As they dig out from under thick snow dropped by back-to-back holiday blizzards, residents of Colorado, northern and central New Mexico, and several Plains states may wonder what the rest of winter holds for them. But at least the patterns behind such weather are predictable. They're typical of the long arm of El Niño, which now reigns in the tropical Pacific.
For residents of the East Coast – not to mention Europe – it's a different story. Record-setting warmth, which gave Muscovites near shirt-sleeve weather, has everyone scratching their heads. So far this season:
•Williamsport, Pa., saw a record high of 70 degrees F. last month and the least snow ever recorded for December.
•In Scotia, N.Y., wet-suited water skiers sped along the Mohawk River – one of their earliest appearances on record.
In Europe, it was more of the same:
•Vienna posted its mildest New Years Day in 155 years. The high reached 57 degrees.
•In mid-December, Muscovites strolled the Russian capital in 47-degree weather – a record wintertime high for the city. Even in frigid western Siberia, temperatures for the first 10 days of December ran six to eight degrees above normal.
Is global warming responsible? Researchers aren't sure. They point instead to a seesaw climate pattern that occurs over the North Atlantic, called the North Atlantic Oscillation or NAO. Less publicly known than El Niño, and certainly less understood, the NAO is getting increasing attention from scientists. Their goal is twofold: to develop useful forecasts of the oscillation and to better estimate whether global warming is exaggerating its effects.
Unlike El Niño, the phenomenon's reach isn't fully globe-circling. But particularly in the winter months, the NAO "is just as important for weather and climate across much of the northern hemisphere," says James Hurrell, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo.
The earliest records for the NAO appear in Scandinavian archives that date back centuries, researchers say. The phenomenon swings between two states. When the NAO is positive, a large region of high pressure appears over the central Atlantic, while a large area of low pressure settles over southern Greenland. Each of these features are stronger than usual, leading to more-severe Atlantic storms that travel on a more northerly track than usual. This leads to wet, mild winters in the Eastern US and northern Europe. When the NAO goes negative, the high and low pressure areas weaken and shift south. Storms are weaker and travel more directly west to east.
The NAO can have wide-ranging ecological effects – from changes to the location, frequency, and intensity of storms and wildfires to shifts in crop and fisheries yields. And it can have a profound effect on energy demand. Energy analysts note that crude oil and natural gas prices have eased, partly because of the mild winter so far.
Much of the rise in average temperatures in the northern hemisphere has been traced to the NAO's persistent positive state for the past three decades. This raises questions about whether global warming is forcing natural swings in climate, such as those provided by the NAO, into one dominant mode and allowing it far fewer forays into its "off" years.
"There is a tremendous amount of certainty on a large global or hemispheric scale that certain things we're seeing are undoubtedly due to human influence on climate, such as the warming of the global oceans," Dr. Hurrell says. "But what people really want to know is: How will the weather in my region change? If seasonal weather is dictated by these patterns of natural variability, then it becomes absolutely critical to understand how those patterns work and how they are going to change in a changing climate."
While scientists have been studying the NAO for decades, meaningful forecasts for the NAO have remained more elusive than for El Niño, Dr. Hurrell explains. El Niño comes and goes with changes in the ocean and the atmospheric temperatures in the western tropical Pacific. The slow pace of change in the ocean, compared with the atmosphere allows forecasters to use marine conditions as harbingers of El Niño's approach. By contrast, the NAO is largely an atmospheric phenomenon. It is far more chaotic, changing on time scales ranging from days to decades.
The oceans may still play a role – even if minor – in NAO's fickleness, Hurrell says. One factor may be the Atlantic itself. In 2005, for instance, Florida State University researcher Lin-Lin Pan published work that suggested that a pattern in North Atlantic sea- surface temperatures and the NAO feed off each other in ways that strengthen both. Over the past four or five years, others have looked at the role that subtle changes in tropical seas as far afield as the Indian Ocean may have on the NAO.
Still others are looking up for answers. For example, a team of researchers in Britain and the US have found that changes in the lower stratosphere appear to play a role in shaping the strength and phase of the NAO. In modeling studies, the team found that the only way to push the NAO into the long-term pattern it has assumed over the past 30 years was to trigger it with patterns found in the lower stratosphere.
Teasing out these relationships is critical to figuring out how global warming may affect weather in specific regions, researchers say.