Abyssal Upwelling in Mid‐Ocean Ridge Fracture Zones

Clement, Louis; Thurnherr, Andreas M.

doi:10.1002/2017gl075872

Cited by 9 publications

(4 citation statements)

References 40 publications

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“…Upwelling currents within narrow canyons can be forced by the density gradient resulting from bottom-enhanced mixing over the sloping topography (e.g., St. Laurent et al, 2001;Thurnherr et al, 2005;Clément and Thurnherr, 2018), which is often predominantly generated by internal tides. Because of topographic blocking, the cross-slope component of the Ekman-like response is suppressed, and the resulting flow is mostly an along-canyon overturning cell with the density field in advective-diffusive balance.…”

Section: Dynamical Origin and Forcingmentioning

confidence: 99%

“…The direct impacts of the (sub-)mesoscale eddies and internal waves on the mean flow in such realistic configuration remain largely unknown as well, while several possible mechanisms have been identified (Holloway and Wang, 2009;Venaille et al, 2011;Grisouard and Bühler, 2012;Xie et al, 2018). For instance, Clément and Thurnherr (2018) recently reported evidence of non-linear interactions between internal waves and the mean flow in a submarine valley, thus showing that these mechanisms could play a significant role in the deep-ocean dynamics.…”

Section: Introductionmentioning

confidence: 99%

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Deep Currents in the Rift Valley of the North Mid-Atlantic Ridge

et al. 2019

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Over mid-ocean ridges, the interaction between the currents and the topography gives rise to complex flows, which drive the transport properties of biogeochemical constituents, and especially those associated with hydrothermal vents, thus impacting associated ecosystems. This paper describes the circulation in the rift valley along the Azores sector of the North Mid-Atlantic Ridge, using a combination of in-situ data from several surveys and realistic high-resolution modeling. It confirms the presence of a mean deep current with an up-valley branch intensified along the right inner flank of the valley (looking downstream), and a weaker down-valley branch flowing at shallower depth along the opposite flank. The hydrographic properties of the rift-valley water, and in particular the along-valley density gradient that results from a combination of the topographic isolation, the deep flow and the related mixing, are quantified. We also show that the deep currents exhibit significant variability and can be locally intense, with typical values greater than 10 cm/s. Finally, insights on the dynamical forcings of the deep currents and their variability are provided using numerical simulations, showing that tidal forcing of the mean circulation is important and that the overlying mesoscale turbulence triggers most of the variability.

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Section: Dynamical Origin and Forcingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Deep Currents in the Rift Valley of the North Mid-Atlantic Ridge

et al. 2019

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“…observations include trapped near-inertial wave packets in a warm-core ring (Joyce et al 2013), dissipation rates elevated in a deep Southern Ocean eddy (Sheen et al 2015), increased shear variance caused by waves trapped in submesoscale cyclonic vortex filaments in the north wall of the Gulf Stream (Whitt et al 2018), and high dissipation rates away from the seafloor inside the midocean fracture zones, caused by transfer of nearinertial wave energy to turbulence in a critical layer (Clément and Thurnherr 2018).…”

Section: Introductionmentioning

confidence: 99%

The Dissipation of Kinetic Energy in the Lofoten Basin Eddy

Fer

Bosse

Ferron

et al. 2018

Journal of Physical Oceanography

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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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“…As a result, the decreasing stratification in such a downstream basin is mostly found at the “benthic thermocline.” This is particularly clear in the “benthic thermocline” of the BB where our analysis reveals a N 2 trend of −12% per decade. This signal extends across the entire basin into the Fracture Zone valleys in the eastern BB where it likely affects the along‐valley transport, upwelling, and diapycnal mixing processes (Clément & Thurnherr, 2018; Thurnherr et al., 2020; Zhao & Thurnherr, 2018). In contrast, where sills are absent or not tall enough to block the AABW, bottom‐reaching negative N 2 trends are observed in the downstream basins (e.g., the SAB).…”

Section: Summary and Discussionmentioning

confidence: 99%

On the Global Decrease in the Deep and Abyssal Density Stratification Along the Spreading Pathways of Antarctic Bottom Water Since the 1990s

Tan

Thurnherr

2023

Geophysical Research Letters

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The density stratification in the ocean is directly related to the diapycnal mixing, which drives the abyssal cell of the Meridional Overturning Circulation (MOC). It is important to understand how stratification has been changing in the world's deep and abyssal oceans under climate change. Using repeat hydrographic data obtained since the 1990s, we find a decreasing stratification associated with changes in the source Antarctica Bottom Water (AABW) properties in its formation basins as well as in basins along its dispersal pathways. Averaged south of 60°S, the squared buoyancy frequency N2 shows a negative trend of −6% per decade in waters deeper than 4,000 m. The observed decadal reduction in stratification is associated with large spatial variability, especially in the Southern Ocean basins with multiple AABW sources. Additionally, there are also significant differences between neighboring basins that are related to the blocking effect of topography.

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Abyssal Upwelling in Mid‐Ocean Ridge Fracture Zones

Cited by 9 publications

References 40 publications

Deep Currents in the Rift Valley of the North Mid-Atlantic Ridge

Deep Currents in the Rift Valley of the North Mid-Atlantic Ridge

The Dissipation of Kinetic Energy in the Lofoten Basin Eddy

On the Global Decrease in the Deep and Abyssal Density Stratification Along the Spreading Pathways of Antarctic Bottom Water Since the 1990s

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