly robust. That makes the gears rev their speed, whipping jet-stream-borne storms on a brisk northeastern path. “The storms pick up warmth and moisture from the Atlantic Ocean, carry it over to Northern Europe, and drop it there as rainfall,” Hurrell says. In fact, his research has shown that the warm air can kick up winter Northern European surface temperatures as much as 3°C during a positive NAO.

The remainder of the air currents swirling around the low and the high act as traffic cops, steering more weather signatures of a positive NAO phase. The northwest edge of the counter-clockwise gear around the Icelandic low hurtles dry, cold air out of the Arctic to the south, freezing Northern Canada and Greenland. Meanwhile, warm air from the Caribbean gets cycled around the Azores high, blessing the US east coast with warmer winters.

In a negative phase, however, the low and high relax. The gears of current slow down, weakening the drive of the westerly jet stream. The reverse effect then occurs: That slack allows storms to drift more naturally toward the Mediterranean and points south, releasing the rain and usual warm weather there instead. The Northwesterly Arctic air is similarly less directed, now allowed to descend further into the northeast US and Northern Europe, bringing with it cold snaps and blustery winters. Northern Canadian and Greenlandic winters are relatively mild. 

What a Ride It’s Been

Keep in mind that we must contend with all that climatic turmoil only during an intensely positive or negative NAO winter. Hurrell cautions that distinct NAO indexes with extreme, telltale activity aren’t always the norm. “In any given winter, or any given month, it may not look like NAO at all,” he says.

“NAO-blah” is precisely what’s been happening in recent memory. The NAO index for the winter of 2003-2004 clocked in at a statistically-zero -.07. The winter of 2002-2003 was a tepid +.2. Such wimpy indexes have made recent northeast US winters seem, well, like typical northeast US winterswith snow, sure; but no real juicy drama to report.

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Still, the trend of the NAO since 1972 has been definitively, positively positive. That includes more peak positive years, like the highest on record, 1989 (+5.08). Prior to that, in the 50s and 60s, the Northern Hemisphere idled in negative phases. (So entertain your elders’ complaints about the hard, cold winters they suffered back then!) These recent long-term trends are unprecedented in the instrumental NAO index, which has been plotted back to 1821.

To be certain these decadal trends are truly a break from tradition, scientists must trace the NAO’s path much, much farther back than 183 years. In fact, a common goal of the recent fervent work on the NAO is to analyze past events to determine if our current positive trend is being goaded by an outside influencenamely, global warming.

Understanding the NAO’s historical behavior is also enormously important for climate prediction. Knowing what the NAO has done in the past (and why) might allow scientists to do the seemingly impossible: forecast what it will do in forthcoming seasons or decades. For more on how scientists are investigating these concepts, click on NAO Data Hunting (And Gathering) or Forecasting the Unpredictable.

Related Links

NCAR­s NAO Spotlight
http://www.eo.ucar.edu/spotlight/nao/index.html

NAO Indices Information
http://www.cgd.ucar.edu/~jhurrell/nao.stat.winter.html#winter
Jim Hurrell's NAO seasonal index since 1860

Winter Weather and the North Atlantic Oscillation
http://earthobservatory.nasa.gov/Study/NAO_200307/

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