Internal tide energy flux over a ridge measured by a co-located ocean glider and moored acoustic Doppler current profiler

Abstract

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 minimize these risks, with u measured by a low-frequency acoustic Doppler current profiler (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 semidiurnal internal tide radiating from the Wyville Thomson 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 subsampling potential density in an idealized internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (<10 %) if all the glider data are 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 realizations within the 95 % confidence interval are underestimates. Over the Wyville Thomson Ridge, where the semidiurnal 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 0.4 % and an error of less than 3 % at the 95 % 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.

Preface: Developments in the science and history of tides

“…This topic is represented in the special issue by a regional study of the predictability of Caribbean internal tides by Zaron (2019a) and an accuracy assessment of various global internal tide models by Carrere et al (2021). The benefits of technology for the local measurement of internal tides using gliders and moored acoustic Doppler current profilers are described by Hall et al (2019).…”

Future Vision for Autonomous Ocean Observations

“…Sensors on autonomous platforms can also measure physical variables, such as current velocity. Acoustic Doppler Current Profilers (ADCP) are widely used on research vessels, moorings, and more recently on AUVs including gliders (Thurnherr et al, 2015) although they are not routinely installed on gliders because of power requirements and data processing challenges (Hall et al, 2019).…”

Time‐Lapse Acoustic Imaging of Mesoscale and Fine‐Scale Variability within the Faroe‐Shetland Channel

“…However, quantification of mean fluxes through the channel is complicated by intense activity at the mesoscale and at finer scales. Developing an understanding of the effects of this activity, which includes eddies, trains of solitary waves, and an internal tide with an amplitude of ∼40 m, is essential to accurately parametrizing climatic models (e.g., Dooley, 1975;Dooley & Meincke, 1981;Hall et al, 2019Hall et al, , 2011Hosegood et al, 2004;Sherwin et al, 1999Sherwin et al, , 2006Sherwin, 1991). Mesoscale and fine-scale fluctuations within the channel have important implications for fish stocks and for the safe operation of deep-sea rigs (e.g., Gallego et al, 2018;Hansen & Jákupsstovu, 1992;Shelton et al, 1997).…”