The influence of subinertial internal tides on near-topographic turbulence at the Mendocino Ridge: Observations and modeling

R. C. Musgrave, J. A. MacKinnon, R. Pinkel, A. F. Waterhouse, J. Nash, S. M. Kelly

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Abstract

Shipboard measurements of velocity and density were obtained in the vicinity of a small channel in the Mendocino Ridge, where flows were predominantly tidal. Measured daily inequalities in transport are much greater than those predicted by a barotropic tide model, with the strongest transport associated with full depth flows and the weakest with shallow, surface-confined flows. A regional numerical model of the area finds that the subinertial K1 (diurnal) tidal constituent generates topographically trapped waves that propagate anticyclonically around the ridge and are associated with enhanced near-topographic K1 transports. The interaction of the baroclinic trapped waves with the surface tide produces a tidal flow whose northward transports alternate between being surface confined and full depth. Full depth flows are associated with the generation of a large-amplitude tidal lee wave on the northward face of the ridge, while surface-confined flows are largely nonturbulent. The regional model demonstrates that, consistent with field observations, near-topographic dissipation over the entire ridge is diurnally modulated, despite the semidiurnal tidal constituent having larger barotropic velocities. It is concluded that at this location it is the bottom-trapped subinertial internal tide that governs near-topographic dissipation and mixing. The effect of the trapped wave on regional energetics is to increase the fraction of converted barotropic-baroclinic tidal energy that dissipates locally.

Original languageEnglish (US)
Pages (from-to)2139-2154
Number of pages16
JournalJournal of Physical Oceanography
Volume47
Issue number8
DOIs
StatePublished - 2017

Bibliographical note

Funding Information:
Ship time was provided by the University of California Ship Funds. Shaun Johnston, Uwe Send, and Janet Sprintall generously loaned instrumentation that made these measurements possible. Thanks to Capt. Murline and the crew of the R/V Melville for their professionalism at sea; Paul Chua for his engineering expertise; and Julie Alvarez, Hanne Beate Skator, Elizabeth Bunin, Vanessa Crandall-Beck, Jonathan Franco, Michael Fuentes, Felipe Gomez, Bob Reed, and Alfredo Wetzel for their invaluable help gathering this data. Computing was carried out on the COMPAS cluster at SIO. We gratefully acknowledge useful conversations with Myrl Hendershott and Ken Brink, and we thank Uriel Zajaczkovski for his help making the 3D visualizations using Paraview, an open source platform for scientific visualization. This manuscript was improved by the comments and suggestions of three anonymous referees. R.C.M. was supported by the Office of Naval Research. A.F.W. was supported by NSF OCE-0968721. J.A.M. was supported by ONR-N00014-091-0273.

Keywords

  • Bottom currents
  • Internal waves
  • Mixing
  • Tides
  • Topographic effects

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