Internal waves in the ocean
Another most important type of waves primarily determining the nonlinear dynamics of the upper ocean is the internal waves (IW) that form in the region of the highest gradients of the vertically inhomogeneous density profile of the ocean water, i.e., in the pycnocline region. The basic role in the pycnocline formation is played by the stratification of the deep profile of temperature. This fact provided a basis for the creation of a largescale laboratory model of the upper ocean, which was successfully implemented in the Large thermostratified tank of the IAP RAS under the supervision of V. I. Talanov.
Study of the interaction between waves of various spatial and temporal scales is prominent among the IAP RAS activities in the field of hydrophysics. The interaction of surface and internal waves is one of the most interesting examples of such an interaction under natural conditions. It is shown by the IAP RAS scientists that the physical mechanisms of the influence exerted by IW and IWgenerated subsurface currents on wind waves are very different in various ranges of wind waves. In the meter and decimeter wavelength ranges, the determining role is played by the mechanism related to the direct effect of an IWgenerated variable subsurface current on surface wave kinematics. An appropriate model for the variation of the wind wave field characteristics affected by IW was developed by the team headed by V. I. Talanov and called the kinematic model. This model has been confirmed by numerous laboratory and field experiments (V. V. Bakhanov) and is well known to specialists. In the highfrequency part of the wave spectrum (centimeter and millimeter ripples), of primary importance are other effects, in particular, the modulation of the damping coefficient of surface waves due to the redistribution of films of surface active substances under the action of IW (S. A. Ermakov). There is experimental evidence for an essential role of the cascade mechanisms of IW action on short (centimeter) wind waves: the current generated on the IW surface transforms meter and decimeter waves and they, in turn, affect centimeter ripples. Besides, the inhomogeneous current field generated by IW varies the wind velocity field over the water surface, which leads to the modulation of the smallscale windwave increment (Yu. I. Troitskaya).
Study of the interaction between waves and inhomogeneous currents is also of great importance in hydrophysical research. Of special interest is the nonlinear interaction of IW with flows in the vicinity of the socalled critical layers, i.e., the resonance levels, in which the phase velocity of waves coincides with the flow velocity. This interaction can give rise to a significant restructuring of the current due to a return action of the wave on the current as a result of the resonance pulse exchange. In the quasilinear model of a turbulent wake behind a body towed in a stratified liquid for large Reynolds and Froude numbers, developed by
Yu. I. Troitskaya, it is shown that the basic mechanism of the wake evolution is the development of a hydrodynamic instability of the jet flow excited by the body. Numerical and laboratory experiments confirmed the proposed model; the relevant results on the velocity field evolution in the wake (average parameters of the wake) are determined from the model with an accuracy of up to several percent.
To carry out a comprehensive study of the phenomena caused by submerged buoyant jets in a stratified fluid, a direct numerical simulation of the dynamics of the fountain formed by the penetration of a vertical jet through the pycnocline in a stratified fluid was made (O. A. Druzhinin). The calculations have shown that for the Froude numbers exceeding the critical value, selfsustained oscillations of the fountain occur, which are accompanied by IW generation in the pycnocline. The possible modes of fountain selfsustained oscillations and the corresponding structures of the generated IW are studied. It follows from the calculation data that the dependence of the oscillation amplitude of the fountain top on the Froude number well agrees with the predicted theoretical model of interacting mode competence in the soft excitation regime.

