Abstract.A higher-order extension of the familiar Korteweg-de Vries equation is derived for internal solitary waves in a density-and current-stratified shear flow with a free surface. All coefficients of this extended Kortewegde Vries equation are expressed in terms of integrals of the modal function for the linear long-wave theory. An illustrative example of a two-layer shear flow is considered, for which we discuss the parameter dependence of the coefficients in the extended Korteweg-de Vries equation.

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