Observations on stratified flow over a bank at low Froude numbers

Jarosz, Ewa; Wijesekera, H. W.; Teague, William J.; Fribance, Diane B.; Moline, Mark A.

doi:10.1002/2014jc009934

Cited by 8 publications

(9 citation statements)

References 56 publications

(91 reference statements)

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“…Since the median value of the two estimates are so similar, we suspect that bin averaging a greater number of observations would result in better agreement between the two methods. Similar ranges were observed by Wijesekera et al (2014) over the East Flower Garden Bank in the Gulf of Mexico, a shallow bank with similar rates of TKE dissipation to what we observed in Bering Strait (Jarosz et al 2014).…”

Section: Estimating the Bottom Drag Coefficientsupporting

confidence: 87%

“…These rates are high for the coastal ocean (e.g., Moum et al 2002;MacKinnon and Gregg 2003;Shao et al 2018) but not uncommon (e.g., Simpson et al 1996;Shao et al 2018). They are similar in magnitude to dissipation rates observed at flow constrictions or over shallow banks and sills where the acceleration of flow approaching an obstacle is known to create turbulence (e.g., Wesson and Gregg 1994;Polzin et al 1996;Moum and Nash 2000;Nash and Moum 2001;Jarosz et al 2014). The diapycnal diffusivities observed here, however, are elevated above those observed in other regions of strong turbulence.…”

Section: B Observed Rates Of Energy Dissipationmentioning

confidence: 56%

“…Equation ( 5) represents the dissipation method for estimating friction velocity within the boundary layer (Dewey and Crawford 1988;Perlin et al 2005b;Wijesekera et al 2014). For each profile, we determine the values of u * using a least squares fit to our measurements of « from the deepest observation to the height above the boundary where « can no longer be approximated by a 1/z profile.…”

Section: Estimating the Bottom Drag Coefficientmentioning

confidence: 99%

“…8, 9). To extend our ADCP measurements (which usually stop 5 or more meters off the bottom) closer to the constant stress layer, we linearly extrapolate the two deepest velocity measurements to a height of 3 m off the bottom to get our value of u 0 (Wijesekera et al 2014). The C d from the dissipation method is 2.3 (60.4) 3 10 23 , and the profile method gives a value of 1.7 (60.6) 3 10 23 (Figs.…”

Section: Estimating the Bottom Drag Coefficientmentioning

confidence: 99%

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Mixing Rates and Bottom Drag in Bering Strait

Couto

Alford

MacKinnon

et al. 2020

Journal of Physical Oceanography

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Three shipboard survey lines were occupied in Bering Strait during autumn of 2015, where high-resolution measurements of temperature, salinity, velocity, and turbulent dissipation rates were collected. These first-reported turbulence measurements in Bering Strait show that dissipation rates here are high even during calm winds. High turbulence in the strait has important implications for the modification of water properties during transit from the Pacific Ocean to the Arctic Ocean. Measured diffusivities averaging 2 × 10−2 m2 s−1 are capable of causing watermass property changes of 0.1°C and 0.1 psu during the ~1–2-day transit through the narrowest part of the strait. We estimate friction velocity using both the dissipation and profile methods and find a bottom drag coefficient of 2.3 (±0.4) × 10−3. This result is smaller than values typically used to estimate bottom stress in the region and may improve predictions of transport variability through Bering Strait.

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Section: Estimating the Bottom Drag Coefficientsupporting

confidence: 87%

Section: B Observed Rates Of Energy Dissipationmentioning

confidence: 56%

Section: Estimating the Bottom Drag Coefficientmentioning

confidence: 99%

Section: Estimating the Bottom Drag Coefficientmentioning

confidence: 99%

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Mixing Rates and Bottom Drag in Bering Strait

Couto

Alford

MacKinnon

et al. 2020

Journal of Physical Oceanography

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“…The one on the bank, M5, was a 600 kHz Workhorse; the others, M1–M4, operated at 300 kHz to examine flow adjacent to the bank. Intensive shipboard measurements by NRL in June 2011 found flow disturbances downstream of the bank, but upstream mixing levels were modest, with diapycnal diffusivities less than or equal to

5 \times 10^{- 5} {normalm}^{2} {normals}^{- 1}

[ Jarosz et al ., ]. Between 5 and 14 November 2011, we sampled flows and scalar fields on and near the bank from R/V Cape Hatteras.…”

