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Internal Waves Strongly nonlinear internal waves of depression are commonly observed on many continental shelves around the world. Packets of high-frequency waves are typically generated by the diurnal tides at the shelf break during stratified conditions (late spring to early fall) and tend to propagate shoreward.  The temporal and spatial separations of these packets suggest that they evolve from internal tides generated by the semi-diurnal barotropic tidal current advecting stratified fluid over the steep topography of the upper slope. In this region the semidiurnal tidal flow is dominated by the principal lunar constituent M2 (12.42 hour period), which propagates shoreward from the North Atlantic. The internal waves within the packets tend to become soliton-like, with large amplitudes, short wavelengths, and high frequencies near the local buoyancy frequency. The physics of the generation, propagation, and dissipation of strongly nonlinear internal waves over the shelf is of intense interest. These waves not only significantly perturb the current and density field, they also initiate bottom sediment resuspension and mix nutrients into the euphotic zone. The perturbations in the density field, specifically the temperature field, can cause large fluctuations in the sound speed and thus affect acoustic propagation over the shelf. Fig.1: This figure shows the shelf region of the Mid-Atlantic Bight off the east coast of the United States. It is well known that satellite synthetic aperture radars (SAR) are very effective at imaging the surface manifestations of oceanic internal waves. Such SAR signatures depend not only on the dynamic characteristics of the interior ocean, but are also strongly dependent on phenomena taking place at the air-sea interface, such as the variability of surface roughness and wave damping by surface films. SAR imagery helped underline internal waves characteristics: the propagation in groups or packets, of few dozen individual waves, with the biggest waves leading the edge of the packet. Wave signatures observed in SAR imagery are a series of alternating light/dark linear or curvilinear bands that represent the crests and troughs of the waves.
Fig.2: This figure shows internal waves observed by the Radarsat SAR on August 13, 2006 in the Mid-Atlantic Bight. The solid circles show moorings and the open shapes show the locations of research vessels in the area at the time of the image. |
