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

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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 surface fluid system with an upper fresh-water and a lower salt-water layer. The interface between the two layers is diffuse and has a finite thickness which grows with the life time of the layered fluid system. Soliton-type disturbances of the interface are generated at one end of the channel. They move along the channel and encounter an obstruction (sill) in the middle, where they break into reflected and transmitted signals. Two types of solitary waves are produced by the interaction: a fast, ground mode soliton and a slower (by a factor of approximately 3), mode-one soliton due to the diffusive nature of the interface. Using the shallow water theory of a Boussinesq fluid with a continuous density profile and Benney's (1996) theoretical description it is demonstrated that the observed fast and slow speed wave signals are indeed due to the given density structure. It is also demonstrated that the ground mode wave speed decreases with increasing thickness of the diffusive interface whereas that of the first higher mode increases with good agreement between theory and experimental observation.