Harvey E. Seim

The importance of aspiration and channel curvature in producing strong vertical mixing over a sill

On the basis of observations of the time-dependent, tidally forced flow over a long sill we find that aspiration and channel curvature set the flow structure and condition the flow to allow intense vertical mixing. Aspiration reduces the potential energy of the water column by thinning it while maintaining its density contrast. Channel curvature induces a cross-channel circulation that can rapidly overturn a stratified flow. Eighteen along-channel sections of density, velocity, and dissipation rate of turbulent kinetic energy ∈ were collected in and around the Tacoma Narrows of Puget Sound, a site suspected of driving a strong vertical circulation in adjoining Main Basin. Rapid inflow to the Narrows on flood from one channel of a triple junction reduces dynamic pressure, allowing dense water from below sill depth to be uplifted, or aspirated, into the Narrows. We estimate water from below 150 m, 3 times the sill depth, is drawn into the Narrows on a 3-m flood tide. Once in the Narrows the flow remains stratified until it passes a 50° bend where a strong secondary circulation overturns the 50-m-deep water column and generates intense turbulent mixing. Cross-channel velocities of up to 0.4 m s−1 are observed, and maximum values of ∈ exceed 10−3 W kg−1. Upon leaving the sill, stratification is reestablished, and turbulence decays. A similar set of sequences occurs on ebb, except that the outflow bypasses the flood inflow channel and instead discharges into Colvos Passage, the third branch of the triple junction. Colvos Passage ultimately discharges the ebb effluent back into Main Basin, enhancing the impact of mixing at the Narrows by discharging the mixed product far from the source. Scaling of the cross-channel momentum equation suggests that, below a threshold value of along-channel velocity, stratification should suppress secondary circulation for a given vertical shear, radius of curvature and channel width. Above the threshold velocity the magnitude of the cross-channel velocity is roughly consistent with predictions for unstratified flow. We estimate the maximum effective eddy diffusivity that aspiration and mixing in the Narrows can produce in Main Basin to be 10−3 m2 s−1.

Detailed observations of a naturally occurring shear instability

Simultaneous profiles of microstructure, horizontal velocity, and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date. Shear increased rapidly after passing through a lateral constriction which formed a hydraulic control. A kilometer-long set of 20-m-tall billows grew on a middepth density interface where the Richardson number fell below 0.25. The velocity interface thickened steadily after the billows formed, consistent with rapid momentum mixing across a shear layer with a Reynolds number of 3×106. The billows generated large density overturns and dissipation rates greater than 10−5 W kg−1, even within the first large overturn, indicating that these structures were fully turbulent early in their development. As the billows grew, a well-mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods, 3 times longer than in laboratory studies at low Reynolds number. Variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where the billows first formed. With data from the large overturns within the shear layer, we find e/νN2 ≈ 3 × 104, an average root-mean-square overturn scale ( Lrms–) of 2.6 m, and a buoyancy scale (Lb) of 2.7 m. Despite having sampled the billows at varying stages of their evolution, we find no indication that the ratio Lrms/Lb is ever significantly different than 1 for this shear instability.

Acoustic Backscatter from Turbulent Microstructure

Abstract Acoustic backscatter has produced spectacular images of internal ocean processes for nearly two decades, but interpretation of the images remains ambiguous because several mechanisms can generate measurable backscatter. The authors present what is thought to be the first simultaneous measurements of calibrated acoustic returns and turbulent microstructure, collected in a set of 20-m-tall billows. The observations are from Admiralty Inlet, a salt-stratified tidal channel near Puget Sound. Scattering due to turbulent microstructure alone is strong enough to explain the measured backscatter at specific sites within the billows. Existing formulations underestimate the strength of acoustic backscatter from turbulent microstructure. Due to a misinterpretation of the high-wavenumber temperature spectrum, some previous formulations underestimate the differential Scattering cross section (σ) when scattering from the viscous-convective subrange. Also, the influence of salinity on refractive-index fluctuati...

