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Bruno Ferron, Florian Kokoszka, Herlé Mercier, and Pascale Lherminier

Abstract

A total of 96 finestructure and 30 microstructure full-depth vertical profiles were collected along the A25 Greenland–Portugal Observatoire de la Variabilité Interannuelle et Décennale en Atlantique Nord (OVIDE) hydrographic line in 2008. The microstructure of the horizontal velocity was used to calculate turbulent kinetic energy dissipation rates ε vmp, where vmp refers to the vertical microstructure profiler. The lowest dissipation values (ε vmp < 0.5 × 10−10 W kg−1) are found below 2000 m in the Iberian Abyssal Plain and in the center of the Irminger basin; the largest values (>5 × 10−10 W kg−1) are found in the main thermocline, around the Reykjanes Ridge, and in a 1000-m-thick layer above the bottom near 48°N. The finestructure of density was used to estimate isopycnal strain and that of the lowered acoustic Doppler current profiler to estimate the vertical shear of horizontal velocities. Strain and shear were used to estimate dissipation rates ε G03 () associated with the internal wave field. The shear-to-strain ratio correction term of the finescale parameterization ε G03 brings the fine- and microscale estimates of the dissipation rate into better agreement as found. The latitude/buoyancy frequency term slightly improves the parameterization for weakly stratified waters. Correction term ε G03 is consistent with ε vmp within a factor of 4.5 over 95% of the profiles. This good consistency suggests that most of the turbulent activity recorded in this dataset is due to the internal wave field. The canonical globally averaged diffusivity value of order 10−4 m2 s−1 needed to maintain the global abyssal stratification () is only reached on the flank of the Reykjanes Ridge and in the region around 48°N.

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Bruno Ferron, Herlé Mercier, Kevin Speer, Ann Gargett, and Kurt Polzin

Abstract

The Romanche Fracture Zone is a major gap in the Mid-Atlantic Ridge at the equator, which is deep enough to allow significant eastward flows of Antarctic Bottom Water from the Brazil Basin to the Sierra Leone and Guinea Abyssal Plains. While flowing through the Romanche Fracture Zone, bottom-water properties are strongly modified due to intense vertical mixing. The diapycnal mixing coefficient in the bottom water of the Romanche Fracture Zone is estimated by using the finestructure of CTD profiles, the microstructure of high-resolution profiler data, and by constructing a heat budget from current meter data.

The finestructure of density profiles is described using the Thorpe scales L T. It is shown from microstructure data taken in the bottom water that the Ozmidov scale L O is related to L T by the linear relationship L O = 0.95L T, similar to other studies, which allows an estimate of the diapycnal mixing coefficient using the Osborn relation. The Thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream (eastward) of the main sill of the Romanche Fracture Zone. In this region, a mean diapycnal mixing coefficient of about 1000 × 10−4 m2 s−1 is found for the bottom water.

Estimates of cross-isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the Romanche Fracture Zone and the nearby Chain Fracture Zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the Romanche Fracture Zone. Although the two fracture zones occupy only 0.4% of the area covered by the Sierra Leone and Guinea Abyssal Plains, the diffusive heat fluxes across the 1.4°C isotherm in the Romanche and Chain Fracture Zones are half that found over the abyssal plains across the 1.8°C isotherm, emphasizing the role of these passages for bottom-water property modifications.

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Ilker Fer, Anthony Bosse, Bruno Ferron, and Pascale Bouruet-Aubertot

Abstract

Ocean microstructure, current, and hydrography observations from June 2016 are used to characterize the turbulence structure of the Lofoten Basin eddy (LBE), a long-lived anticyclone in the Norwegian Sea. The LBE had an azimuthal peak velocity of 0.8 m s−1 at 950-m depth and 22-km radial distance from its center and a core relative vorticity reaching −0.7f (f is the local Coriolis parameter). When contrasted to a reference station in a relatively quiescent part of the basin, the LBE was significantly turbulent between 750 and 2000 m, exceeding the dissipation rates ε in the reference station by up to two orders of magnitude. Dissipation rates were elevated particularly in the core and at the rim below the swirl velocity maximum, reaching 10−8 W kg−1. The sources of energy for the observed turbulence are the background shear (gradient Richardson number less than unity) and the subinertial energy trapped by the negative vorticity of the eddy. Idealized ray-tracing calculations show that the vertical and lateral changes in stratification, shear, and vorticity allow subinertial waves to be trapped within the LBE. Spectral analysis shows increased high-wavenumber clockwise-polarized shear variance in the core and rim regions, consistent with downward-propagating near-inertial waves (vertical wavelengths of order 100 m and energy levels 3 to 10 times the canonical open-ocean level). The energetic packets with a distinct downward energy propagation are typically accompanied with an increase in dissipation levels. Based on these summer observations, the time scale to drain the volume-integrated total energy of the LBE is 14 years.

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Bruno Ferron, Florian Kokoszka, Herlé Mercier, Pascale Lherminier, Thierry Huck, Aida Rios, and Virginie Thierry

Abstract

The variability of the turbulent kinetic energy dissipation due to internal waves is quantified using a finescale parameterization applied to the A25 Greenland–Portugal transect repeated every two years from 2002 to 2012. The internal wave velocity shear and strain are estimated for each cruise at 91 stations from full depth vertical profiles of density and velocity. The 2002–12 averaged dissipation rate 〈ε 2002–2012〉 in the upper ocean lays in the range 1–10 × 10−10 W kg−1. At depth, 〈ε 2002–2012〉 is smaller than 1 × 10−10 W kg−1 except over rough topography found at the continental slopes, the Reykjanes Ridge, and in a region delimited by the Azores–Biscay Rise and Eriador Seamount. There, the vertical energy flux of internal waves is preferentially oriented toward the surface and 〈ε 2002–2012〉 is in the range 1–20 × 10−10 W kg−1. The interannual variability in the dissipation rates is remarkably small over the whole transect. A few strong dissipation rate events exceeding the uncertainty of the finescale parameterization occur at depth between the Azores–Biscay Rise and Eriador Seamount. This region is also marked by mesoscale eddying flows resulting in enhanced surface energy level and enhanced bottom velocities. Estimates of the vertical energy fluxes into the internal tide and into topographic internal waves suggest that the latter are responsible for the strong dissipation events. At Eriador Seamount, both topographic internal waves and the internal tide contribute with the same order of magnitude to the dissipation rate while around the Reykjanes Ridge the internal tide provides the bulk of the dissipation rate.

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