Geoscience Reference
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Figure 4.20.
Positive minus negative
NAO difference field of cyclone
events for the cold season based on
index extremes over the 1966-1993
period. The contour interval is 20.
Positive differences are shown by solid
contours with negative differences
shown by dash-dot contours (from
Serreze et al.,
1997
, by permission
of AMS).
suggest that when the sea ice margin in the Atlantic sector retreats, it invokes a
local change in heating, which alters the North Atlantic storm track. This appears to
involve at least a weak response of the NAO. The observational study of J. Francis
et al. (
2009
) lends some support, finding that summers with low sea ice extent (as
assessed for the Arctic as a whole) tend to be followed by a neutral or negative NAO
phase in autumn and winter. They argue that having large areas of open water at
summer's end leads to strong warming of the lower atmosphere in autumn and win-
ter (part of the process of observed Arctic amplification of recent SAT changes, see
Figure 1.5
), a reduction of static stability (the Arctic temperature inversion becomes
weaker), increased cloudiness, and - because of the outside Arctic warming rela-
tive to lower latitudes - a reduction on the poleward gradient in atmospheric thick-
ness that weakens the polar jet stream. J. Overland and M-Y. Wang (
2010
) come to
similar conclusions. In a follow-on observational study, Francis and Vavrus (
2012
)
argue that the Arctic amplification linked to ice loss contributes to slower easterly
progression of Rossby waves in the upper level flow, influencing middle-latitude
weather patterns.
Although low-frequency variability does not stand out as significant in a spectral
analysis, a number of studies (e.g., Portis et al.,
2001
) highlight multidecadal epochs
of the NAO. Looking back to the station-based index plotted as part of
Figure 4.17
,
there was an epoch from about 1870 through about 1900 when the NAO was mostly
negative, followed by mostly positive values from 1900 to 1950. This was followed
by a negative period from 1960 through 1980, and then a positive epoch from the
1980s through the late 1990s. As realized in a number of studies conducted during
the early 1990s and early 2000s, notably Hurrell (
1995
,
1996
), this multidecadal
trend from the negative to the positive phase on the NAO in large part accounted
for the pattern a pattern of linear trends in winter SAT observed between the two
periods, which, following from
Figure 4.19
, consisted of warming over northern
