The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia. The NAO is a large scale seesaw in atmospheric mass between
the subtropical high and the polar low. The corresponding index varies from year to year, but also
exhibits a tendency to remain in one phase for intervals lasting several years.
Positive NAO Index
The Positive NAO index phase shows a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low.
The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track.
This results in warm and wet winters in Europe and in cold and dry winters in northern Canada and Greenland
The eastern US experiences mild and wet winter conditions
The NAO index is defined as the
anomalous difference between the polar low and the subtropical high during the winter season (December through March)
Negative NAO Index
- The negative NAO index phase shows a weak subtropical high and a weak Icelandic low.
- The reduced pressure gradient results in fewer and weaker winter storms crossing on a more west-east pathway.
- They bring moist air into the Mediterranean and cold air to northern Europe
- The US east coast experiences more cold air outbreaks and hence snowy weather conditions.
- Greenland, however, will have milder winter temperatures
source http://www.ldeo.columbia.edu/NAO by Martin Visbeck