Introduction

While ocean internal waves are critical to acoustic propagation, the topic is also of intense oceanographic interest due to the fact that internal waves provide an important pathway for the transfer of energy and momentum from large scales (of order tens to hundreds of kilometers) to small scales (of order centimeters to meters). In providing this pathway, internal waves form an important link between the largely two-dimensional motions at large scale and the three-dimensional flows known to exist at smaller scale. Indeed, internal-wave breaking and subsequent mixing has been implicated in shaping the smoothly varying ocean thermocline that is observed throughout the world'soceans (Munk, 1966; Munk and Wunsch, 1998). Internal waves at tidal frequencies (internal tides) are generated by barotropic tidal flows over rough or abrupt topography, and are a form of tidal dissipation that leads to perturbations in the earth-moon system (Munk, 1998). Additionally, internal waves radiate momentum and thus exert a stress on large-scale ocean motions, which is fundamentally different from other processes that diffuse momentum (Müller, 1976). Internal waves are also known to disperse and advect chemical and biological tracers, processes that can have large ecological and public health implications (Garrett, 1979; Young et al., 1982; Duda and Farmer, 1999).

Generally speaking, ocean internal waves fit into four broad categories: inertial waves, internal tides, internal solitary waves, and broadband stochastic or random internal waves. Figures 3.1 and 3.2 show typical spectra of displacement and horizontal velocity obtained in deep ocean and continental shelf regions. Inertial waves are the energetic waves evident in the horizontal current spectra near the local inertial frequency f. These waves are largely wind-driven and therefore propagate downward from the sea surface (D'Asaro, 1984). Curiously, however, due to unknown physical processes, observations show that there is nearly as much upward propagating energy as downward propagating energy. Kinematics dictate that inertial waves do not have a strong vertical displacement (see displacement spectra in Figures 3.1 and 3.2), and thus they are considered of little importance to ocean acoustics except in the case where reciprocal transmissions are made so as to isolate horizontal current effects.

Internal tides can be regularly observed near continental shelves and roughabrupt topography and they can propagate large distances across and into the deep ocean basins (Garrett and Kunze, 2007).