Geoscience Reference
In-Depth Information
scale as the Southern Oscillation. Variations in the
intensity of atmospheric circulation at these locations
are known as
North Atlantic Oscillation
(NAO) and
the
North Pacific Oscillation
(NPO). Whereas the
Southern Oscillation characterizes zonal pressure fluc-
tuations in the tropics across the Pacific, the NAO and
NPO are meridional air circulation phenomena reflect-
ing inter-annual variability in the strength of Rossby
waves in the northern hemisphere. The North Atlantic
Oscillation also reflects major temperature differences
in winter. An index of the NAO since 1864 has
been constructed using the normalized atmospheric
pressure between the Icelandic Low, measured at
Stykkisholmur/Reykjavik, Iceland, and the Azores
High, measured at Lisbon (Figure 2.8). Positive values
of the index indicate stronger-than-average westerlies.
Air temperature fluctuations are best characterized by
the winter temperature between Jakobshavn, West
Greenland, and Oslo, Norway. Significant environ-
mental changes throughout the twentieth century have
been associated with the NAO. These include varia-
tions in wind strength and direction, ocean circulation,
sea surface temperatures, precipitation, sea-ice extent,
and changes in marine and freshwater ecosystems. The
North Atlantic Oscillation influences an area from
Siberia to the eastern seaboard of the United States.
Given the magnitude and extent of the NAO, it is
surprising that little attention was given to it until
the 1990s.
The NAO undergoes large variation from year to
year and, more importantly, at decadal time scales.
However, large changes can also occur during a winter
season. Hence it is difficult to characterize northern
hemisphere winters as completely severe or benign.
This probably reflects the fact that climate processes
are random over time. Large amplitude anomalies in
wintertime stratospheric winds precede anomalous
behavior of the NAO by one to two weeks. Changes
to the strength and pathway of mobile polar highs and
the amplitude of Rossby waves in the polar jet stream
also correspond to winter anomalies. Similar to the
Southern Oscillation, short-term alterations are also
preceded by changes in sea surface temperature about
nine months in advance. Together the NAO and SO
account for 50 per cent (34 per cent and 16 per cent,
respectively) of the variation in winter temperatures in
the northern hemisphere outside the tropics. However,
the Southern and North Atlantic Oscillations act inde-
pendently of each other. The eastern United States and
adjacent Atlantic region are the only areas where both
phenomena always act together.
There are two distinct decadal phases associated
with the NAO. When the index is positive, atmospheric
pressure is low around Iceland and high in the Azores.
Sea surface temperatures off the United States eastern
seaboard and western Europe are warmer than
normal, while those in the western north Atlantic are
cooler than average (Figure 2.13a). These aspects
enhance meridional air circulation in the atmosphere,
strengthening westerly winds at temperate latitudes
and forcing warm, moist air from the Atlantic Ocean
over northern Europe. Hence land temperatures are
warmer adjacent to these pools of warmer ocean
waters, leading to milder winters. Temperatures are
cool over North Africa, the Middle East, eastern
Canada, and Greenland. The flux of heat across the
north Atlantic produces more intense and frequent
storms, a fact that has been more obvious since 1970
(Figure 2.14). As a result, wave heights increase in the
north-east Atlantic and decrease south of 40°N
latitude. This is paralleled by changes in precipitation
- wetter over Scandinavia and drier in the Mediter-
ranean. Over northern Africa, more dust moves from
the Sahara into the Atlantic Ocean. Sea-ice also
extends further south over the west Atlantic, while
retreating east of Greenland. These effects are either
wind- or temperature-driven. In the negative phase,
the pressure gradient between Iceland and the Azores
is weakened, the Mediterranean region is wetter and
warmer, and northern Europe is cooler and drier
(Figure 2.13b). While storms off the east coast of the
United States may be more frequent, they are virtually
absent off western Europe. These periods of anom-
alous circulation patterns have persisted over long
periods. For example, the NAO index was positive
from the turn of the twentieth century until about the
1930s. During the 1960s, the NAO index was negative
with severe winters across northern Europe. Since the
early 1970s, circulation has been locked into a positive
phase with warmer temperatures than average over
Europe and cooler ones over eastern Canada and
Greenland.
North Pacific Oscillation
The North Pacific Oscillation (NPO) measures the
strength and position of the Aleutian low-pressure
system, mainly in winter. Because there are few
recording stations in the north Pacific, an index
