A new mechanism is proposed to explain the large changes in radar backscatter observed at high microwave frequencies (X band and C band) near ocean fronts and internal waves. The proposed mechanism involves the interaction of short Bragg waves with intermediate-scale waves having wavelengths of the order of 1 m, as well as the interaction of both sets of waves with the large-scale current gradients associated with the front or internal wave. Intermediate-scale waves that are steepened by the large-scale current field interact more strongly with the short Bragg waves, resulting in an increase in the mean spectral density of the short waves. Calculations of these effects, when combined with a simple Bragg model for the radar backscatter, are found to agree at least qualitatively with the backscatter modulations observed in SAR images collected near the edge of the Gulf Stream. ¿ 1998 American Geophysical Union