Nonlinear motions at the internal tide source

Oceanic internal tidal wave generation is commonly modeled via interaction of a sinusoidal barotropic vertical velocity W with topography. However, general w above slopes are predominantly nonlinear, as appears in some nonhydrostatic models. Here, 1–2 Hz sampled observations are presented from 2–84 m above the bottom (mab) at ∼1400 m at the Bay of Biscay continental slope. Fronts are observed that all propagate upslope having heights >50 mab and more or less, ±5%, occurring at the tidal periodicity. They are accompanied by O(10 min) pulses of w≈0.10 m s −1 ≫W and irregular small‐scale waves superimposed on an asymmetric tidal signal. This nonlinearity is further exemplified by variations in frontal arrival and steepness, height, form, and is causing: i) internal tidal intermittency of ∼10 tidal periods, ii) nonlinear generation of internal tidal higher harmonics at the source. The observations suggest internal tide generation in a broadband instead of a monochromatic beam.