In the present study we consider propagation of a localized internal perturbation in the form of an oscillating wave packet (breather) of the first mode in a three-layer fluid with an uneven bottom shaped as a smoothed step. The study is carried out by methods of numerical simulation within a fully nonlinear two-dimensional (vertical plane) set of NavierStokes equations. A set of calculations was carried out for different widths and heights of the bottom step. Inhomogeneity of the medium leads to transformation of the internal wave field with the formation of weak reflected waves and one or two first-mode breathers passed to the shallow zone. By analyzing linear stability in terms of Richardson and Froude numbers, it was revealed that potentially unstable regions arise at the smallest values of the step width. An amplitude and energy analysis of secondary reflected nonlinear waves was performed. The vertical mode composition of the fully nonlinear wave field is analyzed. It is shown that the first mode makes the largest contribution to the vertical structure of the full-nonlinear packet, though the fourth, second and the third modes also contribute noticeably.

Purpose. The paper is aimed at studying the regional features of the internal waves’ field in the Sea of Japan (the Peter the Great Bay) based on the field data, namely, determination of the internal waves’ statistical characteristics that can be used to predict possibility of generating the waves of extreme amplitudes. Methods and Results. The records of water temperature variability in the Peter the Great Bay (the depth at the point of measurements is 42 m) obtained for October 11 – 20, 2011 were used as the initial data. Sampling frequency of the records was 1 s that permitted to analyze the shape of the short-period internal waves. The data on the salinity vertical distribution near the measurement point was also used. The law of the power density spectrum decay (as applied to the studied record) is well described by the Garrett – Munk model for the Sea of Japan zone being under consideration. The calculated temporal series of density were applied for obtaining the basic statistical characteristics including the statistical moments. Besides, empirical distribution for such parameters as the wave heights, periods and steepness and the wave slope amplitude was approximated by the log-normal distribution law and analyzed. The expected wave heights were forecasted using the Poisson statistics. Conclusions. It is shown that the probabilistic characteristics of the internal waves are described well by the log-normal distribution. Based on repeatability of the internal waves’ heights, probability of appearance of intensive disturbances is estimated. It is shown that within 10 days, occurrence of a short-period wave with the height not less than 7 m is guaranteed at the observation point at the 42 m depth. Keywords intensive internal waves, in situ data, probabilistic characteristics of extreme waves, shelf, Sea of Japan.