Geoscience Reference
In-Depth Information
Fig. 7.4
High-latitude annular modes. The Southern Annular Mode (SAM) or Antarctic
Oscillation (AAO) mode is moderately symmetric about the pole (
left
), but due to the more
complex distribution of the northern continents, the Northern Annular Mode (NAM) or Arctic
Oscillation (AO) is more evident over the North Atlantic and the north Pacific Oceans. Zonal-mean
geopotential height fields are represented with contour intervals of 5 m (modified from Thompson
counterpart, especially over Europe, the Sea of Okhotsk, and northern America. For
example, the summer NAM pattern accounts for many of the anomalous weather
features observed during the summer of 2003. Temperature anomalies over north-
western Eurasia, northeastern Siberia, and Canada during that period exceeded 3
◦
C
The positive polarity of the Southern Hemisphere Annular Mode is associated
with cold anomalies over most of Antarctica. The one notable exception is the
Antarctic Peninsula and southern South America, where the enhanced westerlies
related to the high SAM polarity increase the advection of relatively warm oceanic
toward stronger circumpolar flow is in the same sense as the trends that have domi-
nated the Northern Hemisphere extratropical circulation over the past few decades.
The occurrence of positive trends in both the NAM and SAM suggests that the trends
reflect processes that transcend the high-latitude climate of a particular hemisphere.
7.3 Tree-Ring Records Across the Western Americas
A major result of the Collaborative Research Network has been the consolidation
and expansion of tree-ring collections across the traditional research regions of
North and South America, the focusing on key areas, and the start of many devel-
opments in new regions of Canada, Mexico, Peru, Bolivia, Chile, and Argentina
(Boxes
7.1
,
7.2
,
7.3
,
7.6
and
7.8
). Along the western coasts of North and South
