A non-linear numerical model with wind forcing at a frequency ω and the inertial frequency f is used to examine the processes giving rise to energy at the (ω + f) and ∣ω - f∣ frequencies in a surface frontal region. Calculations show that vorticity associated with shear in the along frontal jet leads to regions of internal wave propagation (wave frequency > local effective inertial) or trapping. In regions of propagation the non-linear term involving vertical velocity at frequency ω and vertical shear of lateral velocity at f is the main source of interaction. In trapped regions horizontal current gradients are the main source of interaction. These calculations in part explain the variability found in internal wave energy spectra.