Mathias Vuille

Insolation-driven changes in atmospheric circulation over the past 116,000 years in subtropical Brazil

During the last glacial period, large millennial-scale temperature oscillations—the ‘Dansgaard/Oeschger’ cycles—were the primary climate signal in Northern Hemisphere climate archives from the high latitudes to the tropics. But whether the influence of these abrupt climate changes extended to the tropical and subtropical Southern Hemisphere, where changes in insolation are thought to be the main direct forcing of climate, has remained unclear. Here we present a high-resolution oxygen isotope record of a U/Th-dated stalagmite from subtropical southern Brazil, covering the past 116,200 years. The oxygen isotope signature varies with shifts in the source region and amount of rainfall in the area, and hence records changes in atmospheric circulation and convective intensity over South America. We find that these variations in rainfall source and amount are primarily driven by summer solar radiation, which is controlled by the Earths precessional cycle. The Dansgaard/Oeschger cycles can be detected in our record and therefore we confirm that they also affect the tropical hydrological cycle, but that in southern subtropical Brazil, millennial-scale climate changes are not as dominant as they are in the Northern Hemisphere.

The climate of the Altiplano: observed current conditions and mechanisms of past changes

Abstract The large-scale controls on the climate of the South American Altiplano are investigated using local observations, reanalysis data and general circulation model experiments. The objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial–interglacial. Our results suggest that, on all timescales, the climatic conditions on the Altiplano are closely related to the upper-air circulation, with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions. Different factors influence the upper-air circulation on the different timescales. Intraseasonal variability is a reflection of the position and intensity of the Bolivian High, which is modulated by Rossby waves emanating from the midlatitude South Pacific. The annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere, rather than direct insolation forcing over the Altiplano or moisture changes in the source area. Interannual variability is primarily related to changes in the mean zonal flow over the Altiplano, 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. Orbitally forced changes in the land–sea contrast drive continental-scale circulation changes, which significantly alter the zonal flow over the Altiplano. On glacial–interglacial timescales, the contrast in heating between northern and southern hemispheres during the glacial leads to upper-air easterly anomalies throughout the tropics. On modern timescales the marked submonthly, seasonal and interannual changes of moisture over the Altiplano cannot be accounted for by moisture changes in the humid tropical lowlands. However, the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands, which counteracts the effect of the upper-level circulation, resulting in little overall change in precipitation. This observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the Altiplano on a wide range of timescales.

20th Century Climate Change in the Tropical Andes: Observations and Model Results

Linear trend analysis of observational data combined with model diagnostics from an atmospheric general circulation model are employed to search for potential mechanisms related to the observed glacier retreat in the tropical Andes between 1950 and 1998. Observational evidence indicates that changes in precipitation amount or cloud cover over the last decades are minor in most regions and are therefore rather unlikely to have caused the observed retreat. The only exception is in southern Peru and western Bolivia where there is a general tendency toward slightly drier conditions. Near-surface temperature on the other hand has increased significantly throughout most of the tropical Andes. The temperature increase varies markedly between the eastern and western Andean slopes with a much larger temperature increase to the west. Simulations with the ECHAM-4 model, forced with observed global sea surface temperatures (SST) realistically reproduce the observed warming trend as well as the spatial trend pattern. Model results further suggest that a significant fraction of the observed warming can be traced to a concurrent rise in SST in the equatorial Pacific and that the markedly different trends in cloud cover to the east and west of the Andes contributed to the weaker warming east of the Andes in the model. The observed increase in relative humidity, derived from CRU 05 data, is also apparent in the model simulations, but on a regional scale the results between model and observations vary significantly. It is argued that changes in temperature and humidity are the primary cause for the observed glacier retreat during the 2nd half of the 20th century in the tropical Andes.

Climate change projections for the tropical Andes using a regional climate model: Temperature and precipitation simulations for the end of the 21st century

[1] High-elevation tropical mountain regions may be more strongly affected by future climate change than their surrounding lowlands. In the tropical Andes a significant increase in temperature and changes in precipitation patterns will likely affect size and distribution of glaciers and wetlands, ecosystem integrity, and water availability for human consumption, irrigation, and power production. However, detailed projections of future climate change in the tropical Andes are not yet available. Here we present first results for the end of the 21st century (2071‐2100) using a regional climate model (RCM) based on two different emission scenarios (A2 and B2). The model adequately simulates the spatiotemporal variability of precipitation and temperature but displays a cool and wet bias, in particular along the eastern Andean slope during the wet season, December‐ February. Projections of changes in the 21st century indicate significant warming in the tropical Andes, which is enhanced at higher elevations and further amplified in the middle and upper troposphere. Temperature changes are spatially similar in both scenarios, but the amplitude is significantly higher in RCM-A2. The RCM-A2 scenario also shows a significant increase in interannual temperature variability, while it remains almost unchanged in RCM-B2 when compared to a 20th century control run. Changes in precipitation are spatially much less coherent, with both regions of increased and decreased precipitation across the Andes. These results provide a first attempt at quantifying future climate change in the tropical Andes and could serve as input for impact models to simulate anticipated changes in Andean glaciation, hydrology, and ecosystem integrity.

