No sailor takes the North Atlantic's winter weather lightly. Little wonder that when Danes began round-trip service to Greenland in the 17th century, they noticed that when the island's winters were mild, Europe's were severe. When Greenland's winters were biting, the Continent's were relatively balmy.
According to research published in the current issue of the journal Science, a phenomenon known as the North Atlantic Oscillation (NAO), which is responsible for that swing, is a prime suspect behind the string of unusually warm, wet winters in Central and Northern Europe during the past decade.
These recent conditions and their effects closely match some computer models of global warming's impact on the region, says James Hurrell, an atmospheric scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.
Yet as he looked at decades-long trends in the atmosphere's winter circulation patterns across the North Atlantic and analyzed their impact on moisture transfer, the NAO emerged as a significant force behind the odd weather patters.
One of the oscillation's strong periods was during the 1920s. ''If you look at the long-term trends in Northern Hemisphere temperatures, you'll see they took a big jump in the 1920s,'' says Harry van Loon, an atmospheric scientist at NCAR and a longtime student of the NAO. ''Most of the increase was measured in the area between Labrador and the ocean north of Norway. The North Atlantic Oscillation dominated the changes in the rest of the hemisphere.''
When scientists looked at a plot of winter temperatures in Europe, the warm areas caught everyone's attention, Dr. Hurrell says. ''A lot of work has gone into explaining [that] as a response to greenhouse-gas forces,'' he says.
Yet given the NAO's duration and its effects on moisture transfer, and hemispheric temperatures, ''it's wrong to look at the picture'' in Europe ''and attribute it to greenhouse-gas forces,'' Hurrell concludes. Yet he doesn't rule out a greenhouse-gas component.
''Perhaps the North Atlantic Oscillation's extreme mode is a response to greenhouse-gas forcing,'' he adds. But that won't become clear, he says, until researchers find out how well their climate models take the NAO into account.
The oscillation shows up as changes in the strength of westerly surface winds over the North Atlantic. The winds arise as air circulates counterclockwise around a low-pressure zone over Iceland and clockwise around a high-pressure zone over the central Atlantic to the south. (During the summer, this high-pressure zone migrates to Bermuda, giving the East Coast of the United States relentless bouts of hot, muggy weather.)
Researchers have noted that when the Icelandic low is strong, so is the subtropic high, intensifying the westerly winds. When Hurrell looked at long-term data on the oscillation, ''I found that the past 10 years have been fairly remarkable. Circulation changes across the Atlantic were stronger than previous decades.''
Hurrell also looked at the NAO's impact on moisture transport as the winds pick up warmth and moisture over seawater, generating intense winter storms.
As the two pressure features strengthen, they shift the storm tracks to the northeast, bringing more moisture to Northern and Central Europe and Scandinavia than to Southern Europe.
''This has implications for the buildup or melting of glaciers,'' Mr. Van Loon says. During periods when the NAO is strong, he says, glaciers in Scandinavia would retreat. Glaciers in Greenland also would not expand much, since they would receive less moisture once the storm track shifts. Hurrell adds that the intensity and duration of swings in the NAO can be correlated with ice core samples taken from Greenland.
While scientists have known about the NAO for more than 100 years, it remains a mystery, says Jeff Rogers, professor of geography at Ohio State University and another long-time NAO researcher.
Like the Southern Oscillation, which gives rise to El Nino in the Pacific Ocean, the North Atlantic Oscillation has a measurable average duration: three to six years for El Nino but ''decades'' for the NAO.
Unlike they could with El Nino, however, scientists have been unable to close in on the conditions causing the NAO. For example, the Southern Oscillation has been traced to sharp variations in sea-surface temperatures and their subsequent effects on atmospheric circulation.
''There seems to be no underlying ocean feature supporting the North Atlantic Oscillation,'' Rogers says. ''One of the big things we have learned is that the North Atlantic Oscillation is only remotely linked to the Gulf Stream.''
Because the factors that give rise to the NAO are so poorly understood, he adds, the NAO is far less predictable than El Nino.