Swiss-NSF 8220050401 P.I.: M.
Vuille
NSF ATM-9707698, P.I.: R.S.
Bradley
Project summary
Altiplano
The main spatiotemporal modes of interannual temperature and
austral summer (DJF) precipitation variability in the Central Andes were
identified based on a two-way principal component analysis (PCA) of 30-year
(1961-1990) monthly station data and related to contemporaneous tropical
Pacific and Atlantic sea surface temperature anomalies (SSTAs). In addition,
various meteorological fields, based on National Centers for Environmental
Prediction / National Center for Atmospheric Research (NCEP/NCAR) reanalysis,
NOAA-Outgoing Longwave Radiation (OLR), Andean radiosonde and station data,
were analyzed during periods of strong positive and negative SSTA and the
respective composites tested for local significance using a Student’s t-test
approach. In addition we showed how the atmospheric circulation over the
Bolivian Altiplano changes during periods of extreme WET and DRY phases
and during HIGH and LOW index periods of the Southern Oscillation, which
is of special importance for the interpretation of the Nevado Sajama ice
core.
Temperature variability in the Central Andes is primarily related to
El Niño – Southern Oscillation (ENSO) and closely follows SSTA in
the central equatorial Pacific with a lag of 1-2 months. In the southern
Altiplano, temperatures have significantly increased since the late 1970s.
DJF precipitation is also primarily related to ENSO, featuring below (above)
average precipitation during El Niño (La Niña). Precipitation
over the dry western part of the Altiplano shows the closest relationship
with ENSO, due to ENSO-induced atmospheric circulation anomalies. Precipitation
variability over the western Altiplano features a decadal-scale oscillation,
related to a similar climatic shift in the tropical Pacific domain in the
late 1970s. Over the northern Altiplano the precipitation signal is reversed
in the austral summer following the peak phase of ENSO, presumably due
to the temporal evolution of tropical Pacific SSTA, rapidly switching from
one state to the other. No evidence for a tropical Atlantic influence on
DJF precipitation was found. SSTAs in the tropical NE Atlantic, however,
presumably are influenced by heating and convection over the Altiplano
through an upper air monsoon return flow, altering the strength of the
NE trades that emanate from the Sahara High.
Ecuador
In the Andes of Ecuador
(1°N - 4°S) the main spatiotemporal modes of seasonal precipitation
and temperature variability were identified in a similar way, 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) 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
Temperature trends in the tropical Andes are determined
over the last six decades (1939-1998) based on a newly established station
database including 268 stations between 1°N and 23°S and from 0
m to 5000 m a.s.l. Annual average temperature time series are determined
using the First Difference method and trends are estimated using both an
ordinary least squares (OLS) and a more robust least absolute residuals
(LAR) approach. Temperatures in the tropical Andes have increased by 0.10
- 0.11°C/decade since 1939. The rate of warming has more than tripled
over the last 25 years (0.32 - 0.34°C/decade). Of the last 13 years
only one (1996) was below average, and the last two years of the series,
associated with the 1997/98 El Niño, were the warmest of the last
six decades. There is a general decrease of the observed warming trend
with increasing elevation, except for the lower eastern slopes of the Andes,
where the trend is insignificant. On the Pacific side, below 1000 m, temperatures
have increased by 0.39 - 0.40°C/decade since 1959, while temperatures
have increased only by 0.09 - 0.16°C/decade above 4000 m. However,
despite the lower rate of warming, the trend toward increased temperatures
is still significant at the 95% confidence level, even at the highest elevations.
Related Publications
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