Soliton generation by internal tidal beams impinging on a pycnocline: laboratory experiments

TL;DR

This study demonstrates through laboratory experiments that internal tidal beams can generate internal solitary waves upon impacting a pycnocline, providing insights into oceanic wave phenomena observed via remote sensing.

Contribution

First laboratory demonstration of internal solitary wave generation by internal tidal beams hitting a pycnocline, linking experimental results to ocean observations.

Findings

Generation of mean flow and higher harmonics at reflection points

Detection of internal solitary waves in the pycnocline

Spectral bulge indicating solitary wave presence

Abstract

In this paper, we present the first laboratory experiments that show the generation of internal solitary waves by the impingement of a quasi-two-dimensional internal wave beam on a pycnocline. These experiments were inspired by observations of internal solitary waves in the deep ocean from synthetic aperture radar (SAR) imagery, where this so-called mechanism of 'local generation' was argued to be at work, here in the form of internal tidal beams hitting the thermocline. Nonlinear processes involved here are found to be of two kinds. First, we observe the generation of a mean flow and higher harmonics at the location where the principal beam reflects from the surface and pycnocline; their characteristics are examined using particle image velocimetry (PIV) measurements. Second, we observe internal solitary waves that appear in the pycnocline, detected with ultrasonic probes; they are further characterized by a bulge in the frequency spectrum, distinct from the higher harmonics. Finally, the relevance of our results for understanding ocean observations is discussed.