Section: Bathymetry and Samplingmentioning

confidence: 99%

Mode-2 hydraulic control of flow over a small ridge on a continental shelf

Gregg

Klymak

2014

J. Geophys. Res. Oceans

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Some of the most intense turbulence in the ocean occurs in hydraulic jumps formed in the lee of sills where flows are hydraulically controlled, usually by the first internal mode. Observations on the outer Texas-Louisiana continental shelf reveal hydraulic control of internal mode-2 lasting more than 3 h over a 20 m high ridge on the 100 m deep continental shelf. When control began the base of the weakly stratified surface layer bulged upward and downward, a signature of mode-2. As the westward flow producing control was lost, large-amplitude disturbances, initially resembling a bore in the weakly stratified layer, began propagating eastward. Average dissipation rates inferred from density inversions over the ridge were 10 28 and 10 27 W kg 21 , one to two decades above local background. Corresponding diapycnal diffusivities, K q , were 10 24 to 10 23 m 2 s 21 . Short-term mixing averages did not evolve systematically with hydraulic control, possibly owing to our inability to observe small overturns in strongly stratified water directly over the ridge.To test the feasibility of our interpretation of the observations, hydrostatic runs with a three-dimensional MITgcm simulated mode-2 control and intense mixing over the ridge below the interface. Details differed from observations, principally because we lacked three-dimensional density fields to initialize the model which was forced with currents observed by a bottom-mounted ADCP several kilometers east of the ridge. Consequently, the model did not capture all flow features around the bank. The principal conclusion is that hydraulic responses to higher modes can dominate flows around even modest bathymetric irregularities. Background and IntroductionSome of the most intense mixing in the ocean occurs in hydraulic jumps downstream of sills lying across channels and straits. Internal modes are hydraulically controlled where their propagation speeds are slower than opposing currents. Control is lost downstream as the current slows and/or mode speeds increase with increases in channel width or depth. The transition from a high-energy to a low-energy state releases energy in the form of strong turbulence. Many cases prominent in the oceanographic literature are forced by M 2 barotropic tides and have controls lasting only a few hours of the tidal cycle [Farmer and Smith, 1980;Armi and Farmer, 1988;Wesson and Gregg, 1994;Ferron et al., 2003;Klymak and Gregg, 2004]. Hydraulic controls, however, also occur at subtidal frequencies. For instance, observations on the Oregon shelf found intense mixing when irregular flows produced mode-1 control atop 20 m high Stonewall Bank [Moum and Nash, 2000;Nash and Moum, 2001]. Controls have also been demonstrated in quasi-steady flows, such as the Bosphorus, where currents vary principally in response to changes in synoptic weather patterns [O guz et al., 1990;Gregg et al., 1999].The introduction to Pratt and Whitehead [2008] provide an extensive discussion of the range of hydraulic flows found in ocean and atmosphere. Much o...

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Coral bleaching and recovery from 2016 to 2017 at East and West Flower Garden Banks, Gulf of Mexico

Johnston

Hickerson

Nuttall³

et al. 2019

Coral Reefs

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East Flower Garden Bank (EFGB) and West Flower Garden Bank (WFGB), part of Flower Garden Banks National Marine Sanctuary (FGBNMS) in the northwestern Gulf of Mexico, support tropical coral reefs that exhibit over 50% living coral cover. These reefs have been monitored annually since 1989, and in 2016 were exposed to higher than normal seawater temperatures leading to a severe bleaching event. Corals at EFGB and WFGB showed no signs of bleaching until September 2016, occurring later in the year compared to other reefs in the Caribbean region. Coral bleaching and subsequent recovery at each bank were documented through a time series of repetitive photographs within previously established long-term monitoring stations. Preceding the event, mean live coral cover within monitoring stations was collectively 64 ± 2%. Prior to signs of bleaching from July to September 2016, seawater temperatures on the reef were above 30°C for a total of 36 d at EFGB and 21 d at WFGB. By October 2016, 67 ± 5% of the coral cover within EFGB monitoring stations and 25 ± 3% within WFGB monitoring stations exhibited signs of bleaching or paling stress, with dissimilarities in the amount of bleaching most likely due to significant differences in thermal profiles between banks. Significantly increasing long-term trends for daily mean seawater temperature indicate that temperatures on the banks have become warmer over time, and calculated bleaching threshold curves suggest that more than 50 d above 29.5°C would initiate a bleaching year at EFGB and WFGB. Even though recovery within monitoring stations at both banks was documented with no significant declines in mean coral cover from 2016 to 2017 (64% and 62%, respectively), it is likely FGBNMS will be subject to additional and more frequent bleaching events in the future as ocean temperatures continue to rise.

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Observations on stratified flow over a bank at low Froude numbers

Cited by 8 publications

References 56 publications

Mixing Rates and Bottom Drag in Bering Strait

Mixing Rates and Bottom Drag in Bering Strait

Mode-2 hydraulic control of flow over a small ridge on a continental shelf

Coral bleaching and recovery from 2016 to 2017 at East and West Flower Garden Banks, Gulf of Mexico

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