The role of dissipation and mixing in exchange flow through a contracting channel

We investigate the transport of mass and momentum between layers in idealized exchange flow through a contracting channel. Lock-exchange initial value problems are run to approximately steady state using a three-dimensional, non-hydrostatic numerical model. The numerical model resolves the large-scale exchange flow and shear instabilities that form at the interface, parameterizing the effects of subgrid-scale turbulence. The closure scheme is based on an assumed steady, local balance of turbulent production and dissipation in a density-stratified fluid. The simulated flows are analysed using a two-layer decomposition and compared with predictions from two-layer hydraulic theory. Inter-layer transport leads to a systematic deviation of the simulated maximal exchange flows from predictions. Relative to predictions, the observed flows exhibit lower Froude numbers, larger transports and wider regions of subcritical flow in the contraction. To describe entrainment and mixing between layers, the computed solutions are decomposed into a three-layer structure, with two bounding layers separated by an interfacial layer of finite thickness and variable properties. Both bounding layers lose fluid to the interfacial layer which carries a significant fraction of the horizontal transport. Entrainment is greatest from the faster moving layer, occurring preferentially downstream of the contraction. Bottom friction exerts a drag on the lower layer, fundamentally altering the overall dynamics of the exchange. An example where bed friction leads to a submaximal exchange is discussed. The external forcing required to sustain a net transport is significantly less than predicted in the absence of bottom stresses.

Operation and application of a regional high-frequency radar network in the Mid-Atlantic Bight

The Mid-Atlantic Regional Coastal Ocean Observing System (MARCOOS) High-Frequency Radar Network, which comprises 13 long-range sites, 2 medium-range sites, and 12 standard-range sites, is operated as part of the Integrated Ocean Observing System. This regional implementation of the network has been operational for 2 years and has matured to the point where the radars provide consistent coverage from Cape Cod to Cape Hatteras. A concerted effort was made in the MARCOOS project to increase the resiliency of the radar stations from the elements, power issues, and other issues that can disable the hardware of the system. The quality control and assurance activities in the Mid-Atlantic Bight have been guided by the needs of the Coast Guard Search and Rescue Office. As of May 2009, these quality-controlled MARCOOS High-Frequency Radar totals are being served through the Coast Guards Environmental Data Server to the Coast Guard Search and Rescue Optimal Planning System. In addition to the service to U.S. Coast Guard Search and Rescue Operations, this data supports water quality, physical oceanographic, and fisheries research throughout the Mid-Atlantic Bight.

Acoustic Backscatter from Salinity Microstructure

The contribution of salinity changes to sound speed fluctuations is often neglected in estimating the scattering cross section at high frequencies (.10 kHz). To examine when salinity might be important, an expression is formulated for the scattering cross section s that includes salinity and an estimate of the cospectrum of temperature and salinity. Profiles from the southern New England shelf, the Bosphorus, and Puget Sound are used to estimate levels of s as a function of depth and acoustic frequency. Salinity can increase s by more than an order of magnitude, particularly at frequencies greater than 100 kHz, when salinity controls the density field. The cospectrum is expected to be large under the same conditions and can potentially negate strong scattering at lower frequencies. An f 11 dependence of s is expected over two decades in frequency when salinity controls density. Multifrequency acoustic systems may be able to distinguish biology and microstructure based on this spectral dependence.

Transport of salt and suspended sediments in a curving channel of a coastal plain estuary: Satilla River, GA

Abstract This study describes the transport of salt and suspended sediment in a curving reach of a shallow mesotidal coastal plain estuary. Circulation data revealed a subtidal upstream bottom flow during neap tide, indicating the presence of a gravitational circulation mode throughout the channel. During spring tide, landward bottom flow weakened considerably at the upstream end of the channel and changed to seaward in the middle and downstream areas of the reach, suggesting the importance of tidal pumping. Salt flux near-bottom was landward at both ends of the channel during neap tide. At spring, however, the salt flux diverged along the bottom of the thalweg suggesting that tidal pumping caused a transfer of salt vertically and laterally into the intertidal zone. Thus, landward flux of salt is maintained even in the presence of subtidal seaward flow along the bottom at the downstream end of the channel. Landward bottom stress is greater than seaward stress, preferentially transporting suspended sediments upstream. Compared with salt, however, the weight of the suspended sediments causes less upward transfer of sediments into the intertidal zone. Flood flow carried more suspended sediments landward at the upstream end compared with the downstream end. We speculate that secondary flow in the curving channel picks up increasing amounts of suspended sediments along the sides during flood and adds them to the axial flow in the thalweg. Since the landward flow along the bottom of the thalweg weakens and even reverses during spring tide, there appears to be a complex re-circulation system for sediments re-suspended in curving channels that complicates the picture of a net transport of sediments landward.

