Abstract

Dynamic response calibrations have been made for temperature and conductivity probes produced by Sea- Bird Electronics. These probes are used on wire-lowered and towed conductivity, temperature, and depth systems (CTDs) as well as on free-fall oceanographic profilers. The response of the conductivity cell was measured for free-flow and pumped configurations. Legendre polynomials have been fitted to the measured transfer functions to provide analytic farina for data correction.

Abstract

A microstructure survey near the head of Monterey Submarine Canyon, the first in a canyon, confirmed earlier inferences that coastal submarine canyons are sites of intense mixing. The data collected during two weeks in August 1997 showed turbulent kinetic energy dissipation and diapycnal diffusivity up to 10³ times higher than in the open ocean. Dissipation and diapycnal diffusivity within 10 km of the canyon head were among the highest observed anywhere (ε = 1.1 × 10⁻⁶ W kg⁻¹; K _ρ = 1.0 × 10⁻² m² s⁻¹). Mixing occurred mainly in an on-axis stratified turbulent layer, with thickness and intensity increasing from neap to spring tide. Strain spectra showed a gentler than k ⁻¹ _z rolloff, suggesting that critical reflection and scattering may push energy into high wavenumbers. Dissipation dependence on shear appears to be much weaker in the canyon than in the open ocean, with indications that the dependence maybe as low as ε ∝ S . Coastal canyons may account for a small but significant fraction of the internal tide energy budget. A crude estimate of global dissipation in canyons is 58 GW, ≈15% of the estimated global M ₂ internal tide dissipation.

Abstract

Near-diurnal internal waves were observed in velocity and shear measurements from a shipboard survey along a 35-km section of the Kaena Ridge, northwest of Oahu. Individual waves with upward phase propagation could be traced for almost 4 days even though the ship transited approximately 20 km. Depth–time maps of shear were dominated by near-diurnal waves, despite the fact that Kaena Ridge is a site of considerable M ₂ barotropic-to-baroclinic conversion. Guided by recent numerical and observational studies, it was found that a frequency of ½M ₂ (i.e., 24.84-h period) was consistent with these waves. Nonlinear processes are able to transfer energy within the internal wave spectrum. Bicoherence analysis, which can distinguish between nonlinearly coupled waves and waves that have been independently excited, suggested that the ½M ₂ waves were nonlinearly coupled with the dominant M ₂ internal tide only between 525- and 595-m depth. This narrow depth range corresponded to an observed M ₂ characteristic emanating from the northern edge of the ridge. The observations occurred in close proximity to the internal tide generation region, implying a rapid transfer of energy between frequencies. Strong nonlinear interactions seem a likely mechanism. Nonlinear transfers such as these could complicate attempts to close local single-constituent tidal energy budgets.

Abstract

During August 2006 aggregations of nekton, most likely small fish, intersected microstructure survey lines in Monterey Bay, California, providing an opportunity to examine biologically generated mixing. Some aggregations filled the water column, 80 m deep, and extended 100–200 m along the survey track. Others were half that size, and some were much smaller. Acoustic energy backscattered from the aggregations was measured with a calibrated echosounder and yielded volume backscattering strength S_v values of −80 to −60 dB re 1 m⁻¹.

Turbulent dissipation rates ϵ were 10⁻⁶–10⁻⁵ W kg⁻¹ in the more intense aggregations. Within these, ϵ was much more uniform than turbulence measured outside the aggregations and varied with S_υ . Three similar aggregations contributed half of the average ϵ in 142 profiles taken along a 5-km-long survey line during a 12.5-h tidal cycle.

