Models of the oceanic internal wave field

Abstract

In recent years, considerable progress has been made in internal wave research by a fruitful combination of experiment and theory. Kinematical models of the wave field appear to be well established, and dynamical models are evolving toward a stage of understanding the energetics and the interrelations of the waves within the oceanic field of motion. This review presents kinematical models of the wave field in terms of vertically progressive waves (WKB waves) as well as standing modes. Some emphasis is attributed to critical layer effects. Spectral models have been successfully developed for the wave field in the main thermocline. This appears to be in a stationary universal state with respect to spectral shape and level, whereas the upper ocean wave field shows considerable temporal and regional variability. Several approaches for separating the internal wave contribution from turbulence and other contaminations in observations have been proposed. This problem is of particular relevance in the transition region between waves and small‐scale turbulence at small vertical wavelengths. An accurate identification of reversible (wave induced) fine structure and irreversible fine structure is needed to determine the dissipation rate of the wave field and the mixing rates of the ocean. The search for dynamical relations of the wave field to environmental conditions has been extensive. The lack of dynamical correspondences between the wave spectrum and possible forcing fields in observations suggests that forcing is weak. Theoretical models of wave generation show that many mechanisms may contribute with equal efficiency. In concert with the observed low dissipation rates in the deep ocean, these results point toward the conclusion that there is no dominant source of energy but weak forcing by many different sources and weak dissipation. Under such conditions the interrelation between forcing and dissipation as well as the spectral form is controlled by internal transfer by wave‐wave interactions which are very efficient in relaxing spectral distortions to the observed universal form.