Turbulence and high‐frequency variability in a deep gravity current outflow

Nash, Jonathan D.; Peters, Hartmut; Kelly, Samuel M.; Pelegrí, Josep Lluís; Emelianov, Mikhail; Gasser, Marc

doi:10.1029/2012gl052899

Cited by 21 publications

(32 citation statements)

References 46 publications

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“…Hence, shear instability can be reasonably assumed to be the mechanism for these overturns. However, the largest overturns (Figure b) and the bulk of the turbulence (Figure c) occur immediately downstream of the shear layers in association with the hydraulic responses to topographic features, suggesting that convective overturning in lee waves triggered by the topography dominates the mixing in this descending gravity current—as found in a numerical model by Özgökmen and Fischer [] and observationally by Nash et al []. The importance of these hydraulic jumps presents a challenge for overflow simulations that currently rely on shear‐based entrainment schemes such as Jackson et al [].…”

Section: Resultsmentioning

confidence: 92%

Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage

Alford

Girton

Voet

et al. 2013

Geophysical Research Letters

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[1] We report the first direct turbulence observations in the Samoan Passage (SP), a 40 km wide notch in the South Pacific bathymetry through which flows most of the water supplying the North Pacific abyssal circulation. The observed turbulence is 1000 to 10, 000 times typical abyssal levels -strong enough to completely mix away the densest water entering the passage-confirming inferences from previous coarser temperature and salinity sections. Accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence. Specifically, the flow accelerates substantially at the primary sill within the passage, reaching speeds as great as 0.55 m s -1 . A strong hydraulic response is seen, with layers first rising to clear the sill and then plunging hundreds of meters downward. Turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill. In addition to the primary sill, other locations along the multiple interconnected channels through the Samoan Passage also have an effect on the mixing of the dense water. In fact, quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers, suggesting that abyssal mixing depends sensitively on bathymetric details on small scales. Citation: Alford, M. H., J. B. Girton, G. Voet, G. S. Carter, J. B. Mickett, and J. M. Klymak (2013), Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage, Geophys. Res. Lett., 40,[4668][4669][4670][4671][4672][4673][4674]

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Section: Resultsmentioning

confidence: 92%

Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage

Alford

Girton

Voet

et al. 2013

Geophysical Research Letters

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“…Nash et al (2012) have pointed out the importance of small-scale features in the bottom topography. The flow appears to be hydraulically controlled at a small topographic constriction, with turbulence and internal waves varying together and increasing dramatically downstream of a choke point.…”

Section: Discussionmentioning

confidence: 99%

Microstructure measurements and estimates of entrainment in the Denmark Strait overflow plume

Пака

Zhurbas²,

Rudels³

et al. 2013

Ocean Sci.

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Abstract. To examine processes controlling the entrainment of ambient water into the Denmark Strait overflow (DSO) plume / gravity current, measurements of turbulent dissipation rate were carried out by a quasi-free-falling (tethered) microstructure profiler (MSP). The MSP was specifically designed to collect data on dissipation-scale turbulence and fine thermohaline stratification in an ocean layer located as deep as 3500 m. The task was to perform microstructure measurements in the DSO plume in the lower 300 m depth interval including the bottom mixed layer and the interfacial layer below the non-turbulent ambient water. The MSP was attached to a Rosette water sampler rack equipped with a SeaBird CTDO and an RD Instruments lowered acoustic Doppler current profiler (LADCP). At a chosen depth, the MSP was remotely released from the rack to perform measurements in a quasi-free-falling mode.Using the measured vertical profiles of dissipation, the entrainment rate as well as the bottom and interfacial stresses in the DSO plume were estimated at a location 200 km downstream of the sill at depths up to 1771 m. Dissipation-derived estimates of entrainment were found to be much smaller than bulk estimates of entrainment calculated from the downstream change of the mean properties in the plume, suggesting the lateral stirring due to mesoscale eddies rather than diapycnal mixing as the main contributor to entrainment. Dissipation-derived bottom stress estimates are argued to be roughly one third the magnitude of those derived from log velocity profiles. In the interfacial layer, the Ozmidov scale calculated from turbulence dissipation rate and buoyancy frequency was found to be linearly proportional to the overturning scale extracted from conventional CTD data (the Thorpe scale), with a proportionality constant of 0.76, and a correlation coefficient of 0.77.

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“…Gasser et al 2011and Nash et al (2012) report observations using moorings and tow-yos in the Mediterranean outflow in the Gulf of Cadiz 70 km west of the Strait of Gibraltar and to the west of the Espartel Sill, the most western sill of the Strait. At this location the dense outflow is confined to a westward flowing layer of water, some 150 m thick and of relatively high salinity, moving westward over the seabed at approximately 1.2 m s -1 .…”

Section: The Mediterranean Outflowmentioning

confidence: 99%

“…In (a) points are taken from in the two channels, NC and SC, of the Red Sea Outflow. (b) shows points taken from Gasser et al 2011and Nash et al (2012) at stations UTS and DTS in the Mediterranean Outflow. The dot-dash line corresponds to an interfacial gradient Richardson number of 1/3.…”

Section: Appendix C Radiation Of Internal Wavesmentioning

confidence: 99%

Application of a model of internal hydraulic jumps

et al. 2017

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A model devised by Thorpe & Li (J. Fluid Mech., vol. 758, 2014, pp. 94–120) that predicts the conditions in which stationary turbulent hydraulic jumps can occur in the flow of a continuously stratified layer over a horizontal rigid bottom is applied to, and its results compared with, observations made at several locations in the ocean. The model identifies two positions in the Samoan Passage at which hydraulic jumps should occur and where changes in the structure of the flow are indeed observed. The model predicts the amplitude of changes and the observed mode 2 form of the transitions. The predicted dissipation of turbulent kinetic energy is also consistent with observations. One location provides a particularly well-defined example of a persistent hydraulic jump. It takes the form of a 390 m thick and 3.7 km long mixing layer with frequent density inversions separated from the seabed by some 200 m of relatively rapidly moving dense water, thus revealing the previously unknown structure of an internal hydraulic jump in the deep ocean. Predictions in the Red Sea Outflow in the Gulf of Aden are relatively uncertain. Available data, and the model predictions, do not provide strong support for the existence of hydraulic jumps. In the Mediterranean Outflow, however, both model and data indicate the presence of a hydraulic jump.

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Turbulence and high‐frequency variability in a deep gravity current outflow

Cited by 21 publications

References 46 publications

Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage

Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage

Microstructure measurements and estimates of entrainment in the Denmark Strait overflow plume

Application of a model of internal hydraulic jumps

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