Turbulence within aggregations differed markedly from that outside in the following three ways: (i) Thorpe scales, that is, root-mean-square overturning lengths, were much smaller than Ozmidov scales, L _OZ ≡ (ϵ/N ³)^1/2, the upper limit for overturns limited by stratification. (ii) Spectra of small-scale shear matched the universal shape only in the viscous high-wavenumber rolloff. At lower wavenumbers, the shear spectra had slopes closer to k ¹ than to the k ^1/3 Kolmogorov slope with a corresponding velocity spectra peak near 8 cpm. (iii) Temperature gradient spectra matched portions of the universal Batchelor spectrum that could be resolved, but their magnitudes were smaller than those in turbulence produced by flow instabilities. Average mixing efficiency Γ was 0.0022 within aggregations, compared with 0.23 outside. This 100-fold decrease in efficiency compensates for a 100-fold increase in ϵ to produce no net change in diapycnal diffusivity K_ρ . Because the composition and behavior of the aggregations were not confirmed—that is, whether the nekton had regular spacing and swimming patterns characterizing schools—it is not possible to know whether one or many turbulent states were sampled within aggregations. If these observations are representative, mixing in aggregations may be more interesting than it is important, but the data are too few for definitive conclusions.

Abstract

Very high turbulent dissipation rates (above ε = 10⁻⁴ W kg⁻¹) were observed in the nonlinear internal lee waves that form each tide over a sill in Knight Inlet, British Columbia. This turbulence was due to both shear instabilities and the jumplike adjustment of the wave to background flow conditions. Away from the sill, turbulent dissipation was significantly lower (ε = 10⁻⁷ to ε = 10⁻⁸ W kg⁻¹). Energy removed from the barotropic tide was estimated using a pair of tide gauges; a peak of 20 MW occurred during spring tide. Approximately two-thirds of the barotropic energy loss radiated away as internal waves, while the remaining one-third was lost to processes near the sill. Observed dissipation in the water column does not account for the near-sill losses, but energy lost to vortex shedding and near-bottom turbulence, though not measured, could be large enough to close the energy budget.

Abstract

Observations and modeling simulations are presented that illustrate the importance of a density contrast and the upstream response to the time dependence of stratified flow over the Knight Inlet sill. Repeated sections of velocity and density show that the flow during ebb and flood tides is quite different: a large lee wave develops early in flood tide, whereas lee-wave growth is suppressed until the second half of ebb tide. There is a large upstream response that displaces as much water as accumulates in the lee wave, one that is large enough to also block the flow at a depth roughly consistent with simple dynamics. There is a large density contrast between the seaward and landward sides of the sill, and a “salty pool” of water is found in the seaward basin that is not found landward. The interface with this salty pool demarks the point of flow separation during ebb, initially suppressing the lee wave and then acting as its lower boundary. A simple two-dimensional numerical model of the inlet was used to explore the important factors governing the flow. A base simulation that included the landward–seaward asymmetry of the sill shape, but not the density difference, yielded a response that was almost symmetric with a large lee wave forming early during both flood and ebb tide. The simulation behaves more like the observations when a salty pool of water is added seaward of the sill. This salty pool induces flow separation in the model and suppresses growth of the lee wave until late in ebb. This effect is termed “density-forced” flow separation, a modification of “postwave” flow separation that allows for a density gradient across an obstacle.

Abstract

An analysis of a moored time series at 0°, 140°W from November to December 1991 found a nighttime enhancement of isotherm displacement variance and of zonal velocity variance below the surface mixed layer at frequencies higher than 1 cph. The nighttime enhancement was generally not seen below the core of the equatorial undercurrent. At 45-m depth, the potential energy and the horizontal kinetic energy of high-frequency waves were strongly correlated and similar in magnitude. The shear production of turbulence kinetic energy calculated from the mooring measurements is strongly correlated with the turbulence kinetic energy dissipation rate observed from the nearby R/Vs Wecoma and Moana Wave during the Tropical Instability Wave Experiment. This suggests a dynamical link between the observed high-frequency internal waves and deep-cycle turbulence.

The relationship between internal waves and turbulence in the thermocline was further explored in a case study of one energetic wave packet. This wave packet propagated westward and downward with a horizontal wavelength of no less than 200 m. The potential energy was similar to the horizontal kinetic energy of the wave packet, with the dominant variance occurring in a frequency band close to the local buoyancy frequency. The estimated vertical flux of the horizontal momentum of waves during the event was 0.3 Pa, three times the surface wind stress. About 2 hours after the wave packet passed the mooring site, an anomalous turbulence dissipation rate with a magnitude similar to that of the estimated shear production of the wave packet was observed from the R/V Wecoma. The observed time tag was likely the result of the spatial separation of the observing platforms.

