General circulation models (GCMs) are capable of simulating certain statistical characteristics of the composite El Ni¿o-Southern Oscillation (ENSO) cycle. Here we demonstrate that the NCAR Climate System Model (CSM), in absence of external forcing and data assimilation, simulates both temporal variations and spatial patterns associated with the observed ENSO cycles during 1980s. The model also produces a distinct decadal change of ENSO variations that resembles the observed shift from 1980s (strong and well-defined cycles) to the early 1990s (weak and short-lived warm phases) followed by the strong 1997 El Ni¿o. The study thus suggests that the ENSO evolution during 1980s and the decadal shift in 1990s might be a result of natural variability intrinsic to the interactive climate system. Given the perceived capability to simulate natural variations, the CSM can be used to supplement observational data to better understand climate variability on interannual-to-decadal timescales. ¿ 1998 American Geophysical Union