Swiss-NSF 8220050401 P.I.: M.
Vuille
NSF ATM-9707698, P.I.: R.S.
Bradley
Project summary
Altiplano
The atmospheric circulation over the Bolivian Altiplano, where an ice
core was recovered in 1997 on Nevado Sajama (Thompson et al., 1998), was
investigated during composite periods of extreme WET and DRY phases and
during HIGH and LOW index situations of the Southern Oscillation, using
daily radiosonde and precipitation data and monthly NCEP (National Centers
for Environmental Prediction) reanalysis data between 1960 and 1998. We
showed how precipitation variability near the ice core drilling site in
the western Bolivian Andes relates to changes in the atmospheric circulation.
Based on Principal Component Analysis (PCA), the influence of the ENSO
phenomenon and Sea Surface Temperature Anomalies (SSTA) in the tropical
Pacific and Atlantic upon precipitation and temperature anomalies were
examined, which is especially important for an accurate calibration and
interpretation of the Sajama ice core.
Ecuador
In the Andes of Ecuador (1°N - 4°S) the main spatiotemporal
modes of seasonal precipitation and temperature variability were identified
based on a PCA of monthly station data. The score time series of the main
modes were correlated with tropical Pacific and Atlantic Sea Surface Temperature
Anomalies (SSTA) at varying time-lags to detect areas of significant oceanic
forcing. To confirm the results, a reverse procedure was applied, by extracting
the main modes of tropical Pacific and Atlantic SSTA, again by means of
PCA. The score time series of these main SSTA modes were then correlated
against station precipitation and temperature anomalies to see whether
similar coherency patterns emerged. In most cases, this two-way approach
strengthened the findings obtained and the results compared very favorably.
A prime result of our analysis is that despite the close proximity to the
Pacific, precipitation variability in the Andes of Ecuador is not related
to SSTA in the tropical Pacific domain alone. The El Niño-Southern
Oscillation (ENSO) influence is most dominant in the northwestern part
of the Andes and associated with below (above) average precipitation during
El Niño (La Niña) years. However, precipitation along the
eastern Andean slope is related to a dipole-like correlation structure
in the tropical Atlantic, featuring positive correlations with SSTA to
the south of the ITCZ, and negative correlations to the north. The proposed
mechanism involves positive SSTA in the tropical South Atlantic and contemporaneous
negative SSTA in the tropical North Atlantic, resulting in increased rainfall
on the eastern Andean slopes. The only region with slightly increased precipitation
during El Niño events is confined to a narrow area along the western
Andean slope between 1°-3°S in close proximity to the Pacific.
Temperature variability in the Andes can largely be explained by SSTA in
the Pacific domain. The temperature response closely follows SSTA in the
NINO3 and NINO3,4 regions with approximately one month lag.
Temperature trends
(Insert Abstract GRL here).
Related Publications
Potential future (yellow) and already drilled (blue)
ice coring sites in the tropical Andes. Interannual precipitation and temperature
variability during the last decades was analyzed along the entire tropical
Andean range to facilitate the interpretation of these ice core records.
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