Richard Dewey

Bottom Stress Estimates from Vertical Dissipation Rate Profiles on the Continental Shelf

Abstract Measurements of the near-bottom distribution of the turbulent dissipation rate on the continental shelf west of Vancouver Island are used to calculate bottom stress. A free-failing vertical profiler with microstructure shear probes was used to measure the dissipation rate, from near the surface to within 0.15 m of the bottom. The shear probes measure velocity gradients at scales within the viscous subrange of the turbulence and therefore directly measure the rate at which kinetic energy is dissipated by viscosity. Friction velocities are computed from the formula uast; = (ϵκz/ρ)⅓, where the dissipation rate ϵ is measured in the constant stress layer. The technique is more reliable than estimates of the dissipation rate obtained by fitting spectral slopes to velocity spectra at scales in the inertial subrange. Near-bottom current measurements indicate that the bottom stress values obtained from the turbulent measurements are well correlated with the current magnitude. An estimate of the drag coeff...

Observations of Biologically Generated Turbulence in a Coastal Inlet

Measurements in a coastal inlet revealed turbulence that was three to four orders of magnitude larger during the dusk ascent of a dense acoustic-scattering layer of krill than during the day, elevating daily-averaged mixing in the inlet by a factor of 100. Because vertically migrating layers of swimming organisms are found in much of the ocean, biologically generated turbulence may affect (i) the transport of inorganic nutrients to the often nutrient-depleted surface layer from underlying nutrient-rich stratified waters to affect biological productivity and (ii) the exchange of atmospheric gases such as CO2 with the stratified ocean interior, which has no direct communication with the atmosphere.

A Microstructure Instrument for Profiling Oceanic Turbulence in Coastal Bottom Boundary Layers

Abstract A free-falling instrument has been built to measure temperature, salinity and turbulent shear from near surface to within 15 cm of the ocean bottom. A probe guard mounted at the lower end of the instrument protects sensitive shear and temperature sensors from bottom sediments. The noise level in the shear signal corresponds to a dissipation rate of 3.0 × 10−7 W m−3, with vibrations of the probe guard providing the largest component of the noise. The signals are transmitted through a neutrally buoyant line to the ship where they are displayed in real time and recorded for later analysis. The profiling technique is capable of 20 profiles per hour through 50 m of water. Simultaneous profiles of turbulent microstructure and density can now be consistently obtained through the entire water column.

The Probability Distribution of the Thorpe Displacement within Overturns in Juan de Fuca Strait

Vertical mixing in the ocean can sometimes be quantified by measurements of the Thorpe overturning scale, LT. In regions of weak mixing and weak density gradients such measurements may be limited by slow sensor response times (or sampling rates) and/or by lack of resolution and noise in the density measurements. On the other hand, the Thorpe scale can be written as LT 5 ( L2P1(L) dL)1/2, where P1(L) is the probability of the ‘ # 0 Thorpe displacement, L. Data from Juan de Fuca Strait, British Columbia, show that, even though the probability of a small Thorpe displacement is much greater than that of a large Thorpe displacement, it is the large and more easily resolved values of L that dominate the Thorpe scale. It is found to be possible to determine LT down to a scale of 0.4 m with a conventional conductivity‐temperature‐depth instrument. This corresponds to values of Ky . 1024 m2 s 21 in summertime if LT (e/N 3)1/2, as is confirmed using velocity and temperature microstructure data. Here P1(L) is a convolution of the probability distribution of overturn height, P2(H), with the probability distribution of the fractional displacement within each overturn, P3(L/H). Data show that P2(H) is dominated by small overturns, consistent with previous work on the thickness of turbulence patches. Finally, the distribution of P3(L/H) is examined and compared with the prediction of a very simple kinematic model. The data show a pattern similar to that predicted by the model, though with more small L/H and fewer medium to large L/H than in the model.

Structure and Dynamics of a Coastal Filament

Repeated microstructure transects across filaments in the coastal transition zone (CTZ) have revealed fundamental structure and dynamics of the complicated features. The measurements allow detailed momentum and vorticity analyses and provide a possible explanations for structural asymmetry of the fronts. Observations made between July 2 and July 23, 1988, along the central meridional CTZ survey line were used to estimate terms in the meridional momentum equation. The analysis indicates geostrophic flow along the axes of the fronts with the acrosg-fr0nt pressure gradient explaining as much as 87% of the variance in the balance. Significant ageostrophic flow in the across-front coordinate was found, with the along-front pressure gradient explaining only 7!% of the variance in the momentum balance. The fronts were found to be asymmetric in relative vorticity, with stronger positive vorticity on the cooler side of the front and weaker negative vorticity on the warm side. Mean vertical velocities were estimated from the repeated transects of acoustic Doppler current profiles and the rapid sampling vertical profiler hydrographic and turbulence measurements. Regions of upwelling and downwelling are likely associated with adjustments in the relative vorticity, resulting in maximum vertical velocities of 40 m d -l . Asymmetry in the near-surface temperature and salinity extrema are explained by cross-frontal exchang e. This cross-frontal exchange modifies the relative roles of salinity and temperature in determining the density away from the coastal upwelling region, a dynamically important characteristic not revealed by advanced very high resolution radiometer imagery.

