Oceanic Microstructure Research Articles

How prevalent is acoustic scattering from oceanic microstructure?

Any oceanic environment with spatial gradients in sound speed and density can result in acoustic scattering hot spots. These acoustic hotspots can be rapidly evolving and vary in their spatial heterogeneity. Here, we review acoustic scattering models for turbulence and present data collected with a variety of split-beam and multi-beam echosounders illustrating the range of environments and spatial scales associated with scattering from physical microstructure, including shear instabilities in estuarine environments, non-linear internal waves on the continental shelf, strong interface scattering due to double-diffusion, scattering from strong gradients or interfaces, and turbulent microstructure at the New England shelf break front. The theoretical acoustic scattering formulations for different types of physical microstructure are applied to these different environments, and recommendations are made for optimal frequency bands to sample the different types of physical microstructure and the optimal measurements for inference of parameters that describe the physical microstructure. Limitations imposed on detection and quantification of scattering from the physical microstructure due to sources including suspended sediment, bubbles, and biological targets, are also discussed. [Work supported by the ONR.]

Ocean Mixed Layer Depth From Dissipation

AbstractThe mixed layer depth (MLD) is a widely used parameter for physical, chemical, and biological oceanography. The MLD delimits that region of the ocean, which is directly influenced by the atmosphere. There is a similar length scale referred to as the mixing layer depth, which is determined from profiles of dissipation rate of turbulent kinetic energy, but is less prevalent due to the requirement for specialized instrumentation. Here, we suggest that the MLD can be estimated using density/temperature (henceforth hD) or using dissipation (henceforth hϵ). Utilizing two data sets in the North Atlantic collected with the autonomous Air‐Sea Interaction Profiler, hD and hϵ in the upper 100 m are compared. A new method based on the shape of the dissipation profile for estimating hϵ is presented. The main sources of turbulence in the upper ocean (i.e., wind, waves, and buoyancy fluxes), which tend to deepen the MLD, are more strongly correlated with hϵ variability for both data sets, confirming that the MLD derived from dissipation measurements represents more accurately the variability in the mixed layer. Given that dissipation measurements have become more operational due to the development of ocean microstructure technology, then it is appropriate that hϵ be adopted in the future for estimation of the MLD. This is additionally supported by the fact that hD typically overestimates the MLD when compared to hϵ, thus having consequences for studies where hD is used to represent the extent of mixing in the upper ocean.

Design and Motion Performance Analysis of Turbulent AUV Measuring Platform.

The use of a multi-functional autonomous underwater vehicle (AUV) as a platform for making turbulence measurements in the ocean is developed. The layout optimization of the turbulence package and platform motion performance are limitation problems in turbulent AUV design. In this study, the computational fluid dynamics (CFD) method has been used to determine the optimized layout position and distance of the shear probe integrated into an AUV. When placed 0.8 D ahead of the AUV nose along the axis, the shear probe is not influenced by flow distortion and can contact the water body first. To analyze the motion of the turbulence AUV, the dynamic model of turbulence AUV for planar flight is obtained. Then, the mathematical equations of speed and angle of attack under steady-state motion have also been obtained. By calculating the hydrodynamic coefficients of the turbulence AUV and given system parameters, the simulation analysis has been conducted. The simulation results demonstrated that the speed of turbulent AUV is 0.5–1 m/s, and the maximum angle of attack is less than 6.5°, which meets the observation requirements of the shear probe. In addition, turbulence AUV conducted a series of sea-trials in the northern South China Sea to illustrate the validity of the design and measurement. Two continuous profiles (1000 m) with a horizontal distance of 10 km were completed, and numerous high-quality spatiotemporal turbulence data were obtained. These profiles demonstrate the superior flight performance of turbulence AUV. Analysis shows that the measured data are of high quality, with the shear spectra being in very good agreement with the Nasmyth spectrum. Dissipation rates are consistent with background shear. When shear velocity is weak, the measurement of dissipation rate is 10−10 W Kg−1. All indications are that the turbulence AUV is suitable for long-term, contiguous ocean microstructure measurements, which will provide data needed to understand the temporal and spatial variability of the turbulent processes in the oceans.

An Integrated Spatio-Temporal Features Analysis Approach for Ocean Turbulence Using an Autonomous Vertical Profiler

Turbulent energy cascade and intermittency are very important characteristics in the turbulent energy evolution process. However, understanding the temporal–spatial features of kinetic energy transfer and quantifying the correlations between different scales of turbulent energy remains an outstanding challenge. To deeply understand the spatial–temporal features in the energy transfer process, an integrated features identification and extraction method is proposed to quantitatively investigate the correlations using the ocean shear turbulence measured by an autonomous vertical reciprocating profiler (AVRP). The proposed integrated method mainly contains two parallel features analysis modules: first, temporal multiscale features structures of the nonlinear and nonstationary turbulent cascade are identified by Variational Mode Decomposition (VMD); then, the ocean microstructure shear fluctuation data are decomposed into a series of intrinsic mode functions (IMFs), which are characterized by different time scales and frequency bandwidths. The local features of energy transfer are identified when the local intermittency peaks overlap and the phase-synchronization case occurs between two neighboring scales; second, the spatial statistical characteristics of the turbulent energy dissipation are quantitatively studied. The cumulative probability distribution functions (CPDFs) of kinetic energy dissipation are approximated well by a normal distribution, indicating that the turbulent dissipation process exhibits a robust spatial scaling correlation and a few intense dissipation locations dominate the integrated process. Finally, the proposed integrated method is evaluated through experiments using an autonomous vertical reciprocating profiler deployed in the South China Sea. Preliminary experimental results show that the proposed novel method is useful to improve our understanding of turbulent energy transfer and the evolution process in the ocean dynamic systems.

