Geoscience Reference
In-Depth Information
summer ice loss. The summer Arctic Dipole Anomaly defined in this way is clearly
linked to the phase of the summer AO as identified by Ogi et al. (
2004
) which is in
turn linked to the strength of the summer Arctic cyclone maximum over the central
Arctic Ocean discussed by Serreze and Barrett (
2008
). Overland et al. (
2012
) show
that the Arctic Dipole Anomaly has been a persistent feature of the summer atmo-
spheric circulation over the 2007-2012 period, especially in June. Whether this pat-
tern will continue to persist remains to be seen.
4.7.5
Pacific North American Pattern
The Pacific North American pattern, commonly referred to as the PNA, refers to the
amplitude of the atmospheric longwave pattern (the “wavetrain”) extending from
the North Pacific Ocean through North America. The positive phase of the PNA
features above-average pressure heights in the vicinity of Hawaii, below-average
heights located south of the Aleutian Islands, above average heights over the inter-
mountain region of North America, and below average heights over the southeastern
United States. When the anomaly pattern in pressure heights is the reverse, such
that the amplitude of the wavetrain is smaller than normal, the PNA is in its neg-
ative phase. The PNA is one of the most important modes of variability affecting
North America in winter, with the positive phase linked to above-average tempera-
tures over western Canada, extending well into the Arctic and including Alaska, and
below-average temperatures across the south-central and southeastern U.S.
The index of the PNA is typically based on an EOF analysis of the geopotential
height field. The PNA is strongly influenced by the phase of ENSO. The positive
phase tends to be associated with El-Nino while the negative phase tends to be
associated with La-Nina. Although the PNA is best expressed in winter, it can be
identified throughout the year. When evaluated on a monthly basis, such as is done
by the NOAA Climate Prediction Center, the troughs and ridges in the wavetrain are
seen to vary in intensity and location through the year. From
Figure 4.25
, it is evi-
dent that the main centers of action of the PNA pattern for the summer months are
weaker than those in winter (the correlation between time series of the PNA index
and pressure heights is lower), are shifted poleward, and are more closely spaced.
During summer, a prominent center of action in lies over the Arctic Ocean.
M. L'Heureux et al. (
2008
) viewed the unusual summer Arctic circulation of
2007 as an expression of an extreme positive phase of the summer PNA wavetrain.
Note from
Figures 4.24
and
4.25
the close relationship between the pronounced
500 hPa ridge extending into the Beaufort Sea, the strong Beaufort Sea High at the
surface lying just east of the ridge axis, and the location of the summer center of
action of the PNA located over the Arctic Ocean. In an attempt to find some com-
mon ground between different studies, Serreze and Barrett (
2010
) showed that a
strong Beaufort High tends to be a feature of the positive phase of the summer PNA,
the negative phase of the summer Northern Annular Mode, as well as the positive
phases of the summer Arctic Dipole Anomaly and Pacific Decadal Oscillation. The
