The effects of slide-gate dithering on surface flow behavior during continuous casting are investigated via mathematical modeling and plant trial measurements. A transient turbulent flow model with a new free surface tracking algorithm using a dynamic mesh was developed, validated, and applied to match different measured level-fluctuation histories. Simulation results explain the mechanism of large sloshing observed in a dithering trial when the dithering frequency matches the mold natural frequency. The associated flow-rate variations then amplify the velocity variations in the circulating fluid. Parametric studies reveal that mold level fluctuations increase with increasing casting speed, and dithering stroke.