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[1] We use two atmospheric general circulation models (AGCMs), the ECHAM-4 and the GISS II models, to analyze the interannual variability of d 18 O in precipitation over the tropical Americas. Several different simulations with isotopic tracers forced with observed global sea surface temperatures (SST) between 1950 and 1998 reveal the influence of varying temperature, precipitation amount, and moisture source contributions on the predicted d 18 O distribution. Observational evidence from climatic (NCEP-NCAR) and sparse stable isotope (IAEA-GNIP) data is used to evaluate model performance. The models capture the essential features of surface climate over the tropical Americas in terms of both their spatial and temporal characteristics. Using a low-resolution model (GISS II), adjusted to provide a more realistic Andean topography, or a higher-resolution model (ECHAM-4 T106) leads to an improved d 18 O distribution over the tropical Americas with an altitude effect comparable to observations. Water vapor transport and gradual rainout and increasingly depleted composition of water vapor along its trajectory are correctly simulated in both models, although the ECHAM model appears to underestimate the continentality effect over the Amazon basin. A significant dependence of d 18 O on the precipitation amount is apparent in both models, in accordance with observations, while the influence of temperature seems to be less significant in most regions and is accurately reproduced by the ECHAM model only. Over most regions, however, the d 18 O signal in precipitation is influenced by a combination of factors, such as precipitation amount, temperature, moisture source variability, and atmospheric circulation changes. Over parts of the tropical Americas, the d 18 O signal is therefore also significantly correlated with ENSO because ENSO is an integrator of many factors affecting the d 18 O composition of precipitation.

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The interannual variability of austral summer [December-January-February-March (DJFM)] convective activity and precipitation in the central Andes (15-30S) 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 T b 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 T b 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 (mid-latitude) 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.