The recent number isotopic records extracted from Andean ice cores (South America) has illustrated the key role such archives can play in past climate reconstructions. Nevertheless, interpreting isotopic archives as quantified climate proxies requires an understanding of which climate parameters control the stable isotopic composition of water. Mesoscale modeling sheds new light on the meteorological mechanisms dominant during austral summer. Here we focus on the variability of the South Atlantic Convergence Zone (SACZ) and its repercussions on upstream regions. The SACZ is a major component of the South American Monsoon System (SAMS). The present study uses the isotopic signature of the SAMS, as simulated by the stable water isotope enabled regional circulation model REMOiso to answer the question: how does the SAMS affect the isotopic composition of precipitation during the wet season? In order to analyze the internal, purely atmospheric variability mode, the model was forced by climatological sea-surface temperatures. We investigate the concurrent intraseasonal variability of meteorological and isotopic parameters at pentad (5 days) interval using empirical orthogonal functions (EOFs). REMOiso reproduces the main meteorological characteristics of the SAMS consistent with observations as well as previous modeling studies. Furthermore, we demonstrate that δ18O integrates both circulation and precipitation variability. This new evidence contributes to the comprehension of the δ18O signal in tropical South America, highlighting the internal atmospheric variability, as opposed to external forcing by Pacific and Atlantic sea-surface temperature.