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要旨

A simulation of the stable water isotope cycle over South America by the regional circulation model REMO_iso is discussed. The performance of the regional model, with a resolution of 0.5° (∼55 km), is compared to simulations by the global circulation model ECHAM_iso at two coarser resolutions and evaluated against observations of precipitation and δ¹⁸O. Here REMO_iso is demonstrated to reproduce reasonably well climatic and isotopic features across South America. This paper explores further insights of δ¹⁸O as a climate proxy, based on REMO_iso’s improvements as compared to ECHAM_iso. In particular, the authors focus on the seasonal variation of the amount effect (δ¹⁸O decrease with precipitation amounts) and the anomalous δ¹⁸O continental gradient across the Amazon basin, as inferred from the REMO_iso, ECHAM_iso, and GNIP datasets. The finer resolution of topography in REMO_iso enables a detailed analysis of the altitude effect: not only the first, but also the second derivative of δ¹⁸O with altitude is considered. It appears that high-altitude grid cells show an isotopic signature similar to Rayleigh distillation, in accordance with experimental studies. Finally, a Lagrangian reference frame is adopted to describe the evolution of δ¹⁸O in precipitation along its trajectory, in order to relate the simulation analysis to the fractionation mechanisms. This confirms that the amount effect, via Rayleigh distillation processes, is dominant during the wet season. During the dry season, the δ¹⁸O in precipitation is controlled by isotopic reequilibration of rain droplets with surrounding vapor, reflecting the impact of nonfractionating transpiration by the vegetation.