Late in the 16th century, a missionary who had traveled back and forth across the Atlantic Ocean for several years noted that mild winter conditions in Greenland often coincided with severe winter conditions in Denmark, and vice versa. The severe-versus-mild phenomenon he described is now recognized as an impact of the North Atlantic Oscillation or NAO. This prominent pattern of climate variability has a strong influence on weather over northeastern North America, Greenland, and Europe. Conditions associated with the two phases of this oscillation directly affect human demand for energy, quality of crop yields, and productivity of fisheries.

Air pressure over two regions drive this oscillation:

• The high latitudes of the North Atlantic Ocean near Greenland and Iceland generally experience lower air pressure than surrounding regions. This zone of low pressure is called the sub-polar low.

• Farther to the south, air pressure over the central North Atlantic Ocean is generally higher than surrounding regions. This atmospheric feature is called the subtropical high.

Phases of the NAO are defined by higher-than-normal air pressure in one of these regions and lower-than-normal air pressure in the other. The anomalous patterns affect weather all around the Atlantic by influencing the intensity and location of the jet stream and the storm tracks that follow it.

NAO Index, 1950 – present

NAO’s positive phase
The NAO is in a positive phase when both the sub-polar low and the subtropical high are stronger than average. During positive NAO phases, the increased difference in pressure between the two regions results in a stronger Atlantic jet stream and a northward shift of the storm track. Consequently, northern Europe experiences increased storminess and precipitation, and warmer-than-average temperatures that are associated with the air masses that arrive from lower latitudes. At the same time, southern Europe experiences decreased storminess and below-average precipitation. In eastern North America, the positive phase of the NAO generally brings higher air pressure, a condition associated with fewer cold-air outbreaks and decreased storminess.

NAO’s negative phase
The NAO is in a negative phase when both the sub-polar low and the subtropical high are weaker than average. During negative NAO phases, the Atlantic jet stream and storm track have a more west-to-east orientation, and this brings decreased storminess, below-average precipitation, and lower-than-average temperatures to northern Europe. Conversely, southern Europe experiences increased storminess, above-average precipitation, and warmer-than-average temperatures. In eastern North America, the negative phase of NAO generally brings lower air pressure, a condition associated with stronger cold-air outbreaks and increased storminess.

Data
A record of NAO phases for 1950 through the present is available from NOAA’s Climate Prediction Center.

References
North Atlantic Oscillation page at Lamont-Doherty Earth Institute, Columbia University, by Ian Bell and Martin Visbeck. Accessed October 14, 2009.

North Atlantic Oscillation, NOAA’s Climate Prediction Center. Accessed October 14, 2009.

Wallace, J. M., and D. S. Gutzler, 1981: Teleconnections in the geopotential height field during the Northern Hemisphere Winter. Mon. Wea. Rev., 109, 784-812.