Geoscience Reference
In-Depth Information
their study were up to 3.1
o
C higher. The latter period also saw greater mean cloud-
iness, higher wind speeds, and higher precipitation totals, attended by lower SLP
and geopotential heights. The lower SLP manifests a deeper Aleutian Low in winter
and spring. Intensification of the Aleutian low increased the advection of relatively
warm and moist air to Alaska during winter and spring. The regime shift has also
strongly influenced surface air temperature trends in Alaska. When analyzing the
total time period from 1951 to 2001, the temperature trends is positive. However,
the twenty-five-year period trend analyses before 1976 (1951-1975) and thereafter
(1977-2001) are both characterized by general cooling.
4.7.4
Arctic Dipole Anomaly
It is widely accepted that the extreme September minimum in Arctic sea ice extent
observed in 2007 (a record minimum at the time as assessed over the satellite rec-
ord) was driven in considerable part by a persistent summer pattern of atmospheric
circulation that favored ice loss. Its key feature in the SLP field was an unusually
high pressure over the northern Beaufort Sea (expressed as a strong Beaufort Sea
High) paired with unusually low pressure over northern Siberia. The strong Beaufort
Sea High is found just east of a pronounced ridge at the 500 hPa level (
Figure 4.24
)
Resulting southerly winds between the pressure anomaly centers at the surface pro-
moted transport of ice away from the coasts of Siberia and Alaska toward the North
Pole, as well as strong melt in the East Siberian and Chukchi seas. The pattern also
led to a strong surface pressure gradient across the Fram Strait (between Greenland
and Svalbard) enhancing wind-driven transport of sea ice out of the Arctic Ocean
and into the North Atlantic.
J. Wang and colleagues (
2009
) developed a framework for understanding this
pattern through an EOF analysis of seasonal mean SLP for the region north of 70
o
N.
They defined the first (leading) EOF mode as the AO (NAM), which for summer
has a seasonal structure very similar to that shown by Ogi et al. (
2004
). The second
mode, which they termed the Arctic Dipole Anomaly, has two centers of action of
opposing sign, one over the northern Beaufort Sea and one over the Kara Sea. The
positive phase of the Arctic Dipole Anomaly is characterized by a strong, north-
ward-shifted Beaufort High and negative SLP anomalies centered over the Kara
Sea. In their framework, the SLP anomaly pattern shown in
Figure 4.24
linked to
the extreme 2007 sea ice minimum (as well as other recent extreme minima in the
record), is consistent with the combination of a positive summer mode of the Arctic
Dipole Anomaly and a negative mode of the summer AO.
While subsequent studies questioned the statistical robustness of the Arctic
Dipole Anomaly pattern, which as discussed was computed using data for a rather
limited spatial domain, the term Arctic Dipole Anomaly (or more simply, Dipole
Anomaly) has nevertheless stuck. In common usage, it refers to a summer circula-
tion pattern in which SLP is anomalously high over the northern Beaufort Sea and
unusually low over northeastern Siberia (positive phase) or vice versa (negative
phase), with the understanding that the positive (negative) phase favors (disfavors)
