Weatherwatch: what is the North Atlantic
Oscillation?
There is uncertainty over the future behaviour of this
important weather phenomenon in a warming climate
Sat 3 Jul
2021 06.00 BST
Jonathan
Harrison (Met Desk)
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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