- 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 trends:
- A positive trend is when there is a low pressure in the north (Greenland) and high pressure in mid-latitude of the North Atlantic so that a large pressure gradient exists.
- It 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.
- Negative trends:
- When both regions have low pressure and the gradient flattens out it is called a negative trend.
- 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