Pathways, Volume Transport, and Mixing of Abyssal Water in the Samoan Passage

Voet, Gunnar; Girton, James B.; Alford, Matthew H.; Carter, Glenn S.; Klymak, Jody M.; Mickett, John B.

doi:10.1175/jpo-d-14-0096.1

Cited by 35 publications

(43 citation statements)

References 50 publications

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“…Our work suggests that lateral advection of tracer over rough topography where residence time is longer can provide a complementary explanation. A recent field programme in the equatorial Pacific Ocean found that over the rough topography of the Samoan Passage the microstructure-based diffusivity was a factor of 2 to 6 smaller than that inferred from a heat budget of the region, a bulk estimate similar in spirit to tracer-based estimates35. While under-sampling of strong turbulence was proposed as an explanation for the large discrepancy, our work suggests an alternative or complementary explanation.…”

Section: Discussionsupporting

confidence: 55%

Topographic enhancement of vertical turbulent mixing in the Southern Ocean

et al. 2017

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It is an open question whether turbulent mixing across density surfaces is sufficiently large to play a dominant role in closing the deep branch of the ocean meridional overturning circulation. The diapycnal and isopycnal mixing experiment in the Southern Ocean found the turbulent diffusivity inferred from the vertical spreading of a tracer to be an order of magnitude larger than that inferred from the microstructure profiles at the mean tracer depth of 1,500 m in the Drake Passage. Using a high-resolution ocean model, it is shown that the fast vertical spreading of tracer occurs when it comes in contact with mixing hotspots over rough topography. The sparsity of such hotspots is made up for by enhanced tracer residence time in their vicinity due to diffusion toward weak bottom flows. The increased tracer residence time may explain the large vertical fluxes of heat and salt required to close the abyssal circulation.

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Section: Discussionsupporting

confidence: 55%

Topographic enhancement of vertical turbulent mixing in the Southern Ocean

et al. 2017

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“…We then use the instantaneous buoyancy gradient N 2 to estimate the thickness‐weighted‐average buoyancy gradient

1 false/ (⟨⟩, 1 false/ N^{2})

, so that the ratio between turbulent buoyancy flux and thickness‐weighted‐average buoyancy gradient expressed by and becomes

κ_{e} = (⟨⟩, \frac{1}{N^{2}}) (⟨⟩, normalΓ ϵ),

where the angle brackets again denote an average in density space, or on an isopycnal. The dependence of the effective squeeze dispersion diffusivity on the average turbulent buoyancy flux ⟨Γ ϵ ⟩ is consistent, for example, with the logic used by Voet et al () to compare the average in situ turbulent heat flux with a bulk estimate of heat flux from the temperature distribution in the Samoan passage. In section , we develop a technique to coarse‐grain the Samoan passage observations reported in Alford et al () and Voet et al () to evaluate equation and interpret its implications for tracer dispersion.…”

Section: Discussionsupporting

confidence: 82%

“…The dependence of the effective squeeze dispersion diffusivity on the average turbulent buoyancy flux ⟨Γ ϵ ⟩ is consistent, for example, with the logic used by Voet et al () to compare the average in situ turbulent heat flux with a bulk estimate of heat flux from the temperature distribution in the Samoan passage. In section , we develop a technique to coarse‐grain the Samoan passage observations reported in Alford et al () and Voet et al () to evaluate equation and interpret its implications for tracer dispersion.…”

Section: Discussionsupporting

confidence: 82%

Squeeze Dispersion and the Effective Diapycnal Diffusivity of Oceanic Tracers

Wagner

Flierl

Ferrari

et al. 2019

Geophysical Research Letters

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We describe a process called “squeeze dispersion” in which the squeezing of oceanic tracer gradients by waves, eddies, and bathymetric flow modulates diapycnal diffusion by centimeter to meter‐scale turbulence. Due to squeeze dispersion, the effective diapycnal diffusivity of oceanic tracers is different and typically greater than the average “local” diffusivity, especially when local diffusivity correlates with squeezing. We develop a theory to quantify the effects of squeeze dispersion on diapycnal oceanic transport, finding formulas that connect density‐averaged tracer flux, locally measured diffusivity, large‐scale oceanic strain, the thickness‐weighted average buoyancy gradient, and the effective diffusivity of oceanic tracers. We use this effective diffusivity to interpret observations of abyssal flow through the Samoan Passage reported by Alford et al. (2013, https://doi.org/10.1002/grl.50684) and find that squeezing modulates diapycnal tracer dispersion by factors between 0.5 and 3.

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“…Most ocean basins are delineated by their distinctive range of bottom densities. For example, the Samoa Passage, a well documented bottleneck of AABW (Roemmich et al, ; Voet et al, ), separates the Southwest Pacific Basin and the Central Pacific Basin with a step like change in bottom density.…”

Section: Definition Of the Wsbblmentioning

confidence: 99%

“…The profiles are shifted in density for clarity. , 1996;Voet et al, 2015), separates the Southwest Pacific Basin and the Central Pacific Basin with a step like change in bottom density.…”

Section: Definition Of the Wsbblmentioning

confidence: 99%

The Weakly Stratified Bottom Boundary Layer of the Global Ocean

2018

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The weakly stratified bottom boundary layer (wsBBL) of the global ocean is currently unmapped; even the definition of the wsBBL layer is yet lacking. However, recent studies point to the wsBBL as a region where most of the abyssal water transformation takes place. In this study, historical high‐resolution density profiles are used to map the properties of the wsBBL in the global ocean. We use a density gradient criteria ( 1×10−5 kg m– 4) to define the top of the layer. The thickness of the wsBBL varies from several meters to over a thousand meters and can be used as a rule of thumb to differentiate basin walls from the basin bottom, respectively. Although the thickness varies greatly, the pressure at the top of the wsBBL varies relatively smoothly allowing us to map its distribution across the ocean along with the density of the wsBBL. The neutral density, γwsBBL, and pressure, PwsBBL, of the upper boundary of the wsBBL are highly correlated within each ocean basin. Diagrams of γwsBBL versus PwsBBL clearly differentiate the different basins, connected by the narrow channels, along the pathways of abyssal water circulation. The diagrams give insight into the different mechanisms of abyssal water transformation and highlight locations where transformation happens: inter‐basin channels and over some parts of mid‐oceanic ridges such as found in the Brazil Basin, in the Guiana Basin, and in the Southwest Pacific Basin.

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Pathways, Volume Transport, and Mixing of Abyssal Water in the Samoan Passage

Cited by 35 publications

References 50 publications

Topographic enhancement of vertical turbulent mixing in the Southern Ocean

Topographic enhancement of vertical turbulent mixing in the Southern Ocean

Squeeze Dispersion and the Effective Diapycnal Diffusivity of Oceanic Tracers

The Weakly Stratified Bottom Boundary Layer of the Global Ocean

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