The North Atlantic Oscillation is a numerical index and a large-scale expression of westerly wind strength over the North Atlantic, often associated with the strength of low pressure over Iceland. Positive values are usually defined by a deep area of low pressure here, while negative values often coincide with a nearby area of high pressure.

In north-west Europe, a positive winter North Atlantic Oscillation (NAO) generally coincides with above-average temperatures, storminess and precipitation due to increased westerly wind strength. A negative winter NAO is often associated with drier and colder conditions owing to more frequent easterlies and northerlies. The summer NAO is less influential as relationships between weather and atmospheric circulation are more localised.

The NAO fluctuates irregularly through each winter. However, it was often positive from the 1900s-20s and late 1980s to present. Conversely, the winter NAO was frequently negative from the 1940s-80s. The causes of these variations are unclear. Some studies suggest anthropogenic climate change has caused the increase in positive NAO winters since the late 1980s.

There is uncertainty over future NAO behaviour under a warming climate, however. The Arctic has been warming at a rate faster than mid-latitudes, and this may be leading to increasingly blocked weather patterns across the northern hemisphere, along with a more erratic NAO. In recent years there have been extreme or record-breaking positive NAO phases (December 2015, February 2020) and negative (December 2010, March 2013).

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