Entrainment and Energy Transfer Variability Along the Descending Path of the Denmark Strait Overflow Plume

North, Ryan P.; Jochumsen, Kerstin; Moritz, Martin

doi:10.1002/2018jc013821

Cited by 10 publications

(13 citation statements)

References 28 publications

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“…Having resolved L T as an indicator of vertical mixing, it is possible to then derive an estimate of turbulence dissipation rate from the overturning length scale (Moum, 1996): Dillon (1982). This ratio has been supported by subsequent observations (Ferron et al, 1998;Moum, 1996;Paka et al, 2013;Stansfield et al, 2001) and is commonly used to infer in the ocean due to the relative ease with which overturns are measured using standard oceanographic instrumentation (Finnigan et al, 2002;Fer, 2004;North et al, 2018). However, a number of other studies have proposed coefficients ranging from 0.6 to 1.2 (Mater et al, 2015;Smyth, 2000) and Ferron et al (1998) found that variability in the ratio can be 60%.…”

Section: Methodsmentioning

confidence: 89%

Turbulent Scales Observed in a River Plume Entering a Fjord

Stevens

O'Callaghan

2019

JGR Oceans

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Observations of turbulent mixing are quantified in the near-field region (first several kilometers) of a controlled river flow entering Doubtful Sound, New Zealand. Measurement techniques include direct turbulence measurements using microstructure profilers in both Eulerian and quasi-Lagrangian perspectives and turbulent overturn analysis. Over 500 profiles of vertical microstructure were collected within the inner fjord. With high flow speeds of the surface plume over 2 m/s and some of the strongest stratification (N 2 ∼ 10 −1 s −2 ) recorded in the coastal ocean, turbulent overturning (Thorpe, L T ) scales are used to indirectly infer turbulence dissipation rates ( ) and compared with direct measurements in a quasi-idealized laboratory setting. There exists an inverse relationship between directly measured estimates of and distance from the plume discharge point. Peak values of >10−2 W/kg are observed within 500 m from the discharge point, among the highest turbulence dissipation rates recorded in oceanic shear flows. Thorpe scales range from below 1 cm in the pycnocline to a few meters in the weakly stratified ambient water below the plume. Stratification within the surface layer constrains the vertical dimension, limiting L T . Therefore, L T is an unreliable estimator of L 0 , where L O is the Ozmidov scale and is used to infer . Care must thus be taken when applying this method of overturn analysis in an environment where stratification or physical boundaries limit the vertical extent of overturns.Direct turbulence measurements in estuaries and river plumes have been used to examine stratified-shear flow instabilities as a mechanism of mixing (Geyer et al.

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Section: Methodsmentioning

confidence: 89%

Turbulent Scales Observed in a River Plume Entering a Fjord

Stevens

O'Callaghan

2019

JGR Oceans

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“…High‐resolution modeling studies could provide answers, although topographic features may very well play a role, which are a challenge to capture realistically in models. The lifetime, pathway, and dissipation of the sill eddies will be addressed in further studies, eventually linking the features to downstream overflow modification (North et al, ) and eddy features off Greenland (von Appen et al, ).…”

Section: Discussionmentioning

confidence: 99%

Mesoscale Eddies Observed at the Denmark Strait Sill

et al. 2019

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The Denmark Strait overflow is the major export route of dense water from the Arctic Mediterranean into the North Atlantic. At the Strait's shallow sill, the overflow is a bottom-intensified cold and dense plume, bound to the east by a thermal front formed with the warmer, northward flowing North Icelandic Irminger Current. More than two decades of observations at the sill show strong fluctuations of volume flux on daily time scales. To better understand the source of this variability, a five-mooring array was installed at the sill, capturing nearly 1 year of velocity and bottom temperature measurements at a high temporal and spatial resolution. Bottom temperature fluctuations that exceed 4 • C indicate a meandering of the front between the plume and the North Icelandic Irminger Current. Current vector rotation shows trains of alternating cyclones and anticyclones at the sill. An eddy crosses the sill every 3 to 6 days with a mean velocity of 0.4 m/s and a typical diameter of 30 to 40 km. The results suggest that anticyclones, with centers passing through the deepest part of the sill, may be responsible for periods of increased volume flux-also referred to as boluses and pulses in previous studies. Although the relationship between eddies, pulses, and boluses is still unclear, the results show that eddies are directly linked to fluctuations in the strength, thickness, and position of the overflow plume. Plain Language SummaryThe southward flow of dense water from the Arctic ocean plays a crucial role in global ocean circulation but is almost immediately constrained on its way south by a submarine ridge that connects Greenland, Iceland, the Faroe Islands, and Scotland. The southward flow is therefore forced to pass through several straits and up and over relatively shallow sills. Most of the flow passes over a sill in the Denmark Strait, located between Greenland and Iceland. In this study, we present observations from an array of instruments, which measure the southward flow as it passes over the Denmark Strait sill. The flow is characterised by trains of eddies (vortices), with an alternating sense of rotation; meaning a counterclockwise eddy is usually followed by a clockwise eddy. The eddies are 30 to 40 km wide and need about 1 day to pass over the sill. These eddies help to explain pronounced changes in the flow across the sill, as they can help either to speed up the flow or slow it down. The results of this study contribute to understanding mesoscale fluctuations, which influence local mixing processes and water mass transports.

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“…(Price and Baringer 1994;Legg 2012). Although the mixing in overflows is highly localized (North et al 2018), it plays a significant role in influencing the largescale ocean circulation (Koszalka et al 2017). Studying the DSO helps us understand common features of climatologically important overflow processes in other parts of the global ocean.…”

Section: Introductionmentioning

confidence: 99%

Lagrangian Perspective on the Origins of Denmark Strait Overflow

Saberi

Haine

Gelderloos

et al. 2020

Journal of Physical Oceanography

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The Denmark Strait Overflow (DSO) is an important contributor to the lower limb of the Atlantic meridional overturning circulation (AMOC). Determining DSO formation and its pathways is not only important for local oceanography but also critical to estimating the state and variability of the AMOC. Despite prior attempts to understand the DSO sources, its upstream pathways and circulation remain uncertain due to short-term (3–5 days) variability. This makes it challenging to study the DSO from observations. Given this complexity, this study maps the upstream pathways and along-pathway changes in its water properties, using Lagrangian backtracking of the DSO sources in a realistic numerical ocean simulation. The Lagrangian pathways confirm that several branches contribute to the DSO from the north such as the East Greenland Current (EGC), the separated EGC (sEGC), and the North Icelandic Jet (NIJ). Moreover, the model results reveal additional pathways from south of Iceland, which supplied over 16% of the DSO annually and over 25% of the DSO during winter of 2008, when the NAO index was positive. The southern contribution is about 34% by the end of March. The southern pathways mark a more direct route from the near-surface subpolar North Atlantic to the North Atlantic Deep Water (NADW), and needs to be explored further, with in situ observations.

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Entrainment and Energy Transfer Variability Along the Descending Path of the Denmark Strait Overflow Plume

Cited by 10 publications

References 28 publications

Turbulent Scales Observed in a River Plume Entering a Fjord

Turbulent Scales Observed in a River Plume Entering a Fjord

Mesoscale Eddies Observed at the Denmark Strait Sill

Lagrangian Perspective on the Origins of Denmark Strait Overflow

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