Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across theWestern Americas - Dendroclimatology - page 181

eastern Pacific (the classic key ENSO region) and is not narrowly confined along the
equator. The documented decadal oscillatory mode of Pacific SST shows anomalies
in the western Pacific that extend to the northeast and southeast into the American
subtropics.
Overall, the atmospheric expressions of the ENSO-like climate variations on
both interannual and decadal timescales are remarkably symmetric about the equa-
tor, especially on the Pacific coast of western Americas (Fig. 7.3c , d ) . Positive
variations in the CT and GR are associated with equatorward diversions of the west-
erlies, enhancement of the low-pressure systems, and storms from the midlatitude
Pacific basin toward North and South America subtropical latitudes (Dettinger et al.
2001 ) .

7.2.3 Annular Modes

The Northern Hemisphere (NAM) and Southern Hemisphere (SAM) Annular
Modes dominate extratropical climate variability throughout their respective hemi-
spheres (Thompson and Wallace 2000 ) . The NAM is alternatively referred to
as the North Atlantic Oscillation (NAO) (Hurrell and van Loon 1997 ) and the
Arctic Oscillation (Thompson and Wallace 2000 ) ; whereas the SAM is alternatively
referred to as the High-Latitude Mode (Karoly 1990 ) and the Antarctic Oscillation
(Gong andWang 1999 ; Thompson andWallace 2000 ) .The structures of the Northern
Hemisphere and Southern Hemisphere Annular Modes are shown to be remarkably
similar, not only in the zonally averaged geopotential height and zonal wind fields,
but in the mean meridional circulations as well. Both annular modes are associated
with equivalent barotropic vacillations in the strength of the zonal flow between cen-
ters of action located at ~35 ◦ − 60 ◦ latitude. The SAM is moderately

symmetric about the pole, but due to the more complex distribution of the northern
continents, the NAM is more evident over the north Atlantic and the north Pacific
Oceans (Fig. 7.4 ) .
Periods when the zonal flow along ~55 ◦ −

40 ◦ and ~55 ◦ −

60 ◦ latitude is anomalously westerly
(the so-called high-index polarity of the annular modes) are characterized by lower
than normal geopotential heights and temperatures over the polar cap, and by higher
than normal geopotential heights and temperatures in the middle latitudes centered
at ~45 ◦ . As atmospheric variability in the Northern Hemisphere is largest in winter,
the spatial pattern of the conventional NAM mostly reflects the winter variability
(Thompson and Wallace 2000 ) . The positive polarity of the winter AO is associ-
ated with positive surface air temperature anomalies throughout the high latitudes
of Eurasia and much of North America, and negative anomalies over extreme eastern
Canada, North Africa, and the Middle East. This zonally asymmetric pattern of SAT
anomalies is evident throughout the year except during the Northern Hemisphere
summer months (Thompson and Wallace 2000 ) . The leading mode of the Empirical
Orthogonal Function (EOF) in summer months has a smaller meridional scale than
the conventional NAM. Associated summertime low-level temperature anomalies
show more extended warm anomalies over the midlatitudes than the winter NAM