Environmental Engineering Reference
In-Depth Information
The North Atlantic Oscillation (NAO):
This is the name given to year-to-year variations in the relative strengths and locations of the
Azores High and the Icelandic Low. A large difference in pressure between the two systems
(known as an index of NAO
+
) leads to increased moist westerly winds, giving cool summers
and mild wet winters in Europe. Alternatively, when the pressure difference is low (NAO
−
),
the moist westerly winds weaken, resulting in cold winters in Europe. Cyclones then track
further south into the Mediterranean and over North Africa.
Under NAO
+
conditions, northern Canada and Greenland are colder and drier, but this
situation is reversed during a NAO
−
phase. The NAO also exerts an influence on winter
weather over much of eastern North America. When the index is high (NAO
+
), the Icelandic
Low reflects a strong south-westerly circulation of air over this region at the expense of Arc-
tic air moving south. Particularly during El Niño years, this results in warmer winters over
the north-eastern United States and south-eastern Canada. During NAO
−
years, cold air is
able to penetrate further south and the eastern and south-eastern North American continent
experiences winter cold outbreaks as far south as Florida.
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