Geoscience Reference
In-Depth Information
This emphasis on the winter stratosphere is also relevant to the interpretation of
low-frequency variability. Low frequency variability in the NAO is thought of as
being forced from the surface by slowly varying North Atlantic or tropical SSTs, the
latter finding support from recent modeling studies. By contrast, in the AO frame-
work, thinking turns to linkages with stratospheric ozone depletion or the buildup
of greenhouse gases that are associated with stratospheric cooling. The basic idea
is that such changes can “spin up” the polar stratospheric vortex, with the effects
propagating down to the surface,
The concept of the AO as more fundamental than the NAO was not met with
universal enthusiasm. Although many in the research community were quick to hail
the AO as the new paradigm, others were skeptical. C. Deser (
2000
) concluded that
the correlation between the Pacific and Azores centers of action is not significant
and that the AO cannot therefore be viewed as reflecting such a teleconnection.
M. Ambaum, B. Hoskins, and D. Stephenson (
2001
) find that even the correla-
tion between the Pacific and Icelandic-Arctic centers of action is not significant.
They argue that the AO is mainly a reflection of similar behavior in the Pacific and
Atlantic basins. Other studies contributing to the debate include J. Fyfe (
2003
) and
Ambaum and Hoskins (
2002
). Regardless, from a practical view, indices of the win-
ter AO and NAO are very highly correlated and for many applications, one can use
either paradigm.
However, a different picture emerges when attention is focused on seasonal changes
in the structure of the pattern. Recall from discussion of the NAO that the while the
Icelandic Low and the Azores High exhibit seasonal variations in their strength, there
are also seasonal shifts in the locations of these two centers of action. This is what
drove Portis et al. (
2001
) to develop their seasonally mobile index of the NAO. In a
similar vein, M. Ogi, K. Yamazaki, and Y. Tuchibana (
2004
), defined the AO separately
for each calendar month through an empirical orthogonal function EOF analysis of
geopotential height fields from 1,000 hPa to 200 hPa, poleward of 40
o
N, an approach
argued to better capture seasonal changes in the structure of the mode. Based on their
approach, the summer NAM has a smaller meridional scale than its winter counter-
part, with the Arctic center of action lying over the central Arctic Ocean.
4.7.3
The Pacific Decadal Oscillation
Coupling between the atmosphere and the oceans is implicated in climate variations
on scales of several years to decades. As just discussed, such coupling can help
to explain low frequency variability in the NAO. The best-known coupled atmo-
sphere-ocean phenomenon is ENSO, which results from interactions between the
atmosphere and ocean in the tropical Pacific and operates on time scales of three
to seven years. ENSO is associated with variability in the atmospheric circulation
far from the tropical heating sources. ENSO can in turn be linked in various ways
to “ENSO-like” patterns in the climate system, for which multi-decadal signals are
prominent. Climate signals are best expressed in the extratropics, especially the
northwest Pacific, including Alaska.
