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 directly determine, for the first time, the nonhydrostatic 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 90N/m). 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 nonhydrostatic 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.