Baroclinic solitary water waves in a two-layer fluid system with diffusiveinterface

Experiments are described that have been performed in a 10 m long and 0.33 m wide channel on the baroclinic wave propagation in a two-layered free sur face fluid system with an upper fresh-water and a lower saltwater layer. Th e interface between the two layers is diffuse and has a finite thickness wh ich grows with the life time of the layered fluid system. Soliton-type dist urbances of the interface are generated at one end of the channel. They mov e along the channel and encounter an obstruction (sill) in the middle, wher e they break into reflected and transmitted signals. Two types of solitary waves are produced by the interaction: a fast, ground mode soliton and a sl ower (by a factor of approximately 3), mode-one soliton due to the diffusiv e nature of the interface. Using the shallow water theory of a Boussinesq f luid with a continuous density profile and Benney's (1996) theoretical desc ription it is demonstrated that the observed fast and slow speed wave signa ls are indeed due to the given density structure. It is also demonstrated t hat the ground mode wave speed decreases with increasing thickness of the d iffusive interface whereas that of the first higher mode increases with goo d agreement between theory and experimental observation.