Direct stress measurements in a shallow, sinuous estuary

Abstract Observations from a 4 element mooring array collected in a bend of a shallow, sinuous estuary are used to describe the flow, density structure and momentum balance over a 10-day period. In general, the flow in the lower 3 m is stratified on ebb and unstratified on flood and shear is concentrated near the bed on flood and nearly uniform throughout the water column on ebb. At spring tides stratification is reduced and the flows 1 m above bottom (mab) are consistently greatest at the downstream end of the bend. The along-channel density gradient is weakest during spring tides owing to zero gradient over most of ebb flow. At neap tides vertical stratification is strong enough to raise the gradient Richardson number well above 0.25 for most of the ebb tide. Currents are weaker and do not display a regular along-channel pattern. The variation in density and current structure is interpreted to result from variations in cross-channel circulation associated with the channel bend. At spring tides, the cross-channel circulation appears to be strong enough to overturn the water column whereas at neap tides stratification is strong enough to halt the overturning. Reynolds stress measured with a Benthic Acoustic Stress Sensor undergoes a four-fold increase between neap and spring tide. The drag coefficient relative to flow at 1 mab is 0.0015–0.0025. Bed stress in the bend is estimated using this drag coefficient and the maximum instantaneous velocity at 1 mab over the array. Because of the along-channel variability in current speed, the estimated bed stress is roughly twice as large as the measured Reynolds stress in the middle of the bend. The estimated bed stress is found to balance the horizontal pressure gradient and local acceleration, implying that a depth-averaged linear momentum balance adequately describes the dynamics on the bend when the impact of the cross-channel circulation is taken into account in the estimate of the bottom stress.

Simulation of non-hydrostatic, density-stratified flow in irregular domains

A numerical model has been developed for simulating density-stratified flow in domains with irregular but simple topography. The model was designed for simulating strong interactions between internal gravity waves and topography, e.g. exchange flows in contracting channels, tidally or convectively driven flow over two-dimensional sills or waves propagating onto a shoaling bed. The model is based on the non-hydrostatic, Boussinesq equations of motion for a continuously stratified fluid in a rotating frame, subject to user-configurable boundary conditions. An orthogonal boundary fitting co-ordinate system is used for the numerical computations, which rely on a fourth-order compact differentiation scheme, a third-order explicit time stepping and a multi-grid based pressure projection algorithm. The numerical techniques are described and a suite of validation studies are presented. The validation studies include a pointwise comparison of numerical simulations with both analytical solutions and laboratory measurements of non-linear solitary wave propagation. Simulation results for flows lacking analytical or laboratory data are analysed a posteriori to demonstrate satisfaction of the potential energy balance. Computational results are compared with two-layer hydraulic predictions in the case of exchange flow through a contracting channel. Finally, a simulation of circulation driven by spatially non-uniform surface buoyancy flux in an irregular basin is discussed.

SEA-COOS: A model for a multi-state, multi-institutional regional observation system

The SouthEast Atlantic Coastal Ocean Observing System (SEA-COOS) is a regional partnership that has initiated an integrated coastal ocean observing system for a 4-state (North Carolina, South Carolina, Georgia, Florida) region of the southeast U.S. The long-term intent of SEA-COOS is to establish a regional coastal ocean observing network that will be part of the coastal component of the national Integrated Ocean Observing System (IOOS). This article briefly describes the essential elements of an observing system, the region-wide observations, overlapping circulation models, data management capabilities, and outreach and education activities of SEA-COOS, at present and planned for the coming years. Development of a governance system has also been pursued, and an initial structure is in place for SEA-COOS.