The effects of temporal and spatial smoothing of wind forcing were evaluated in a model simulation of the tropical Pacific Ocean variability during the onset phase of the 1997/1998 El Ni¿o. A total of 16 experiments were performed using the NASA scatterometer wind data smoothed at time intervals from 1 to 30 days and on spatial scales from 1¿ to 10¿. A major effect of the temporal smoothing of winds is to warm sea surface temperature (SST) by reducing the energy input for vertical turbulent mixing. When the daily wind forcing was replaced by the monthly average, the mean SST increased by 0.5¿ to 1¿ over most of the tropical Pacific. The spatial smoothing of winds is not as effective as the temporal smoothing in causing SST warming, but it has a more severe influence on dynamical ocean response for smoothing scales above 5¿. The onset of the 1997/1998 El Ni¿o can be successfully simulated using the wind forcing averaged to monthly intervals and 2¿ squares. For climate models the spatial smoothing of wind forcing on scales larger than the width of the equatorial waveguide is a more serious limitation than the temporal smoothing on scales up to 1 month. ¿ 1999 American Geophysical Union