Laboratory experiments on the influence of stratification and a bottom sill on seiche damping

Abstract. The damping of water surface standing waves (seiche modes) and the associated excitation of baroclinic internal waves are studied experimentally in a quasi-two-layer laboratory setting with a topographic obstacle at the bottom, representing a seabed sill. We find that topography-induced baroclinic wave drag indeed contributes markedly to seiche damping in such systems. Two major pathways of barotropic-baroclinic energy conversions were observed: the stronger one – involving short-wavelength internal modes of large amplitudes – may occur when the node of the surface seiche is situated above the close vicinity of the sill. The weaker, less significant other pathway is the excitation of long waves, internal seiches along the pycnocline that may resonate with the low frequency components of the decaying surface forcing.