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be described as a meridional shifting of the atmospheric mass, north and south
across the North Atlantic Ocean.
An index used to measure the strength and phase of the NAO is similar
to the SOI in that it is based on surface pressure differences between two
locations. In contrast to the SOI, which uses locations at similar latitudes but
different longitudes, the NAO index is based on stations at similar longitudes
but different latitudes. The northern location is Stykkisholmur, Iceland (65°N,
23°W). For the southern location, different studies have used Ponta Delgada
(Azores), Lisbon, and Gibraltar. The NAO index is positive when both the Ice-
landic low and the North Atlantic subtropical high are stronger than normal;
a negative NAO index indicates that both are weaker than normal. The posi-
tive phase of the NAO is often associated with warm winters over the eastern
United States, warm and wet winters over northern Europe, and dry winters
over southern Europe as the North Atlantic storm track (Fig. 2.14) shifts to
the north.
Figure 3.9a shows the NAO index for 1950-2011 and the propensity for
one phase or another to dominate on decadal time scales. For example, the
negative phase of the NAO occurred more frequently than the positive phase
through the 1950s and 1960s, but the positive phase was dominant from the
mid-1980s until the mid-1990s. Recently, the NAO has become more neutral,
and even negative.
The NAO can be seen as part of a larger pattern of variability in the North-
ern Hemisphere known as the Arctic Oscillation (AO). The negative phase of
the AO is associated with anomalously high pressure over polar regions and
anomalously low pressures in middle latitudes, which leads to cold air intru-
sions into western Europe and the central United States. The positive phase of
the AO index (Fig. 3.9b ) is associated with anomalously low pressures over the
Arctic, with wetter conditions in northern Europe and drying in Spain, northern
Africa, and the Middle East. Like the NAO index, the AO index was positive
during the later 1980s and early1990s but fairly low and variable in the 2000s.
Another example of decadal variability in the climate system is the Pacific.
Decadal Oscillation (PDO) . PDO anomalies are similar in structure to ENSO
anomalies except they are much longer lived and have smaller magnitudes
(~0.5 K). Values for more than 100 years of a PDO index are shown Figure
3.10. A positive, La Niña-like phase of the PDO occurred from 1890-1924,
characterized by anomalously warm sea surface temperatures in the western
Pacific (especially at middle and high latitudes) and cool temperatures in the
eastern Pacific. A warm PDO phase persisted from 1925 until 1946 and was
replaced by another cool phase from 1947 until 1976. The warm phase that
followed completed two cycles of the PDO in about a century.
Warm phases of the PDO are correlated with increased marine ecosystem
activity and productivity in Alaska and reduced activity off the west coast of
the United States farther south. This pattern is reversed during the cool phase
of the PDO. The causes of the PDO are not completely understood, but rec-
ognizing its existence aids prediction because of the long persistence of the
pattern.
Another type of decadal variability within the climate system occurs in
the form of persistent drought. Over the central United States, the Dust Bowl
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