Journal of Fluid Mechanics

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Low-mode internal tide generation by topography: an experimental and numerical investigation

PAULA ECHEVERRIa1 c1, M. R. FLYNNa2 p1, KRAIG B. WINTERSa3 and THOMAS PEACOCKa4

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

a2 Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

a3 Scripps Institution of Oceanography and Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093, USA

a4 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Abstract

We analyse the low-mode structure of internal tides generated in laboratory experiments and numerical simulations by a two-dimensional ridge in a channel of finite depth. The height of the ridge is approximately half of the channel depth and the regimes considered span sub- to supercritical topography. For small tidal excursions, of the order of 1% of the topographic width, our results agree well with linear theory. For larger tidal excursions, up to 15% of the topographic width, we find that the scaled mode 1 conversion rate decreases by less than 15%, in spite of nonlinear phenomena that break down the familiar wave-beam structure and generate harmonics and inter-harmonics. Modes two and three, however, are more strongly affected. For this topographic configuration, most of the linear baroclinic energy flux is associated with the mode 1 tide, so our experiments reveal that nonlinear behaviour does not significantly affect the barotropic to baroclinic energy conversion in this regime, which is relevant to large-scale ocean ridges. This may not be the case, however, for smaller scale ridges that generate a response dominated by higher modes.

(Received December 15 2008)

(Revised April 11 2009)

(Accepted April 13 2009)

Correspondence:

c1 Email address for correspondence: paulae@mit.edu

p1 Present address: Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G8.

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