Internal hydraulic flows on the continental shelf: High drag states over a small bank

Nash, Jonathan D.; Moum, James N.

doi:10.1029/1999jc000183

Cited by 140 publications

(125 citation statements)

References 37 publications

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“…[as documented in other similar coastal systems, e.g., Farmer and Armi, 1999;Nash and Moum, 2001;Gregg, 2001, 2004;Armi and Farmer, 2002;Cummins et al, 2003;Palmer et al, 2013]. Observational evidence of lee waves has already been reported to occur in this area downstream of the sill as a result of ebb currents [see Saucier and Chasse, 2000, Figure 15].…”

Section: The Nutrient Pump Mechanismmentioning

confidence: 96%

Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

et al. 2015

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Turbulent vertical nitrate fluxes were calculated using new turbulent microstructure observations in the Lower St. Lawrence Estuary (LSLE), Canada. Two stations were compared: the head of the Laurentian Channel (HLC), where intense mixing occurs on the shallow sill that marks the upstream limit of the LSLE, and another station located about 100 km downstream (St. 23), more representative of the LSLE mean mixing conditions. Mean turbulent diffusivities and nitrate fluxes at the base of the surface layer for both stations were, respectively (with 95% confidence intervals): K HLC Observations suggest that the interplay between large isopleth heaving near the sill and strong turbulence is the key mechanism to sustain such high turbulent nitrate fluxes at the HLC (two to three orders of magnitude higher than those at Station 23). Calculations also suggest that nitrate fluxes at the HLC alone can sustain primary production rates of 3:4ð0:6; 11Þ g C m 22 mo 21 over the whole LSLE, approximately enough to account for a large part of the phytoplankton bloom and for most of the postbloom production. Surfacing nitrates are also believed to be consumed within the LSLE, not leaving much to be exported to the rest of the Gulf of St. Lawrence.

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Section: The Nutrient Pump Mechanismmentioning

confidence: 96%

Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

et al. 2015

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“…This is possibly due to the relatively long, narrow part of the Little Belt in which the flow may stay supercritical for a long distance. In comparison, most of the other examples of observations of hydraulic control known to us are concerned with a steep, relatively short sill with a dense, supercritical overflow spilling down the lee side of the sill (Farmer and Smith, 1980;Farmer and Denton, 1985;Farmer and Armi, 1999;Nash and Moum, 2001;Inall et al, 2004;Klymak and Gregg, 2004;Staalstrøm et al, 2015). In these examples mixing is confined to a relatively limited area downstream of the sill.…”

Section: Discussionmentioning

confidence: 99%

“…Existing observations of hydraulic control in a twolayered system (or with respect to the first internal mode) in a strait include Farmer and Smith (1980), Farmer and Denton (1985), Farmer and Armi (1999), Nielsen (2001), Inall et al (2004), Klymak and Gregg (2004), and Staalstrøm et al (2015). Other observations of control with respect to the first internal mode were made by Nash and Moum (2001), who studied the stratified flow over an isolated bank on the continental shelf. Direct observations of control of higher internal modes have also been made, although these are more subtle and play a lesser role with respect to dynamics, circulation, and water mass formation (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…An important aspect of the Little Belt is that the topography is subject to relatively smooth variations in terms of both breadth and depth, in contrast to most of the other situations in nature in which internal hydraulic control has been observed directly (Farmer and Smith, 1980;Farmer and Denton, 1985;Farmer and Armi, 1999;Nash and Moum, 2001;Inall et al, 2004;Klymak and Gregg, 2004;Staalstrøm et al, 2015). This implies that the Little Belt may be compared with and understood in terms of some simplified, existing theoretical and laboratory studies, such as Armi (1986) and Lawrence (1993).…”

Section: Introductionmentioning

confidence: 99%

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Internal hydraulic control in the Little Belt, Denmark – observations of flow configurations and water mass formation

Nielsen¹,

Vang

Lund-Hansen

2017

Ocean Sci.

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Abstract. Internal hydraulic control, which occurs when stratified water masses are forced through an abrupt constriction, plays an enormous role in nature on both large and regional scales with respect to dynamics, circulation, and water mass formation. Despite a growing literature on this subject surprisingly few direct observations have been made that conclusively show the existence of and the circumstances related to internal hydraulic control in nature. In this study we present observations from the Little Belt, Denmark, one of three narrow straits connecting the Baltic Sea and the North Sea. The observations (comprised primarily of along-strait, detailed transects of salinity and temperature; continuous observations of flow velocity, salinity, and temperature at a permanent station; and numerous vertical profiles of salinity, temperature, fluorescence, and flow velocity in various locations) show that internal hydraulic control is a frequently occurring phenomenon in the Little Belt. The observations, which are limited to south-going flows of approximately twolayered water masses, show that internal hydraulic control may take either of two configurations, i.e. the lower or the upper layer being the active, accelerating one. This is connected to the depth of the pycnocline on the upstream side and the topography, which is both deepening and contracting toward the narrow part of the Little Belt. The existence of two possible flow configurations is known from theoretical and laboratory studies, but we believe that this has never been observed in nature and reported before. The water masses formed by the intense mixing, which is tightly connected with the presence of control, may be found far downstream of the point of control. The observations show that these particular water masses are associated with chlorophyll concentrations that are considerably higher than in adjacent water masses, showing that control has a considerable influence on the primary production and hence the ecosystem in the area.

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“…Flow-topography interactions caused by stratified flow over, around, and through rough topography are important for mixing in the deep abyssal ocean (e.g., Ledwell et al 2000), over the shelf (e.g., Nash and Moum 2001), and in fjords (e.g., Arneborg and Liljebladh 2009). Fjord entrances with sills are typical locations of strong flowtopography interactions due to strong barotropic and baroclinic currents caused by tides and exchange processes between the fjord and the coastal water (e.g., Arneborg 2004).…”

Section: Introductionmentioning

confidence: 99%

Observations of Turbulence Caused by a Combination of Tides and Mean Baroclinic Flow over a Fjord Sill

Staalstrøm

Arneborg

Liljebladh

et al. 2015

Journal of Physical Oceanography

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This study investigates the dissipation rates and flow conditions at the Drøbak Sill in the Oslofjord. The area was transected 13 times with a free-falling microstructure shear probe during 4 days in June 2011. At the same time, an ADCP was deployed inside the sill. During most tidal cycles, internal hydraulic jumps with high dissipation rates were found on the downstream side of the sill. However, the internal response varied strongly between different tidal cycles with similar barotropic forcing. In the beginning of the observational period, ebb tides had no hydraulic jumps, and in the end one of the flood tides did not have a hydraulic jump. During the same period, the mean baroclinic exchange flow changed from inflow to outflow in the bottom layer. The authors conclude that the conditions at the sill are on the edge of forming hydraulic jumps and that the mean baroclinic exchange may push the flow above or below the limit of a hydraulic jump depending on the situation. This conclusion is supported by two-layer hydraulic theory. The volume-integrated dissipation rates within 500 m from the sill crest compare well with estimates of energy loss in the lower layer calculated from the Bernoulli drop under the assumption of no energy loss in the upper layer. Finally, the mean dissipation rate at the sill was compared with the radiation of internal tidal energy away from the sill, and it was found that about 60%-90% of the total energy loss was dissipated locally.

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Internal hydraulic flows on the continental shelf: High drag states over a small bank

Cited by 140 publications

References 37 publications

Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

Internal hydraulic control in the Little Belt, Denmark – observations of flow configurations and water mass formation

Observations of Turbulence Caused by a Combination of Tides and Mean Baroclinic Flow over a Fjord Sill

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