Turbulence in the presence of internal waves in the bottom boundary layer of the California inner shelf

Allen, Rachel M.; Simeonov, Julian; Calantoni, Joseph; Stacey, Mark T.; Variano, Evan

doi:10.1007/s10236-018-1147-7

Cited by 4 publications

(2 citation statements)

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“…Measured dissipation levels at this 250 m deep site were as high as 10 −4 m 2 s −3 (Figure 9), with dissipation at 0.49 m ASB consistently a factor of two larger than at 1.4 m ASB (9c). These observed dissipation levels are high relative to the few prior observations of internal wave forced boundary turbulence made at comparable heights ASB, though in much shallower total water depth (e.g., Allen et al, 2018;Becherer et al, 2020;Klymak & Moum, 2003;Moum et al, 2007;Walter et al, 2014). The very high values suggest that boundary-turbulence may be an appreciable sink of energy for freely propagating waves in relatively deep-water such as our shelf site, though a thorough investigation involving net losses integrated over the boundary layer is beyond the scope of the present study.…”

Section: Overview Of the Turbulence Observationsmentioning

confidence: 52%

“…The presence of stable density stratification also limits the growth of the turbulent boundary layer, and the dynamics thus differ greatly from the tidal boundary layers of the vertically well‐mixed coastal ocean (e.g., Milne et al., 2017). The combined effects of stratification and unsteady forcing is seen in the transient boundary layers that can form beneath aperiodic shoaling waves of elevation or internal boluses (e.g., Allen et al., 2018; Davis & Monismith, 2011; Jones et al., 2020; Klymak & Moum, 2003; Moum et al., 2007; Walter et al., 2014), where significant density overturns can be seen well away from the bottom. However, such transient phenomena are distinct from the perennial stably stratified bottom mixing‐layers of interest in the present study.…”

Section: Introductionmentioning

confidence: 99%

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Mean and Turbulent Characteristics of a Bottom Mixing‐Layer Forced by a Strong Surface Tide and Large Amplitude Internal Waves

et al. 2022

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We present 15 days of both mean and turbulent field observations bottom mixing‐layer at a gently sloping 250 m deep continental shelf site, energized by tides and nonlinear internal waves (NLIWs). The tidal frequency forcing was due to the combined effects of the barotropic tide and a mode‐1 internal tide (IT), while the NLIWs were predominantly mode‐1 waves of depression. The bottom mixing‐layer thickness varied at both semidiurnal and sub‐tidal ∼O(10)d frequencies, with an average thickness of around 10 m. Compression and expansion of the mixing‐layer by both the IT and NLIWs affected the mean velocity profiles in the mixing‐layer, while the phasing between the barotropic and baroclinic flows led to an asymmetry in mean velocity profiles between periods of rising and falling isotherms. With the exception of periods of flow reversal, the turbulent kinetic energy balance and turbulent stress observations were consistent with the existence of an inertial‐sublayer with thickness of approximately 10%–15% of the mixing‐layer thickness ( ∼1 m), even beneath NLIWs. In the outer portion of the mixing‐layer—that is, above the inertial‐sublayer—NLIWs modulated the local turbulence spectra. We discuss the observations in the context of a predictive model for mixing‐layer thickness. The analysis suggests that the high‐frequency variability in mixing‐layer thickness was dominated by internal wave pumping, though strength of the ambient stratification and the frequency of the forcing were important controls on the time‐averaged (sub‐tidal) variation.

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Section: Overview Of the Turbulence Observationsmentioning

confidence: 52%

Section: Introductionmentioning

confidence: 99%

Mean and Turbulent Characteristics of a Bottom Mixing‐Layer Forced by a Strong Surface Tide and Large Amplitude Internal Waves

et al. 2022

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The Inner-Shelf Dynamics Experiment

Kumar

Lerczak

et al. 2021

Bulletin of the American Meteorological Society

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The inner shelf, the transition zone between the surf zone and the mid shelf, is a dynamically complex region with the evolution of circulation and stratification driven by multiple physical processes. Cross-shelf exchange through the inner shelf has important implications for coastal water quality, ecological connectivity, and lateral movement of sediment and heat. The Inner-Shelf Dynamics Experiment (ISDE) was an intensive, coordinated, multi-institution field experiment from Sep.-Oct. 2017, conducted from the mid shelf, through the inner shelf and into the surf zone near Point Sal, CA. Satellite, airborne, shore- and ship-based remote sensing, in-water moorings and ship-based sampling, and numerical ocean circulation models forced by winds, waves and tides were used to investigate the dynamics governing the circulation and transport in the inner shelf and the role of coastline variability on regional circulation dynamics. Here, the following physical processes are highlighted: internal wave dynamics from the mid shelf to the inner shelf; flow separation and eddy shedding off Point Sal; offshore ejection of surfzone waters from rip currents; and wind-driven subtidal circulation dynamics. The extensive dataset from ISDE allows for unprecedented investigations into the role of physical processes in creating spatial heterogeneity, and nonlinear interactions between various inner-shelf physical processes. Overall, the highly spatially and temporally resolved oceanographic measurements and numerical simulations of ISDE provide a central framework for studies exploring this complex and fascinating region of the ocean.

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Turbulence Asymmetries in Bottom Boundary Layer Velocity Pulses Associated with Onshore-Propagating Nonlinear Internal Waves

Becherer

Moum

Colosi

et al. 2020

Journal of Physical Oceanography

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The inner shelf is a region inshore of that part of the shelf that roughly obeys Ekman dynamics and offshore of the surf zone. Importantly, this is where surface and bottom boundary layers are in close proximity, overlap, and interact. The internal tide carries a substantial amount of energy into the inner shelf region were it eventually dissipates and contributes to mixing. A part of this energy transformation is due to a complex interaction with the bottom, where distinctions between nonlinear internal waves of depression and elevation are blurred, indeed, where polarity reversals of incoming waves take place. From an intensive set of measurements over the inner shelf off central California, we identify salient differences between onshore pulses from waves with properties of elevation waves and offshore pulses from shallowing depression waves. While the velocity structures and amplitudes of on/offshore pulses 1 m above the seafloor are not detectably different, onshore pulses are both more energetically turbulent and carry more sediments than offshore pulses. Their turbulence is also oppositely skewed: onshore pulses slightly to the leading edges, offshore pulses to the trailing edges of the pulses. We consider in turn three independent mechanisms that may contribute to the observed asymmetry: propagation in adverse pressure gradients and the resultant inflection point instability, residence time of a fluid parcel in the pulse, and turbulence suppression by stratification. The first mechanism may largely explain higher turbulence in the trailing edge of offshore pulses. The extended residence time may be responsible for the high and more uniform turbulence distribution across onshore compared to offshore pulses. Stratification does not play a leading role in turbulence modification inside of the pulses 1 m above the bed.

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Turbulence in the presence of internal waves in the bottom boundary layer of the California inner shelf

Cited by 4 publications

References 34 publications

Mean and Turbulent Characteristics of a Bottom Mixing‐Layer Forced by a Strong Surface Tide and Large Amplitude Internal Waves

Mean and Turbulent Characteristics of a Bottom Mixing‐Layer Forced by a Strong Surface Tide and Large Amplitude Internal Waves

The Inner-Shelf Dynamics Experiment

Turbulence Asymmetries in Bottom Boundary Layer Velocity Pulses Associated with Onshore-Propagating Nonlinear Internal Waves

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