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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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[1] The reasons for the accelerated glacier retreat observed since the early 1980s in the tropical Andes are analyzed based on the well-documented Chacaltaya glacier (Bolivia). Monthly mass balance measurements available over the entire 1991-2001 decade are interpreted in the light of a recent energy balance study performed on nearby Zongo glacier and further put into a larger-scale context by analyzing the relationship with oceanatmosphere dynamics over the tropical Pacific-South American domain. The strong interannual variability observed in the mass balance is mainly dependent on variations in ablation rates during the austral summer months, in particular during DJF. Since high humidity levels during the summer allow melting to be distinctly predominant over sublimation, net all-wave radiation, via albedo and incoming long-wave radiation, is the main factor that governs ablation. Albedo depends on snowfall and a deficit during the transition period and in the core of the wet season (DJF) maintains low albedo surfaces of bare ice, which in turn leads to enhanced absorption of solar radiation and thus to increased melt rates. On a larger spatial scale, interannual glacier evolution is predominantly controlled by sea surface temperature anomalies (SSTA) in the eastern equatorial Pacific (Niño 1+2 region). The glacier mass balance is influenced by tropical Pacific SSTA primarily through changes in precipitation, which is significantly reduced during El Niño events. The more frequent occurrence of El Niño events and changes in the characteristics of its evolution, combined with an increase of near-surface temperature in the Andes, are identified as the main factors responsible for the accelerated retreat of Chacaltaya glacier.