Interannual climate variability in the Central Andes and its relation to tropical Pacific and Atlantic forcing

The main spatiotemporal modes of interannual temperature and austral summer (DJF) precipitation variability in the Central Andes are identified based on a two-way principal compo- nent 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 me- teorological fields, based on National Centers for Environmental Prediction / National Center for Atmospheric Research CNCEP/NCAR) reanalysis, NOAA-Outgoing Longwave Radiation (OLR) and station data, are analyzed during periods of strong positive and negative SSTA and the respec- tive composites tested for local significance using a Students t-test approach. Temperature vari- ability in the Central Andes is primarily related to E1Nifio - 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 E1 Nifio (La Nifia). 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 re- versed 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 up- per air monsoon return flow, altering the strength of the NE trades that emanate from the Sahara High.

Atmospheric circulation over the Bolivian altiplano during DRY and WET periods and extreme phases of the southern oscillation

The atmospheric circulation over the Bolivian Altiplano during composite WET and DRY periods and during HIGH and LOW index phases of the Southern Oscillation was investigated using daily radiosonde data from Antofagasta (Chile), Salta (Argentina), Lima (Peru) and La Paz (Bolivia), daily precipitation data from the Bolivian/Chilean border between 18° and 19°S and monthly NCEP (National Centers for Environmental Prediction) reanalysis data between 1960 and 1998. In austral summer (DJF) the atmosphere during WET periods is characterized by easterly wind anomalies in the middle and upper troposphere over the Altiplano, resulting in increased moisture influx from the interior of the continent near the Altiplano surface. The Bolivian High is intensified and displaced southward. On the other hand, westerly winds usually prevail during DRY summer periods, preventing the moisture transport from the east from reaching the western Altiplano. Precipitation tends to be deficient over the western Bolivian Altiplano during LOW index summers and above average during HIGH and LOW+1 summers, but the relation is weak and statistically insignificant. LOW summers feature broadly similar atmospheric circulation anomalies as DRY periods and can be regarded as an extended DRY period or as a summer with increased occurrence of DRY episodes. HIGH summers, and to a lesser degree LOW+1 summers, are characterized by broadly opposite atmospheric characteristics, featuring a more pronounced Bolivian High located significantly further south, and easterly wind anomalies over the Altiplano. In winter (JJA) precipitation events are rare; these are associated with increased northerly and westerly wind components, reduced pressure and temperature, and increased specific humidity over the entire Altiplano. Atmospheric circulation anomalies during LOW periods are less pronounced in austral winter (JJA) than in summer, but generally feature similar changes (increased temperatures and a vertically expanded troposphere). However, the significance of these anomalies, especially with regard to the wind pattern, varies depending on station and pressure level. Accordingly, precipitation during austral winter shows no relationship with the extremes of the Southern Oscillation. Copyright

Mean annual temperature trends and their vertical structure in the tropical Andes

Mean annual temperature trends in the tropical Andes were determined over the last six decades (1939–1998), to investigate the apparent inconsistency between the observed glacier retreat and the reported slight cooling trend in the lower tropical troposphere after 1979. Our results indicate that temperature in the tropical Andes has 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) and the last two years of the series, associated with the 1997/98 El Nino, were the warmest of the last six decades. Temperature trends vary with altitude and show a generally reduced warming with increasing elevation. 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. Clearly high elevation surface stations in the Andes do not reflect the slight cooling trend observed in the tropical lower-troposphere.

Climate Variability in the Andes of Ecuador and Its Relation to Tropical Pacific and Atlantic Sea Surface Temperature Anomalies

The main spatiotemporal modes of seasonal precipitation and temperature variability in the Andes of Ecuador (18N-48S) and their relation to tropical Pacific and Atlantic sea surface temperature anomalies (SSTAs) between 1963-92 are identified based on rotated principal component analysis and cross-correlation techniques. Outgoing longwave radiation composites are analyzed during periods of strong oceanic forcing to confirm the proposed physical mechanisms. 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 Nino-Southern Oscillation influence is most dominant in the northwestern part of the Andes during December-February (DJF) and in the eastern Cordillera during June-August (JJA) and in both cases associated with below- (above-) average precipitation during El Nino (La Nina) years. During most of the year precipitation variability over the eastern Andes is related to a dipolelike 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 over the eastern Cordillera. The only region with slightly increased precipitation during El Nino events is confined to a narrow area along the western Andean slope between 18 and 38S in close proximity to the Pacific. However, this relationship is weak and only apparent in DJF. Temperature variability in the Andes can largely be explained by SSTA in the tropical Pacific domain. The temperature response closely follows SSTA in the Nino-3 and Nino-3.4 regions with approximately one-month lag. The northernmost part of the Andes (north of 0.58N) is the only region where temperatures are significantly correlated with tropical North Atlantic SSTA.

