Mixing caused by internal waves and turbulence: a comparative analysis

Abstract

Physical definitions of currents, internal waves and turbulence, and estimates of criteria for discriminating between them are given. It is shown that internal gravity waves can be taken as effective and universal mechanisms to cause such phenomena as fine-structure and large global vertical fluxes of energy and buoyancy in the ocean. Formation of vertical fine-structure (FS) is a natural result of interaction of internal waves (IW) with initially monotonous density profiles. In the process of the interaction, IW and FS are mutually adjusting and expanding their spectra to high frequencies and wave-numbers. In regions with large horizontal gradients of density, IW can induce vertical diffusion with effective coefficients of order 10 −4–10 −3 m 2 s −1. In most parts of the ocean, where horizontal gradients are small, «slow» IW form background shears and density gradients for «fast» IW, and their interaction can lead to vertical fluxes with effective diffusion coefficients of order 10 −5–10 −3 m 2 s −1. The interaction of quasihorizontal turbulence with long wavelength IW leads to vertical diffusion of the same order. Parametric interactions of IW and their interaction with quasihorizontal turbulence in the ocean interior can meet the requirements of global energy and mass vertical transport, and compensate for turbulent fluxes in the upper homogeneous layer of the ocean.