We analyze the interannual variability of the summer monsoon rainy season in South America and its relationship with SST as simulated by the ocean-atmosphere coupled model ECHAM5-OM for present-day conditions (1961--1990) and future A2 emission scenario (2071--2100). The first mode of model precipitation variability, both in spring and summer, is associated with El Ni¿o-Southern Oscillation (ENSO). In both seasons it features a dipole of anomalies between northern and southeastern South America. These modes correspond, with some differences, to the first variability mode of observed spring precipitation, and to the third variability mode of observed summer precipitation, which are also associated with ENSO. The relationship between ENSO events and precipitation variability in southeastern South America weakens for the A2 scenario, especially in spring, which is presently the season with strongest ENSO-related impact. The weakened teleconnection is probably due to the reduction of the SST subtropical latitudinal gradient in the ENSO mode.