Equations of stationary long waves on the interface between a homogeneous fluid and an exponentially stratified fluid are considered. An equation of the second-order approximation of the shallow water theory inheriting the dispersion properties of the full Euler equations is used as the basic model. A family of asymptotic submodels is constructed, which describe three different types of bifurcation of solitary waves at the boundary points of the continuous spectrum of the linearized problem.
Introduction.Equations of an inviscid two-layer fluid with a piecewise-constant density experiencing a jump at the interface between the layers are used (see [1,2]) as a mathematical model of internal waves in a pynocline. In this model, solitary waves and smooth bores are described by the equation of the second-order approximation of the shallow water theory, which was derived by Ovsyannikov [3]. Such an equation was obtained in [4] for the case with no slipping of the layers in the main flow. A similar approximation for long waves in a fluid with a piecewise-constant Brunt-Väisälä frequency was considered in [5]. It was noted [6] that asymptotic series for solitary waves are highly sensitive to small perturbations of the density field. The main objective of the present work is to estimate the influence of weak continuous stratification on the parameters of nonlinear waves on the interface. The behavior of the critical parameters of wave motion with stratification vanishing in one of the layers is studied. The density of the fluid in the second layer is assumed to be constant. The basic feature of this limit transition is the concentration of the spectra of higher modes in a narrow band of the boundary-layer type in the plane of bifurcation parameters. The presence of such a layer substantially affects the asymptotic behavior of solitary waves of the leading mode. In particular, a region of parameters is found, where the branching of solutions at the points of the spectrum boundary differs from the bifurcation of solitary waves to plateau-and bore-type waves for the model of a two-layer fluid [7,8].