The Pennsylvania State University/National Center of Atmospheric Research Mesoscale Model 5 (MM5) is forced by the observed evolution of sea surface temperature and the National Center for Environmental Prediction (NCEP) reanalyses of the atmospheric flow on the lateral boundaries. The MM5 reproduces many of the general features of the large-scale circulation found in the NCEP reanalyses. It does, however, develop a stronger regional meridional circulation with higher precipitation rates in the Intertropical Convergence Zone (ITCZ), stronger cross-equatorial flow in the boundary layer, and greater divergent flow aloft. The model ITCZ is displaced too far north and tends to be located over the highest sea surface temperature. Mesoscale circulation features, such as diurnal land/sea breezes and gap flows across Mexico and Central America, are captured. The observed latitudinal transition from the shallow planetary boundary layer (PBL) with stratiform clouds over the cool waters of the equatorial Pacific to a relatively deeper PBL further north is simulated by the model. More tropical cyclones develop in the MM5 than were observed and they were comparatively shorter-lived and weaker. Also, the model, as presently configured, does not realistically simulate the year-to-year variations in tropical cyclones. The regional circulation during Summer 1997 was dominated by the planetary- scale El Nino/Southern Oscillation phenomenon. The normally cool waters along the equator were unusually warm with enhanced convection to the south of the mean position of the ITCZ. Large-scale subsidence over Central America and the extreme eastern Pacific Ocean led to drought conditions in that region. The number and intensity of tropical cyclones observed during summer 1997 in the eastern equatorial Pacific were reduced despite the fact that the most intense hurricane of the eight seasons occurred during that year.