Experimental study of superharmonic internal wave resonant triads in finite-depth nonuniform stratifications

We present an experimental study on the excitation of superharmonic internal wave by resonant interaction of internal wave modes in finite-depth nonuniform stratifications. In a nonuniform stratification, the primary wave field composed of a single horizontally propagating vertical mode or combination of two horizontally propagating vertical modes is forced from one end of the tank using the wave generator at a forcing frequency omega(0). The spontaneous excitation of a superharmonic wave at frequency 2 omega(0) by resonant interaction of the internal wave modes of the primary wave field forced at frequency omega(0) is observed. We experimentally demonstrate that even an isolated single mode could resonantly interact with itself to excite a superharmonic wave at frequency 2 omega(0) in a nonuniform stratification, which is inhibited in the case of a uniform stratification. The resonant peak frequencies at which the strength of superharmonic signal is maximum, and the corresponding vertical structure of the superharmonic internal wave mode, qualitatively agree well with the theoretical predictions obtained from the weakly nonlinear analysis. The paper concludes with a discussion on experiments performed to observe superharmonic triadic resonant interactions in an oceanlike nonuniform stratification profile, for which the excited superharmonic internal wave modes were found to be trapped within the pycnocline region.