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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Mar 2019

Research article | 05 Mar 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

Internal tide energy flux over a ridge measured by a co-located ocean glider and moored ADCP

Rob Hall1, Barbara Berx2, and Gillian Damerell1 Rob Hall et al.
  • 1Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, NorwichResearch Park, Norwich, NR4 7TJ, UK
  • 2Marine Scotland Science, Marine Laboratory, 375 Victoria Road, Aberdeen, AB11 9DB, UK

Abstract. Internal tide energy flux is an important diagnostic for the study of energy pathways in the ocean, from large-scale input by the surface tide, to small-scale dissipation by turbulent mixing. Accurate calculation of energy flux requires repeated full-depth measurements of both potential density (ρ) and horizontal current velocity (u) over at least a tidal cycle and over several weeks to resolve the internal spring-neap cycle. Typically, these observations are made using full-depth oceanographic moorings that are vulnerable to being fished-out by commercial trawlers when deployed on continental shelves and slopes. Here we test an alternative approach to minimise these risks, with u measured by a low-frequency ADCP moored near the seabed and ρ measured by an autonomous ocean glider holding station by the ADCP. The method is used to measure the M2 internal tide radiating from the Wyville Thompson Ridge in the North Atlantic. The observed energy flux (4.2 ± 0.2 kW m−1) compares favourably with historic observations and a previous numerical model study.

Error in the energy flux calculation due to imperfect co-location of the glider and ADCP is estimated by sub-sampling potential density in an idealised internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (< 10 %) if all the glider data is contained within a watch circle with a diameter smaller than 1/8 the mode-1 horizontal wavelength of the internal tide. Energy flux is biased low because the glider samples density with a broad range of phase shifts, resulting in underestimation of vertical isopycnal displacement and available potential energy. The negative bias increases with increasing watch circle diameter. If watch circle diameter is larger than 1/8 the mode-1 horizontal wavelength, the negative bias is more than 3 % and all energy fluxes within the 95 % confidence limits are underestimates. Over the Wyville Thompson Ridge, where the M2 mode-1 horizontal wavelength is ≈ 100 km and all the glider dives are within a 5 km diameter watch circle, the observed energy flux is estimated to have a negative bias of only 0.5 % and an error less than 4 % at the 99 % confidence limit. With typical glider performance, we expect energy flux error due to imperfect co-location to be < 10 % in most mid-latitude shelf slope regions.

Rob Hall et al.
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Rob Hall et al.
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Publications Copernicus
Short summary
Internal tides are subsurface waves generated by tidal flows over ocean ridges. When they break they create turbulence that drives an upward flux of nutrients from the deep ocean to the nutrient-poor photic zone. Measuring internal tides is problematic because oceanographic moorings are often fished-out by commercial trawlers. We show that autonomous ocean gliders and acoustic Doppler current profilers can be used together to accurately measure the amount of energy carried by internal tides.
Internal tides are subsurface waves generated by tidal flows over ocean ridges. When they break...