Turbulent Mixing in a Deep Fracture Zone on the Mid-Atlantic Ridge

Abstract

Midocean ridge fracture zones channel bottom waters in the eastern Brazil Basin in regions of intensified deep mixing. The mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones, whether affected by the local or the nonlocal canyon topography, are still subject to debate. To discriminate those mechanisms and to discern the canyon mean flow, two moorings sampled a deep canyon over and away from a sill/contraction. A 2-layer exchange flow, accelerated at the sill, transports 0.04-0.10-Sv (1 Sv [ 10 6 m 3 s 21) up canyon in the deep layer. At the sill, the dissipation rate of turbulent kinetic energy « increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy. Cross-sill density and microstructure transects reveal an overflow potentially hydrauli-

Structure of the Deep Spillway in the Western Part of the Romanche Fracture Zone

“…If the depth range corresponding to the rise of the isotherms in the direction of the jump is above the level of the upper thermistor, then shifting the more weakly stratified waters from the Southern Depression toward the sill or termination of the hydraulic control overflow would cause the temperature increase at the upper thermistor. This situation of positive correlation of the lower and upper thermistors was observed, for example, in the first half of October 2013 (Figure ). Clément et al () showed the determinative influence of spring‐neap modulation (14‐day cycle) on turbulent mixing and overflow regime in the canyon narrows/sills in the South Atlantic Ridge fractures in the Brazil basin. To illustrate the role of this modulation in the Romanche FZ, we show in Figure d the distribution of the absolute values of the total tidal velocity on the basis of TPXO 7.2.…”

Changes in Bottom Water Physical Properties Above the Mid‐Atlantic Ridge Flank in the Brazil Basin

“…Thurnherr et al () argue that hydraulic processes associated with the subinertial along‐canyon flow passing across sills on the canyon floor contribute significantly to the elevated diapycnal mixing in the canyon. This inference has recently been confirmed by data from the Dynamics of Mid‐Ocean Ridge Experiment (DoMORE) project (Clément et al, ).…”

“…The mean abyssal flow within the canyon in the 1990s was found to be eastward based on moored current meters at 17.5°W (Thurnherr et al, ; Toole, ). More recent observations recorded by current meters and McLane Moored Profilers deployed at the sill near 14.6°W and at 13.7°W also show eastward flow below 3,900 m in 2014/2015 (Clément et al, ). The available data thus confirm persistent eastward flow within the canyon, which is essential to maintain a quasi‐steady state in the presence of strong diapycnal mixing (L. C. St. Laurent et al, ).…”

“…The study region is a deep FZ canyon on the western flank of the MAR near 21°S (Figure ). The canyon intersects the ridge flank and extends from the bottom of the Brazil Basin below 5,500 m near 26°W to the MAR crest near 12°W, where the connection to the eastern basin is blocked below 3,000 m. An important bathymetric feature in the FZ canyon is a prominent sill/constriction near 14.6°W where Thurnherr et al () and Clément et al () found the largest horizontal near‐bottom along‐canyon density gradient and the strongest diapycnal mixing. Two hydrographic surveys were carried out with stations along the canyon in 1997 and 2014 during the BBTRE and DoMORE projects, respectively.…”

“…The wave number k can be inferred from rotary spectra derived from the observed velocity shear near the canyon. The upward energy propagation in the semidiurnal frequency band is dominated by wavelengths between 90 m and about 560 m (Clément et al, ). The corresponding horizontal wavelength range inferred from the linear dispersion relationship is 680–4,250 m. Assuming U = 0.05 m/s, h = 200 m, and taking the mean values at 4,000 m in year 1997 and 2014 for N (i.e., versus ), the parameterized becomes about 3.89–20.26 in 1997 and 4.43–23.1 in 2014.…”