Ocean Mixing Process Study 2018

The cruise KB 2018616 aboard the Research Vessel Kristine Bonnevie was the first research cruise of the Nansen Legacy project. The cruise aimed at collecting ocean stratification, currents, and microstructure profiles along selected transects across the Spitsbergen shelf and slope, thus providing the background for the Kronprins Haakon cruise scheduled in late September 2018. This report provides an overview of the methods employed and the data collected during KB 2018616.

Do Strong Winds Impact Water Mass, Nutrient, and Phytoplankton Distributions in the Ice‐Free Canada Basin in the Fall?

AbstractIn general, strong wind events can enhance ocean turbulent mixing, followed by episodic nutrient supply to the euphotic zone and phytoplankton blooms. However, it is unclear whether such responses to strong winds occur in the ice‐free Canada Basin, where the seasonal pycnocline is strong and the nutricline is deep. In the present study, we monitored a fixed‐point observation (FPO) station in the Canada Basin for about 3 weeks in the fall of 2014 to examine the oceanic and biological responses to strong winds. At the FPO site, oceanic microstructure measurements, hydrographic surveys, and water sampling were performed with high temporal resolution, recording internal wave propagation, eddy passage, and water mass changes. Strong winds and internal wave propagation significantly enhanced the mixing above and at the seasonal pycnocline, but their effects were diminished at the nutricline, which was much deeper than the seasonal pycnocline. Therefore, wind‐induced mixing did not increase the upward nutrient supply from the nutricline and did not impact phytoplankton (chlorophyll a) distribution in the surface layer of the FPO site. The temporal evolution of the chlorophyll a concentration was most closely related to water mass changes. We also observed prominent subsurface chlorophyll a maxima with abundant large‐sized phytoplankton that were likely carried by warm‐core eddies to the FPO site. Phytoplankton biomass may have been sustained by the high concentration of ammonium within the eddy and ammonium regeneration at the seasonal pycnocline, where particulate organic matter likely accumulated.

Broadband acoustic scattering from oblate hydrocarbon droplets.

Improved in situ quantification of oil in the marine environment is critical for informing models of fate and transport and evaluating the resiliency of marine communities to oil spills. Broadband acoustic backscatter has been used to quantify a variety of targets in the water column; from fish and planktonic organisms to gas bubbles and oceanic microstructure, and shows promise for use in quantifying oil droplets. Quantifying water column targets with broadband acoustic backscatter relies on accurate models of a target's frequency dependent target strength (TS), a function of the target's acoustic impedance, shape, and size. Previous acoustic quantification of oil droplets has assumed that droplets were spheres. In this study, broadband (100.5-422 kHz) acoustic backscatter from individual oil droplets was measured, and the frequency dependent TS compared to a model of acoustic scattering from fluid spheres and two models for more complex shapes. Droplets of three different crude oils, two medium oils, and one heavy oil were quantified and all droplets were oblate spheroids. The impact of the deviation from sphericity on the accuracy of each model was determined. If an inversion of the model for spherical droplets was used to estimate flux from acoustic observations, errors in the predicted volume of a droplet were between 30% and 50%. The heavy oil also showed deviations in predicted volume of 20%-40% when using the two models for more complex shapes.

Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

AbstractLow‐mode internal waves propagate over large distances and provide energy for turbulent mixing when they break far from their generation sites. A realistic representation of the oceanic energy cycle in ocean and climate models requires a consistent implementation of their generation, propagation, and dissipation. Here we combine the long‐term mean energy flux from satellite altimetry with results from a 1/10° global ocean general circulation model that resolves the low modes of internal waves and in situ observations of stratification and horizontal currents to study energy flux and dissipation along a 1000 km internal tide beam in the eastern North Atlantic. Internal wave fluxes were estimated from twelve 36‐ to 48‐hr stations in along‐ and across‐beam direction to resolve both the inertial period and tidal cycle. The observed internal tide energy fluxes range from 5.9 kW m−1 near the generation sites to 0.5 kW m−1 at distant stations. Estimates of energy dissipation come from both finestructure and upper ocean microstructure profiles and range, vertically integrated, from 0.5 to 3.3 mW m−2 along the beam. Overall, the in situ observations confirm the internal tide pattern derived from satellite altimetry, but the in situ energy fluxes are more variable and decrease less monotonically along the beam. Internal tides in the model propagate over shorter distances compared to results from altimetry and in situ measurements, but more spatial details close the main generation sites are resolved.

