Tilted Baroclinic Tidal Vortices

Topographically generated eddies and internal waves have traditionally been studied separately even though bathymetry that creates both phenomena is abundant in coastal regions. Here a numerical model is used to understand the dynamics of eddy and wave generation as tidal currents flow past Three Tree Point, a 1 km long, 200 m deep, sloping headland in Puget Sound, WA. Bottom pressure anomalies due to vertical perturbations of the sea surface and isopycnals are used to calculate form drag in different regions of the topography to assess the relative importance of eddies versus internal waves. In regions where internal waves dominate, sea surface and isopycnal perturbations tend to work together to create drag, whereas in regions dominated by eddies, sea surface, and isopycnal perturbations tend to counteract each other. Both phenomena are found to produce similar amounts of form drag even though the bottom pressure anomalies from the eddy have much larger magnitudes than those created by the internal waves. Topography like Three Tree Point is common in high latitude, coastal regions, and therefore the findings here have implications for understanding how coastal topography removes energy from tidal currents.

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“…Canals et al . [] show that the velocity remains almost entirely horizontal. TTP is not the only place to spawn tilted eddies.…”

Section: Physical Mechanisms That Create Form Dragmentioning

confidence: 99%

“…[], who details its time evolution, and by Canals et al . [], who describe the isopycnal structure resulting from the eddy tilt. The form drag at TTP is as much as 50 times larger than frictional drag over a flat bottom of equivalent area [ Edwards et al ., ].…”

Section: Introductionmentioning

confidence: 99%

The dynamics of pressure and form drag on a sloping headland: Internal waves versus eddies

Warner

MacCready

2014

J. Geophys. Res. Oceans

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“…Precise bottom pressure sensors (Ppods) (St€ ober and Moum 2011) were deployed across the bathymetry near Three Tree Point (TTP), a headland in Puget Sound, Washington, with predictable tidal currents. TTP has been studied extensively in the past: Edwards et al (2004) and McCabe et al (2006) quantified the internal and external form drag, respectively, Canals et al (2009) described the tilted eddies present there, and Warner and MacCready (2009, hereafter WM09) showed with a numerical model that there is an additional part of the form drag that arises in oscillatory flow situations (''inertial'' form drag). In this study, total form drag from bottom pressure sensors, ''bulk'' drag estimated from the shallow water momentum equation, internal form drag, inertial form drag (arising from tidal time-dependence), frictional drag, and turbulent dissipation are all quantified.…”

Section: Introductionmentioning

confidence: 99%

“…At peak floods, the spatially integrated dissipation is only 25%-50% of the total power. Assuming that our turbulence observations have adequately captured the total dissipation, this suggests that not all of the energy is dissipated locally, but instead is carried away from the topography by either eddies (McCabe et al 2006;Canals et al 2009) or internal waves (section 5a) that do not break directly above TTP. The percentage of local energy dissipation at TTP can be compared to other sites such as Knight Inlet where Klymak and Gregg (2004) found that one-third of the energy lost (Fig.…”

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confidence: 95%

Measurement of Tidal Form Drag Using Seafloor Pressure Sensors

Warner

MacCready

Moum

et al. 2013

Journal of Physical Oceanography

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As currents flow over rough topography, the pressure difference between the up-and downstream sides results in form drag-a force that opposes the flow. Measuring form drag is valuable because it can be used to estimate the loss of energy from currents as they interact with topography. An array of bottom pressure sensors was used to measure the tidal form drag on a sloping ridge in 200 m of water that forms a 1-km headland at the surface in Puget Sound, Washington. The form drag per unit length of the ridge reached 1 3 10 4 N m 21 during peak flood tides. The tidally averaged power removed from the tidal currents by form drag was 0.2 W m 22, which is 30 times larger than power losses to friction. Form drag is best parameterized by a linear wave drag law as opposed to a bluff body drag law because the flow is stratified and both internal waves and eddies are generated on the sloping topography. Maximum turbulent kinetic energy dissipation rates of 5 3 10 25 W kg 21 were measured with a microstructure profiler and are estimated to account for 25%-50% of energy lost from the tides. This study is among the first to measure form drag directly using bottom pressure sensors. The measurement and analysis techniques presented here are suitable for periodically reversing flows because they require the removal of a time-mean signal. The advantage of this technique is that it delivers a continuous record of form drag and is much less ship intensive compared to previous methods for estimation of the bottom pressure field.

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“…This is in agreement with the new unstable mode found by Candelier, Le Dizès & Millet (2011) in a tilted jet using a linear stability analysis. The existence of tilted vortices in a strongly stratified fluid is made possible because the streamlines remain horizontal but centred around a tilted axis (Boulanger, Meunier & Le Dizès 2006;Canals, Pawlak & MacCready 2009). However, this structure generates a critical layer at the radius where the angular velocity equals the Brunt-Väisälä frequency, leading to strong axial shears which are unstable with respect to the Kelvin-Helmholtz instability (Boulanger, Meunier & Le Dizès 2007).…”

Section: Introduction and Contextmentioning

confidence: 99%

Three-dimensional instabilities of a stratified cylinder wake

Bosco

Meunier²

2014

J. Fluid Mech.

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International audienceThis paper describes experimentally, numerically and theoretically how the three-dimensional instabilities of a cylinder wake are modified by the presence of a linear density stratification. The first part is focused on the case of a cylinder with a small tilt angle between the cylinder's axis and the vertical. The classical mode A well-known for a homogeneous fluid is still present. It is more unstable for moderate stratifications but it is stabilized by a strong stratification. The second part treats the case of a moderate tilt angle. For moderate stratifications, a new unstable mode appears, mode S, characterized by undulated layers of strong density gradients and axial flow. These structures correspond to Kelvin–Helmholtz billows created by the strong shear present in the critical layer of each tilted von Kármán vortex. The last two parts deal with the case of a strongly tilted cylinder. For a weak stratification, an instability (mode RT) appears far from the cylinder, due to the overturning of the isopycnals by the von Kármán vortices. For a strong stratification, a short wavelength unstable mode (mode L) appears, even in the absence of von Kármán vortices. It is probably due to the strong shear created by the lee waves upstream of a secondary recirculation bubble. A map of the four different unstable modes is established in terms of the three parameters of the study: the Reynolds number, the Froude number (characterizing the stratification) and the tilt angle

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Tilted Baroclinic Tidal Vortices

Cited by 25 publications

References 35 publications

The dynamics of pressure and form drag on a sloping headland: Internal waves versus eddies

The dynamics of pressure and form drag on a sloping headland: Internal waves versus eddies

Measurement of Tidal Form Drag Using Seafloor Pressure Sensors

Three-dimensional instabilities of a stratified cylinder wake

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