Journal of Fluid Mechanics

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

Matthieu J. Merciera1a6 c1, Manikandan Mathura2a3a1, Louis Gostiauxa2, Theo Gerkemaa4 c1, Jorge M. Magalhãesa5, José C. B. Da Silvaa5 and Thierry Dauxoisa6

a1 Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

a2 Laboratoire des Ecoulements Geophysiques et Industriels (LEGI), UMR 5519 CNRS-UJF-INPG, 21 rue des Martyrs, 38000 Grenoble, France

a3 Laboratoire de Météorologie Dynamique, École Polytechnique, 91128 Palaiseau, France

a4 Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, The Netherlands

a5 CIMAR/CIIMAR, Centro Interdisciplinar de Investigacão Marinha e Ambiental and Departamento de Geociências, Ambiente e Ordenamento do Território, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal

a6 Laboratoire de Physique de l’École Normale Supérieure de Lyon, Université de Lyon, CNRS, 46 Allée d’Italie, F-69364 Lyon CEDEX 07, France

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.

(Received August 04 2011)

(Reviewed February 02 2012)

(Accepted April 14 2012)

(Online publication June 29 2012)

Key Words:

  • internal waves;
  • ocean processes;
  • solitary waves

Correspondence:

c1 Email addresses for correspondence: mmercier@mit.edu, Theo.Gerkema@nioz.nl

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