The pressure disturbance of a nonlinear internal wave train

Observations from a lander fixed to the seafloor over the continental shelf in 124 m of water provide highly resolved velocity measurements through nonlinear internal waves of elevation. From these measurements we determine, for the first time, the non-hydrostatic pressure disturbance ( pₙₕ ) in nonlinear internal waves. For near-bottom waves of elevation ranging in amplitude, a , from 12 to 33 m, the value of pₙₕ evaluated at the seafloor changes sign from \,>\,0 to \,<\,0 and back in accordance with weakly nonlinear theory; peak values of pₙₕ range from 25 to 90 N m ⁻² . The external hydrostatic pressure disturbance due to the surface displacement ( ηH ) is inferred from horizontal accelerations. For elevation waves, ηH\,<\,0 ; peak values range from 0.1 to 9 mm (1 to 90 N m ⁻² ). The internal hydrostatic pressure perturbation ( pWh ), caused by isopycnal displacement, is inferred from measured streamlines and an ambient density profile. Its value at the seafloor is \,>\,0 for elevation waves; peak values range from 100 to 300 N m ⁻² . ηH and seafloor values of pₙₕ , pWh all increase monotonically with a . Since pₙₕ and pWh increase at roughly the same rate with a , no clear trend arises in the degree to which waves become more or less non-hydrostatic as a changes. A distinct bottom pressure signature is determined for bottom-trapped nonlinear waves of elevation, a wave train consisting of a sequence of positive pressure perturbations (dominated by pWh ). By inference, a train of surface-trapped nonlinear internal waves of depression will consist of a sequence of negative pressure perturbations. A result of this analysis is that significant properties of the waves can be discerned from a simple adequately resolved bottom pressure measurement.