NAO: Driving Climate Across the Atlantic

How NAO Does Its Thing

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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.

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 40°-45° 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 winters—with snow, sure; but no real juicy drama to report.

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 influence—namely, 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.