A medium resolution ocean general circulation model of the tropical Pacific Ocean is used to explore current structure and variability on the equator for the period from 1992 to 1997. The model assimilates surface and subsurface temperature data from expendable bathythermographs and the Tropical Ocean Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) moorings and altimetry data from the TOPEX/Poseidon satellite. Currents from the model are compared with current observations from the TOGA-TAO moorings at 110 ¿W, 140 ¿W, and 165 ¿E. Since current data are not assimilated, these data provide an independent verification of the model results. The comparison shows that the model produces accurate currents over a wide range of spatial and temporal scales. In particular, the model correctly resolves temporal variability from instability waves with periods of less than a month to interannual changes with periods of several years. The model results are used to explore zonal velocity changes associated with the onset of the 1997--1998 El Ni¿o-Southern Oscillation (ENSO) event. The event began with a series of equatorial Kelvin waves which were excited by westerly wind bursts in the western Pacific and propagated to the eastern Pacific. The passage of these waves is associated with an initial strengthening of the Equatorial Undercurrent (EUC) followed by a rapid shutdown of the EUC. The weakening of the Undercurrent during ENSO is consistent with previous descriptions and results from the collapse of the easterly trade winds in the western Pacific. ¿ 1999 American Geophysical Union