The object of this modeling study is to identify the physical mechanisms responsible for seasonal variability in sea surface Δ 14C for the equatorial Pacific Ocean. Analyses of Δ 14C in corals from Guam, Galapagos, Fanning, and Canton reveal seasonal variability between 30 and 50 per mil during the 1970s and early 1980s. Given that this variability occurs on seasonal timescales, whereas air-sea isotopic equilibration occurs on a timescale of 5 to 10 years, the variability must be due to seasonal variability in the physical circulation of the ocean. We use the primitive equation ocean circulation model of Gent and Cane <1989>, along with the hybrid mixed layer model of Chen et al. <1994a>, to study the dynamical mechanisms responsible. Upwelling in the eastern equatorial Pacific brings up Δ 14C-depleted waters, and air-sea exchange creates high Δ 14C in the western equatorial Pacific, establishing horizontal gradients in sea surface Δ 14C. Seasonally varying lateral advection, acting on these gradients, is the dominant mechanism for Δ 14C variability in the equatorial Pacific. In addition to the runs which were forced with seasonally varying winds, a run which used interannual winds between 1971 and 1985 was performed. The substantial interannual Δ 14C variability present in this run is associated with advective anomalies in the equatorial waveguide.¿ 1997 American Geophysical Union