Experimental study of parametric subharmonic instability for internal waves

TL;DR

This paper experimentally investigates parametric subharmonic instability in internal waves, analyzing how primary wave properties influence energy transfer to secondary waves, with implications for ocean mixing and energy transport.

Contribution

It provides the first detailed experimental analysis of PSI in internal waves, including measurements of growth rates and wave vectors, and compares results with theoretical models.

Findings

PSI leads to energy transfer from large to smaller scales in internal waves.

Growth rates of secondary waves depend on primary wave frequency and amplitude.

Finite size effects influence the selection of unstable modes.

Abstract

Internal waves are believed to be of primary importance as they affect ocean mixing and energy transport. Several processes can lead to the breaking of internal waves and they usually involve non linear interactions between waves. In this work, we study experimentally the parametric subharmonic instability (PSI), which provides an efficient mechanism to transfer energy from large to smaller scales. It corresponds to the destabilization of a primary plane wave and the spontaneous emission of two secondary waves, of lower frequencies and different wave vectors. Using a time-frequency analysis, we observe the time evolution of the secondary waves, thus measuring the growth rate of the instability. In addition, a Hilbert transform method allows the measurement of the different wave vectors. We compare these measurements with theoretical predictions, and study the dependence of the instability with primary wave frequency and amplitude, revealing a possible effect of the confinement due to the finite size of the beam, on the selection of the unstable mode.