Detailed internal wave mixing above a deep-ocean slope

Abstract

Turbulent vertical eddy diffusivity (K z) and dissipation rate (ε) are estimated between 0.5 and 50 m above the sloping side of Great Meteor Seamount, Canary Basin, using 101 moored temperature sensors, 1-mK precision, sampling at 1 Hz. Effectively, detailed observed time-depth temperature images are split in two: a statically stable and a turbulence image. Tides dominate the temperature variations, but the local bottom slope is supercritical to motions at semidiurnal frequencies. Averaged over a fortnight, the observed overall time-depth mean K z = 3±1×10 −3 m 2 s −1 and ε = 1.5±0.7× 10 −7 W kg −1. Variations with time and depth are large, by up to four orders of magnitude. Although variations do occur having tidal periodicity, shorter-scale variations are more intense. A particular tidal period shows multiple vigorous overturning events, the largest found away from the bottom during the downslope phase but just prior to arrival of an upslope moving, equally vigorous bore. The strength of the bore may be controlled by the intensity of the mixing just prior to it. The bore itself is turbulent from the bottom upward, up to some 40 m above it. Its mixing is most efficient providing large fluxes in extremely thin layers. Parameterizations of turbulence estimates are inconclusive using powers of N, as they show different relationships for different depths, time-ranges and averaging.

Bottom-pressure observations of deep-sea internal hydrostatic and non-hydrostatic motions

“…A similar result is obtained after comparison of the turbulent part of the observed bottom pressure, p IWTT , with pressure independently estimated from dissipation rates using high-resolution temperature data between 0.5 and 50 m above the bottom (for the method, see van Haren & Gostiaux (2012a)). Under the assumption of isotropic turbulence and high-Reynolds-number flows, Kolmogorov scaling yields at wavenumber k for the, presumably free stream rather than bottom or wall, pressure spectrum (Tsuji & Ishihara 2003) , and U and L the velocity and length scales of the flow.…”

“…Van Haren & Gostiaux (2012a) demonstrate that the spectrum of heat flux transits from the canonical internal wave −2 slope to the turbulence −5/3 slope at the frequency where the eddy diffusivity and the pressure spectra transit from their IWC slope to the IWT bulge. Furthermore, the spectral shape of variance and particular time series of intermittent turbulence pressure and vertical currents resemble turbulence observations in the atmospheric boundary layer.…”

Dissipation of Internal Wave Energy Generated on a Critical Slope

“…Efforts over the Mid-Atlantic Ridge (Polzin et al 1997) and Hawaii (Klymak et al 2006) have demonstrated increased dissipation over deep, rough topography. Similar observations have been made elsewhere in a growing body of literature (e.g., Gemmrich and van Haren 2002;Nash et al 2007;Alford et al 2011Alford et al , 2014van Haren and Gostiaux 2012). Progress has also been made in predicting the dissipation observed near topography.…”

“…Progress has also been made in predicting the dissipation observed near topography. For topography with characteristic slopes that are less steep than internal tide beams, a priori estimates of the expected dissipation are possible from linear models of the generation (St. Laurent et al 2002;Polzin 2009). A fraction of the upward-radiating energy is assumed to dissipate because of wave-wave interactions.…”

Stratified turbulence and small-scale internal waves above deep-ocean topography

“…10 When many are used, say 100 at 1 m intervals, time-depth images can be obtained that resolve internal waves and the larger, most energetic turbulence scales, provided the sensors' noise level and precision are sufficiently small (typically 0.001 • C or less). The advantage of measuring temperature over even better spatially resolving acoustics is that the former can quantify the associated turbulence, 11 whereas the latter can only provide qualitative, quasi-synoptic images due to calibration problems. (Nonetheless, such shipborne acoustic images demonstrate very detailed overturning including secondary turbulence along the rim of the largest Kelvin-Helmholtz overturn in high Reynolds number flows in shallow seas).…”