Horizontal variability of high‐frequency nonlinear internal waves in Massachusetts Bay detected by an array of seafloor pressure sensors

Abstract

AbstractA two‐dimensional array of 14 seafloor pressure sensors was deployed to measure properties of tidally generated, nonlinear, high‐frequency internal waves over a 14 km by 12 km area west of Stellwagen Bank in Massachusetts Bay during summer 2009. Thirteen high‐frequency internal wave packets propagated through the region over 6.5 days (one packet every semidiurnal cycle). Propagation speed and direction of wave packets were determined by triangulation, using arrival times and distances between triads of sensor locations. Wavefront curvature ranged from straight to radially spreading, with wave speeds generally faster to the south. Waves propagated to the southwest, rotating to more westward with shoreward propagation. Linear theory predicts a relationship between kinetic energy and bottom pressure variance of internal waves that is sensitive to sheared background currents, water depth, and stratification. By comparison to seafloor acoustic Doppler current profiler measurements, observations nonetheless show a strong relationship between kinetic energy and bottom pressure variance. This is presumably due to phase‐locking of the wave packets to the internal tide that dominates background currents and to horizontally uniform and relatively constant stratification throughout the study. This relationship was used to qualitatively describe variations in kinetic energy of the high‐frequency wave packets. In general, high‐frequency internal wave kinetic energy was greater near the southern extent of wavefronts and greatly decreased upon propagating shoreward of the 40 m isobath.