Generation of Vertical Fine Structure by Internal Waves on a Shear Flow

Purpose. The work is purposed at studying the wave mechanism of fine structure generation, as well as determining the vertical wave fluxes of mass. Methods and Results. In contrast to the previously used mechanism of forming a fine structure by internal waves due to breaking, a new approach based on determining the vertical wave fluxes of mass in the field of an inertia-gravity internal wave without breaking is proposed. The inertia-gravity internal waves on the Black Sea northwestern shelf are considered on a flow with a vertical velocity shift. The flow is assumed to be geostrophically balanced with the vertical velocity shifts compensated by a horizontal density gradient. The f-plane approximation is used. Thus, the classical scheme for describing a wave field by the hydrodynamic equations is applicable with regard to the nonlinear effects. A weakly nonlinear approach is used. In the linear approximation, the eigenfunction and the dispersion relation are found by solving numerically the boundary value problem which determines the vertical structure of a mode in the presence of an average flow. In this case, the wave frequency is of a complex character since the coefficients in the differential equation of the specified boundary value problem are complex. Depending on the wave period and the mode number, either weak attenuation of a wave or its weak amplification is possible. The eigenfunction of internal waves is also complex. Therefore, the vertical wave fluxes of mass and the vertical component of the Stokes drift velocity are nonzero and lead to the generation of vertical fine structure which is irreversible. Conclusions. In the presence of a two-dimensional shear flow, taking into account the horizontal inhomogeneity of the average density field enhances the effect of generation of a vertical fine structure by the inertia-gravity internal waves. The vertical wave fluxes of mass also increase. The indicated fluxes and generated fine structure for the waves of different frequencies are close and the effect is enhanced in the presence of the waves of different frequencies.