Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across theWestern Americas - Dendroclimatology - page 178

Decadal Oscillation in the extratropical north Pacific, the Pacific Interdecadal Mode
in the whole Pacific basin, and as the Global Residual (GR) index on a global scale
(Mantua et al. 1997 ; Garreaud and Battisti 1999 ; Enfield and Mestas-Nuñez 2000 ) .
Decadal variability in the climate of the Atlantic basin has also been identified
(Deser and Blackmon 1993 ) , but its interhemispheric climate effects on the Western
Cordilleras are less well known.
The Arctic and Antarctic Oscillations are the dominant modes of climate vari-
ability at the highest latitudes in both hemispheres. The positive state of these
annular modes is associated with intensified subtropical highs and strong polar lows,
which drive a strong extratropical circulation. They also exhibit short- and long-term
modes of variability.
7.2.1 El Niño/Southern Oscillation (ENSO)
Modern interannual variations in the Pacific basin and their interhemispheric effects
on the Western Cordilleras have been extensively documented. The pattern of sea
surface temperature associated with ENSO, measured as the SST anomalies from
6 ◦ Nto6 ◦ S, 180 ◦ to 90 ◦ W (the Cold Tongue [CT] index of Deser and Wallace
1990 ) , are indicated by correlations mapped in Fig. 7.2 . Positive correlations indi-
cate regions where the ocean is warmer when the index is positive. As can be
expected by the nature of the CT index, the strongest correlation with SST occurs in
the tropical Pacific. The subtropical north and south Pacific Oceans are dominated
by anomalies out of phase with the ones occurring in the tropical Pacific, forming a
symmetric pattern about the equator.
The continental effects of the ENSO-related climate variations in terms of sur-
face air temperature (SAT) and precipitation along the Western Cordilleras are
remarkably symmetric about the equator (Fig. 7.3a , b ) . Positive temperatures in
the tropical Pacific are associated with deeper than normal Aleutian lows, and
the resulting steep north-south gradient in the middle latitudes brings storms and
precipitation to the southwestern and southeastern parts of North America. In a
rough parallel to the circulation changes in the Northern Hemisphere, the steeper
gradient in pressure over the southeastern Pacific related to El Niño events, corre-
sponds to deflections of the low-pressure systems and the associated storms towards
the subtropical belt of South America, increasing precipitation in central Chile
(Fig. 7.3b ) . Positive CT indices are associated with warmer than normal surface
conditions all along the American Cordilleras (Fig. 7.3a ) . El Niño brings cool
temperatures to the southeastern United States and to the eastern Amazon basin
(Dettinger et al. 2001 ) .
7.2.2 Pacific Interdecadal Mode
Using instrumental records, several studies have reported Pacific decadal-scale
oscillatory modes (Trenberth and Hurrell 1994 ; Mantua et al. 1997 ; Zhang et al.