"Some scientists argue that the impact of the NAO on global climate is comparable to El Nino," says Dr. Sukyoung Lee, associate professor of meteorology. "However, most of the scientific community's analyses to date have been of monthly or seasonal averages which fail to reveal the intrinsic nature of the NAO." The fundamental dynamic process of the North Atlantic Oscillation is on a two-week scale, says Dr. Christian Franzke, postdoctoral fellow in meteorology, referencing an earlier work by Dr. Steven Feldstein, senior research associate, Penn State's Environmental Institute. Looking at seasonal data does not really say anything about the causes or mechanisms of the phenomenon. Franzke presents this research at the fall meeting of the American Geophysical Union today (Dec. 9).
The NAO is best known as a pressure difference between the air over Iceland and the air over the Azores – located in the middle of the Atlantic on a latitude with Lisbon, Portugal. If pressure is higher than usual over Iceland, it is colder in Europe during the winter and there is more rain in the Mediterranean. If pressure is anomalously low over Iceland, there are more storms and precipitation in Europe, a milder winter and there is less rain in the Mediterranean.
"The NAO has a strong influence on European weather and is an economic issue as well for fish production and agriculture," says Franzke.
The NAO occurs in the tropopause, the areas of the atmosphere between the troposphere and the stratosphere at between 7 to 10 miles above the Earth's surface. A stream of air called the Polar Front Jet Stream moves from the Pacific, across North America and over the Atlantic Ocean.
During the typical winter season, there are about two or three separate NAO incidents, some positive and some negative.
Franzke, Lee and Feldstein looked at 40 years of daily weather data gathered by the National Center for Atmospheric Research. This data includes measures of temperature, pressure, wind speed and direction, and surface pressure. They used existing climate models to recreate the patterns associated with the NAO and compared those to observational data.
In the Atlantic, they found that the stream sometimes moves in a breaking wave pattern and is oriented either from the northwest to the southeast or from the southwest to the northeast. Each breaking wave takes about six days to form and then disappear and is usually followed by a second wave, completing the two-week pattern. The wave crest can actually collapse and become flat just as ocean waves do, initiating a period of no NAO. When the wave breaks with an orientation from the southwest to northeast, the pattern is considered positive and produces low pressure over Iceland making European winters warmer. When the wave breaks in the opposite direction – southeast to northwest – higher than usual pressure forms over Iceland and European winter is cold. This is a negative incident.
"The recent trend toward global warming shows more bias toward positive phase NAOs," says Franzke. "There are more positive than negative events occurring during a winter."
This would tip the winter weather pattern toward one of milder winters in Europe.
The researchers also found that episodes of NAO are linked to storms in the Pacific Ocean. A storm in the Pacific will create one breaking wave pattern. If the storm is in the northern Pacific, than there will likely be a negative NAO. If the storm starts farther south, the NAO probably will be positive. If a series of storms follows each other, then an NAO episode will last longer. If there are no storms entering the North Atlantic, the NAO dies out.
"Because of this connection to Pacific storms, it might be possible to predict the changes in European weather four or five days in advance," says Franzke.
With so much interest in global warming, we definitely need to understand the mechanism of NAO before we begin to make any predictions, says Feldstein.
The researchers have not yet looked at the effects of the NAO on North American weather, but Franzke states that there is an influence on Eastern Seaboard weather. With Pacific storms implicated in NAO formation, and the Pacific the controlling factor for El Nino, there might be a strong link between the two phenomena. However, to fully understand the global links, the North Atlantic Oscillation must be fully understood.
The National Science Foundation funded this research.