Extensive moored current meters, atmospheric observations, and hydrographic surveys were obtained off central California during the 1997--1998 El Ni¿o. In this study the temperature observations from current meter moorings and hydrographic surveys are assimilated into a coastal ocean model to study the seasonal circulation in the Santa Barbara Channel--Santa Maria Basin (SBC-SMB). Mean coastal circulation was dominated by a poleward jet in fall 1997 at the peak of the El Ni¿o but returned to the normal surface convergent pattern in fall 1998. The data-assimilated model reproduces well the observed flow patterns in both years. The interannual flow variability is examined through sensitivity analysis of the effects of local forcing and open ocean influence. It is shown that the poleward jet is associated with enhanced California Countercurrent during El Ni¿o, while the surface convergence is driven by local wind stress curl. Taking advantage of the extensive data set, the climatology is compared with the synoptic states. It is found that the climatology could cause significant negative impact.