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the major features may be identified. These are often seasonal in nature. For
example, in the North Atlantic the East Atlantic Pattern exists from September to
April, and over Eurasia the Polar/Eurasian Pattern may be discerned from
December to February. An analysis of the many identified patterns is available
from the Climate Diagnostics Center (www.cdc.noaa/ClimateIndices).
2.2 The North Atlantic Oscillation (NAO)
The North Atlantic Oscillation is defined as the difference between sea level
pressure at two stations representing the centers of actions that occur over
Iceland and the Azores. In calculating the pressure difference between the two
pressure systems, data from Stykkisholmur in Iceland have been evaluated using
various other stations to represent the more southerly center of action. Rogers
(
1997
) used Ponta Delgarda, while Lisbon, Portugal, has been used by Hurrel
(
1995
) and Gibraltar by Jones et al.(
1997
). Generally, US analyses (e.g. NCAR)
use the Hurrell index while those in Europe (e.g. East Anglia Climate Research
Unit) use that by Jones et al.
The acquired pressure differences are used to derive an index that identifies the
phase of the oscillation. A positive phase is represented by a stronger than usual
subtropical high pressure and a deeper than normal Icelandic Low. Accordingly,
the negative phase shows a weak subtropical high and a weak Icelandic Low. The
NAO Index is defined using the winter season, December through March.
In its positive phase, the increased pressure difference between the two
centers of action results in frequent strong winter storms tracking in a more
northerly path across the North Atlantic Ocean. Conversely, the negative phase
sees fewer and weaker storms passing in a more southerly track across the North
Atlantic. These two modes have a marked influence on the nature of winter
climates in both Western Europe and eastern North America.
In Western Europe, a positive NAO Index results in warm, wet winters, for the
storm tracks are carried to the north and air from the subtropical highs will
prevail. At the same time, the eastern parts of the United States will probably
experience mild and possibly wet winters. In contrast, during the negative phase,
eastern North America will experience more cold air invasions and so will
Western Europe, especially the Mediterranean.
Comparisons between the NAO and the Southern Oscillation (SO) suggest
that while the SO is driven by sea surface temperature, NAO is an atmospheric
phenomenon. Additionally, the NAO is of much longer time scale, with changes
in phase often taking decades. It is thus of particular interest for long-term
climatological analysis.
Figure
2.1
shows values of the NAO from the late 1800s to the late 1900s. The
cold European winters of the 1940s and the 1960s, which happened to include
one of the coldest decade periods on record, each coincided with negative
phases. A high positive NAO Index in the 1980s and early 1990s was in periods
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