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 fluctuations can be as large as or greater than that of temperature when the density stratification is dominated by salinity. Using temperature alone to estimate σ in coastal and estuarine waters may lead to significant underestimates. A simple formulation is derived that takes these two factors into account. Because of high ambient scattering from zooplankton in Admiralty Inlet, the acoustic data are conditionally sampled along modeled profiler trajectories to avoid using bulk statistics. Scalar dissipation is greatest in the bounding surfaces of the billows, consistent with these surfaces producing the most intense scattering. Acoustic backscatter can be used to remotely sense the spatial structure of scalar dissipation in turbulent events where σ due to turbulent microstructure exceeds the background level set by scattering from biology. In lakes and the deep ocean where scattering from zooplankton is expected to be negligible, scattering from microstructure may be the dominant mechanism. The largest uncertainties in the comparison result from the very large difference in sampling volume of the acoustic system and microstructure profiler.

Abstract

A four and one-half day time series of upper-ocean shear and density observations was collected in the tropical Pacific Ocean in November 1984. The measurements were made on the equator at 139°50′W during a period when the equatorial undercurrent was well developed and 20–30 day period velocity fluctuations were prominent. Shear observations were collected with a ship-mounted acoustic-Doppler velocity profiler; density data were obtained from a loosely tethered microstructure instrument. The mean shear profile during the series strongly reflected the structure of the undercurrent; however, the meridional component contributed significantly to the magnitude of the total shear. The mean Richardson number was large near the undercurrent core, but fell to values less than 0.5 25 m above and below the core, and was below 0.25 in the upper 40 m for most of the sample period. Buoyancy frequency varied on a diurnal time scale in the upper 50 m owing to the solar heating cycle, but a compensating diurnal shear cycle was found only above 24 m. Consequently, the Richardson number varied diurnally in the depth range of 25–50 m. The shear and density fluctuations at depths greater than 50 m were not clearly connected to the diurnal near-surface features and exhibited no dominant periodicity. As has been seen in previous internal wave studies, the data below the diurnal surface layer exhibited a cutoff at Ri ∼ 0.25, perhaps indicative of shear mixing control of the Richardson number distribution.

Abstract

Measurements by the Multi-Scale Profiler (MSP) at seven stations spanning the Straits of Florida characterize levels and patterns of internal wave activity and mixing in this vertically sheared environment. Contrasting properties suggest five mixing regimes. The largest and most vast is the interior regime, where the background flow has an inverse Richardson number (Ri⁻¹) ranging up to 0.55, shear is dominated by fluctuations that are 1–4 times stronger than in the open ocean, and turbulent diffusivities are similarly moderate at (1–4) × 10⁻⁵ m² s⁻¹. The high-velocity core of the current, near the surface at midchannel, is associated with weak mixing. To its west is a zone of high mean shear, where strong stratification results in background Ri⁻¹ of only 0.4, fluctuations are weak, and diffusivity is moderate. Intermittent shear features beneath the core have mean Ri⁻¹ > 1 and strong turbulence. Two regimes are related to channel topography. Adjacent to the steep eastern slope, finescale shear is predominately cross-channel, and turbulence varies from nearly the weakest to nearly the strongest. Within 100 m of the channel floor, turbulent stratified boundary layers are mixing at (2–6) × 10⁻⁴ m² s⁻¹ to account for one-half of the section-averaged diffusivity. Using existing finescale parameterizations, observed dissipation rates can be predicted within a factor of 2 for most of this dataset, despite significantly strong mean shear and generally anisotropic and asymmetric fluctuations. The exceptions are in the high mean shear zones, where total rather than fluctuating shear yields reasonable estimates, and in some of the more turbulent regions, where shear underestimates mixing. Given its overall moderate levels of turbulence and finescale shear, the Florida Current is not a hot spot for oceanic mixing.