Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across theWestern Americas - Dendroclimatology - page 206

7.3.2.1 Subtropical Precipitation and ENSO

Precipitation variations in the United States-Mexico, the Bolivian Altiplano, and
central Chile are related to climatic changes in the tropical Pacific. It is well
known that there is a strong teleconnection between SST changes in the tropi-
cal Pacific and precipitation anomalies in the southern United States and northern
Mexico (Ropelewski and Halpert 1986 ; Kiladis and Diaz 1989 ; Cole and Cook
1998 ) . Warmer SSTs in the tropical Pacific typically result in increased precipi-
tation anomalies in this region. Spatial correlations between different indices of
tropical Pacific circulation and the grid point PDSI series over the conterminous
United States show that the geographic location of the highest correlation field is
the southwestern United States (Cook et al. 2000 ) , a finding that is consistent with
the patterns identified by using instrumental records.
Interannual variability in precipitation over the altiplano is primarily related
to changes in the mean zonal flow, reflecting changes in meridional baroclinicity
between tropical and subtropical latitudes, which in turn is a response to sea surface
temperature changes in the tropical Pacific (Garreaud et al. 2003 ) . There is a general
agreement that a significant fraction of interannual variability in summer precipita-
tion is related to ENSO (e.g., Aceituno 1988 ; Lenters and Cook 1999 ; Vuille 1999 ;
Vuille et al. 2000 ; Garreaud et al. 2003 ) . All of these studies concluded that El Niño
years tend to be dry, whereas La Niña years are often associated with wet conditions
on the altiplano. However, dry La Niña years and wet El Niño years are not com-
pletely uncommon, which indicates that the relationship between SST anomalies in
the tropical Pacific and precipitation in the central Andes is not simple (Garreaud
et al. 2003 ) . Finally, relationships between SSTs in the equatorial Pacific and precip-
itation anomalies in central Chile (30º 35ºS) have been reported by several authors

(Quinn and Neal 1983 ; Aceituno 1988 ; Rutlland and Fuenzalida 1991 ; Aceituno
and Montecinos 1996 ; Montecinos and Aceituno 2003 ) . Positive rainfall anomalies
in central Chile are associated with warmer SSTs in the tropical Pacific. Conversely,
cold SSTs correspond quite closely to dry conditions in the area.
To gain insights into the long-term relationships between SST in the tropical
Pacific and precipitation in the southwestern United States, the Bolivian Altiplano,
and central Chile, we compared precipitation-sensitive records from these three
regions with a multiproxy-based reconstruction of SST for the El Niño-3 region
(Mann et al. 2000 ) . Through exploiting the complementary information shared by
a wide network of different types of proxy climate indicators, the multiproxy El
Niño-3 reconstruction reduces the weaknesses in any individual type or location
of indicator and makes use of the mutual strength of the diversity in the records.
The reconstructed eastern equatorial Pacific Niño-3 areal-mean SST index has been
previously used as a direct indication of ENSO itself for the past 400 years (Mann
et al. 2000 ) . A large proportion of the tree-ring chronologies from the southwestern
United States and Mexico have been used as predictors of both the Niño-3 index and
PDSI reconstructions, which make the reconstructions not statistically independent.
In contrast, neither the El Asiento nor the Bolivian Altiplano chronologies have been
included in the Niño-3 index reconstruction.

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