Geoscience Reference
In-Depth Information
Figure 7.24
(A) Mean 700-mb
contours (in tens of feet) for December
1957, showing a fast, westerly, small-
amplitude flow typical of a high
zonal index. (B) Mean 700-mb zonal
wind speed profiles (m s
-1
) in the
western hemisphere for December
1957, compared with those of a nor-
mal December. The westerly winds
were stronger than normal and dis-
placed to the north.
Source
: After Dunn (1957).
A
B
near Iceland than to the south giving easterlies across
western Europe and the eastern North Atlantic.
The NAO appears to be the major component of
a wider pressure oscillation between the north polar
region and mid-latitudes - the Arctic Oscillation (AO).
However, the mid-latitude zone responds with varying
intensity both geographically and temporally. There is
a much weaker mid-latitude signature of the Arctic
Oscillation over the North Pacific Ocean than over the
North Atlantic. Nevertheless, in the southern hemi-
sphere there is a corresponding Antarctic Oscillation
between the polar region and southern mid-latitudes.
For this reason, some researchers consider the two
zonally symmetric modes to be more fundamental fea-
tures of the global circulation. They also extend upward
throughout the troposphere. In the twentieth century,
the NAO index (of south-north pressure difference)
was generally low from 1925 to 1970. Air temperatures
in the northern hemisphere were above normal and
cyclones along the east coast of North America tended
to be located over the ocean, thus causing longer, drier
east coast summers. Prior to 1925, a regime of colder
climatic conditions was associated with a higher NAO
index. Since 1989, the NAO has been mostly positive,
except for the winters of 1995 to 1996 and 1996 to
1997. This recent phase has given rise to winters that,
compared to normal, are warmer over much of Europe,
wetter (drier) over northern Europe-Scandinavia
(southern Europe-Mediterranean), in association with
a northward shift of storm tracks.
