  Ocean motions at frequencies of the internal wave band are generally
associated with freely propagating waves that are supported by stable vertical
stratification in density. Previous analyses of yearlong current observations
from the Bay of Biscay showed that a finestructure of semidiurnal tidal and
near-inertial higher harmonics fills the spectrum. Here, a simple model is
presented of forced nondispersive motions with forward energy cascade. The
model fits the spectral shape of higher harmonics well within statistical
significance and shows that such interactions imply maximum wave steepness in a
balance between forcing and turbulent mixing. The single fitting parameter is
related to the barotropic tidal flow speed, which thereby sets a nonlinear
limit to baroclinic current scales without generating non-linear higher
harmonics directly.