Holocene paleohydrology and glacial history of the central Andes using multiproxy lake sediment studies

Here we document at century to millennial scale the regional changes of precipitation^evaporation from the late Pleistocene to present with multiproxy methods on a north^south transect of lake sites across the eastern cordillera of the central Andes. The transect of study sites covers the area from V14‡S to 20‡S and includes core studies from seven lakes and modern calibration water samples from twenty-three watersheds analyzed to constrain the down-core interpretations of stable isotopes and diatoms. We selected lakes in different hydrologic settings spanning a range of sensitivity to changes in the moisture balance. These include: (1) lakes directly receiving glacial meltwater, (2) overflowing lakes in glaciated watersheds, (3) overflowing lakes in watersheds without active glaciers, and (4) lakes that become closed basins during the dry season. The results of our current work on multiple lakes in the Bolivian Andes show that while the overall pattern of Holocene environmental change is consistent within the region, conditions were not always stable over centennial to over millennial timescales and considerable decadal- to centuryscale climate variability is evident [Abbott et al., Quat. Res. 47 (1997) 70^80, Quat. Res. 47 (1997) 169^180, Quat. Sci. Rev. 19 (2000) 1801^1820; Polissar, Master’s thesis, University of Massachusetts (1999)]. Comparison of the paleoclimate record from one well-studied site, Lago Taypi Chaka Kkota (LTCK), with others within the region illustrates a consistent overall pattern of aridity from the late glacial through the middle Holocene. Previous work noted a difference between the timing of water-level rise in Lake Titicaca V5.0^3.5 ka B.P. [Abbott et al., Quat. Res. 47 (1997) 169^180; Cross et al., Holocene 10 (2000) 21^32; Rowe et al., Clim. Change 52 (2002) 175^199] and the onset of wetter conditions at 2.3 ka B.P. in LTCK, a lake that drains into the southern end of Lake Titicaca [Abbott et al., Quat. Res. 47 (1997) 70^80]. Sedimentary and oxygen isotope evidence from Paco Cocha (13‡54PS) located in the northern reaches of the expansive 57 000 km 2 Titicaca watershed, which spans V14‡S to 17‡S, indicates that

Interannual Variability of Summertime Convective Cloudiness and Precipitation in the Central Andes Derived from ISCCP-B3 Data

The interannual variability of austral summer [December‐January‐February‐March (DJFM)] convective activity and precipitation in the central Andes (158‐308S) is investigated between 1983 and 1999 based on in situ rain gauge measurements, International Satellite Cloud Climatology Project (ISCCP) reduced radiance satellite data (the B3 dataset), and National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) reanalysis data. Twice-daily ISCCP-B3 calibrated infrared data, corrected for limb-darkening effects and representing equivalent blackbody temperatures Tb emitted by clouds are used to derive seasonal composites of fractional cold cloud coverage F*. Comparison of in situ rain gauge measurements with F* show a good correlation when a temperature threshold Tb 5 240 K is used to derive F*. A rotated empirical orthogonal function (REOF) applied to the seasonal estimates of F* yielded three spatially separated modes of convective activity in the south, northwest, and northeast of the central Andes. Results indicate that precipitation variability in the central Andes shows less spatial coherence than previously thought, with many years showing an antiphasing of wet/dry conditions between the northern and southern part of the study area. Regression analyses confirm the crucial role of both intensity and location of upper-air circulation anomalies with easterly wind anomalies favoring wet conditions, and westerly winds producing dry conditions. Two different forcing mechanisms are identified as main causes of upper-air zonal wind anomalies in the northern and southern part of the central Andes, respectively. Easterly wind anomalies during wet summers in the northern part are in geostrophic balance with reduced meridional baroclinicity due to low-latitude (midlatitude) cooling (warming), consistent with earlier studies. Farther to the south, easterly wind anomalies during wet summers are the result of an upper-air anticyclonic anomaly centered over southeastern South America, leading to a relaxation of the upper-air westerly winds and episodic easterly transport of humid air toward the subtropical Andes. This pattern is similar to one of the leading modes of intraseasonal variability, related to extratropical Rossby wave dispersion and modulation of the position of the Bolivian high. Correlation analysis of F* with near-surface specific humidity reveals that humidity variations in the lowlands to the east are not relevant on interannual time scales for the more humid northern part of the Altiplano. In the southern Altiplano, however, there is a significant correlation between convective activity and precipitation at high elevation and the low-level humidity content to the southeast of the Andes.