Turbulence and mixing: Sources of nutrients on the Vancouver island continental shelf

Abstract Estimates of the rate of dissipation of turbulent energy,?v, were made on the continental shelf off the southwest coast of Vancouver Island throughout the diurnal tidal cycle at six stations. If turbulent mixing takes place away from boundaries, and a constant fraction of the energy supplied to turbulence is converted into potential energy, this dissipation rate per unit volume, ?v, is shown to be proportional to the rate of turbulent mass and nutrient flux across the isopycnals. We compute these fluxes to determine the source of nutrients in the upper mixed layer on the shelf. It is found that tidal mixing on the shelf throughout the water column contributes less than 10% of the flux of nutrients supplied by the estuarine outflow out of Juan de Fuca Strait. Strong winds during upwelling events supply nutrients at rates greater than those due to tidal mixing, but at rates that are likely smaller than the Juan de Fuca source. Therefore, the nutrient‐rich waters observed in the euphotic zone in spr...

Stratified Tidal Flow over a Bump

The interaction of a stratified flow with an isolated topographic feature can introduce numerous disturbances into the flow, including turbulent wakes, internal waves, and eddies. Measurements made near a “bump” east of Race Rocks, Vancouver Island, reveal a wide range of phenomena associated with the variable flow speeds and directions introduced by the local tides. Upstream and downstream flows were observed by placing two acoustic Doppler current profilers (ADCPs) on one flank of the bump. Simultaneous shipboard ADCP surveys corroborated some of the more striking features. Froude number conditions varied from subcritical to supercritical as the tidal velocities varied from 0.2 to 1.5 m s 1 . During the strong ebb, when the moored ADCPs were located on the lee side, a persistent full-water-depth lee wave was detected in one of the moored ADCPs and the shipboard ADCP. However, the placement of the moorings would suggest that, by the time it appears in the moored ADCP beams, the lee wave has been swept downstream or has separated from the bump. Raw ADCP beam velocities suggest enhanced turbulence during various phases of the tide. Many of the three-dimensional flow characteristics are in good agreement with laboratory studies, and some characteristics, such as shear in the bottom boundary layer, are not.

On Turbulence Production by Swimming Marine Organisms in the Open Ocean and Coastal Waters

Abstract Microstructure and acoustic profile time series were collected near Ocean Station P in the eastern subarctic North Pacific and in Saanich Inlet at the south end of Vancouver Island, British Columbia, Canada, to examine production of turbulent dissipation by swimming marine organisms. At Ocean Station P, although a number of zooplankton species are large enough to generate turbulence with Reynolds numbers Re > 1000, biomass densities are typically less than 103 individuals per cubic meter (<0.01% by volume), and turbulent kinetic energy dissipation rates e were better correlated with 16-m vertical shear than acoustic backscatter layers. In Saanich Inlet, where krill densities are up to 104 individuals per cubic meter (0.1% by volume), no dramatic elevation of dissipation rates e was associated with dusk and dawn vertical migrations of the acoustic backscatter layer. Dissipation rates are a factor of 2 higher [〈e〉 = 1.4 × 10−8 W kg−1, corresponding to buoyancy Re = 〈e〉/(νN 2) ∼ 140] in acoustic bac...

Mooring Design & Dynamics—a Matlab® package for designing and analyzing oceanographic moorings

Abstract Mooring Design and Dynamics is a set of Matlab® routines that can be used to assist in the design and configuration of single point oceanographic moorings, the evaluation of mooring tension and shape under the influence of wind and currents, and the simulation of mooring component positions when forced by time-dependent currents. The static model will predict the tension and tilt at each mooring component, including the anchor, for which the safe mass will be evaluated in terms of the vertical and horizontal tensions. Predictions can be saved to facilitate mooring motion correction. Time-dependent currents can be entered to predict the dynamic response of the mooring. The package includes a preliminary database of standard mooring components which can be selected from pull down menus. The database can be edited and expanded to include user specific components, frequently used fasteners/wires etc., or unique oceanographic instruments. Once designed and tested, a draft of the mooring components can be plotted and a list of components, including fasteners can be printed.

Reynolds Stresses and Secondary Circulation in a Stratified Rotating, Shear Flow

Abstract A transverse secondary circulation should result from vertical mixing of momentum in a stratified shear flow influenced by the earths rotation. A rigidly mounted acoustic Doppler current profiler was deployed at the bottom of Juan de Fuca Strait, a stratified estuary on the west coast of North America, for 21 days, to examine this dynamical process and its variation over a spring–neap tidal cycle. The data demonstrate a significant fortnightly modulation of the estuarine exchange flow, with stronger along-channel flow, less dynamic stability, and much greater cross-strait currents at neap tide than at spring tide. Reliable estimates of the Reynolds stresses in the stratified shear flow away from boundaries were also obtained. They were small except for several days around neap tide when they were associated with times of reduced Richardson number. These stresses are not related in any simple way (such as with a smooth, positive, eddy viscosity) to the shear of the mean flow but do have a pattern...