Interannual tropical-ocean variability and interactions with subtropical/extra-tropical atmospheric forcing are diagnosed in the NCAR Community Climate System Model Version 2.0 (CCSM2). The CCSM2 tropical Pacific ocean-atmosphere system exhibits much stronger biennial variability than is observed. However, a canonical correlation analysis technique decomposes the simulated tropical Pacific boreal-winter sea-surface temperature (SST) evolution into two modes of variability, both of which are related to atmospheric variability during the preceding boreal winter. The first mode of ocean/atmosphere variability is related to the strong biennial oscillation in the simulated equatorial SSTs, and appears primarily driven by an unimpeded delayed-oscillator mechanism. The second mode better captures both the spatial and temporal characteristics of the observed El Niņo/Southern Oscillation (ENSO) system. Investigation of the second mode of simulated variability indicates that basin-wide SST anomalies during the boreal winter are associated with subtropical/extra-tropical sea-level pressure (SLP) anomalies over the central and eastern North Pacific 12 months earlier. Further diagnosis of this mode of subtropical/extra-tropical atmospheric variability and its influence on the evolution of the ENSO system suggests that initially the overlying SLP and trade-wind anomalies produce a basin-scale increase in the isotherm depths through Ekman pumping; at the same time they initiate a change in the slope of the isotherms across the Pacific basin. The combination of these two influences dramatically increases the overall isotherm-depth anomalies over the eastern equatorial Pacific, resulting in the onset and maintenance of El Niņo conditions into boreal summer and fall. Further investigation of the non-stationarity in the correlation statistics between the modes of sub-tropical/extratropical SLP variability and the subsequent development of ENSO events indicate that part of the inter-decadal variability in the observed ENSO system may be due to changes in the variance of subtropical SLP-related ENSO forcing discussed here.