Better together—Combining acoustics and environmental DNA to understand ecosystems

Acoustic signals have historically been and presently are the state-of-the-art for sensing the ocean at small to large spatial scales. Passive acoustics non-invasively assess sound levels, surface conditions, human activity, and the distribution of vocalizing marine life. Echosounders provide acoustic backscatter information that contribute not only critical information on biology but also physical components of the water column which has supplied invaluable knowledge on the community structure, organism size and distribution, and oceanic microstructure. Advances in DNA methods present an opportunity to harness a new technology and fundamentally improve our capacity to monitor habitats, communities, and individual species. Environmental DNA (eDNA) includes whole microorganisms, tissue fragments, reproductive and waste products, and other cellular material in a sample. eDNA methods, such as acoustics, allow for the identification of species without having to physically capture animals. While eDNA and acoustics are both powerful methods for identifying and describing organisms present in the environment, no single survey method can fully represent the “real” condition. We envision a future where remote platforms can gather and relay data in near-real time and can elicit a targeted response to information, such as launching a drone to survey additional locations or increasing the sampling frequency when key species are present.

Measurement and Numerical Study of Vertical Mixing Microstructure in the Bohai Strait

ABSTRACT Zhang, Y.; Liang, S., and Sun, Z., 2017. Measurement and numerical study of vertical mixing microstructure in the Bohai Strait. Vertical mixing plays an important role in three-dimensional hydrodynamic and water quality models. The purpose of this study is to analyze the characteristics of vertical mixing in strong tidal waters and the simulation capability of turbulence closure models under the influence of flow velocity variations and temperature distribution. The distribution characteristics of thermocline and vertical mixing in the Bohai Strait is analyzed through field observations conducted using the Turbulence Ocean Microstructure Acquisition Profiler in summer. Based on the measured temperature and mixing data, three-dimensional numerical models are established with various temperature distribution and turbulence closure schemes. It is concluded that the vertical eddy viscosity Km and the thermal-diffusion coefficient Kh, using the Mellor and Yamada level 2.5 (MY-2.5) and the k-e turbulen...

Vibration Analysis of the Free-Falling Microstructure Profiler

Free-falling microstructure profiler (FFMP) is the most effective platform for measuring ocean microstructure turbulence. Vibration is the key factor of influencing the accuracy of the measurement of the shear sensor mounted on the leading end of the FFMP. In the present work, vibration behavior of an FFMP called FFMP1000 was studied through fluid–structure interaction (FSI) simulations and field trials. Vibration characteristics and mechanism of the FFMP1000 were also discussed. Results showed that motion of the FFMP was like a compound pendulum oscillation, and was caused by vortex shedding at the trailing end of the FFMP. Empirical formulas used to predict the oscillation of the FFMP were deduced based on the characteristics of motion behavior and confirmed through sea trials. The present achievement provides scientific guidance for designing optimal hydrodynamic hull shape of the FFMP. It is also useful to estimate the low end detection limit of the FFMP and to modify the turbulence kinetic energy dissipation rate during ocean observations.

Validation of Thorpe-scale-derived vertical diffusivities against microstructure measurements in the Kerguelen region

Abstract. The Thorpe scale is an energy-containing vertical overturning scale of large eddies associated with shear-generated turbulence. This study investigates indirect estimates of vertical diffusivities from the Thorpe scale method in the polar front region east of the Kerguelen Islands based on fine-scale density profiles gathered during the 2011 KEOPS2 (KErguelen Ocean and Plateau compared Study 2) cruise. These diffusivities are validated in comparison with diffusivities estimated from the turbulence dissipation rate directly measured via a TurboMAP (Turbulence ocean Microstructure Acquisition Profiler) microstructure profiler. The results are sensitive to the choice of the diffusivity parameterization and the overturn ratio Ro, and the optimal results have been obtained from the parameterization by Shih et al. (2005) and the Ro = 0.25 criterion, rather than the parameterization by Osborn (1980) and the Ro = 0.2 criterion originally suggested by Gargett and Garner (2008). The Thorpe-scale-derived diffusivities in the KEOPS2 region show a high degree of spatial variability, ranging from a canonical value of O(10−5) m2 s−1 in the Winter Water layer and in the area immediately north of the polar front to a high value of O(10−4) m2 s−1 in the seasonal thermocline between the surface mixed layer and the Winter Water. The latter high diffusivities are found especially over the shallow plateau southeast of the Kerguelen Islands and along the polar front that is attached to the escarpment northeast of the islands. The interaction of strong frontal flow with prominent bottom topography likely causes the observed elevated mixing rates.

A novel platform to study the effect of small-scale turbulent density fluctuations on underwater imaging in the ocean

Optical signal transmission is an important component of numerous underwater applications, including visibility and electro-optical (EO) communication. In addition to the well-studied effect of particle backscatter, underwater optical signal transmission can be limited by changes in the index of refraction (IOR) due to small-scale variations in temperature and salinity, sometimes called “optical turbulence”. These variations in IOR, which are associated with oceanic turbulence, can lead to the blurring of an underwater optical target, particularly at high spatial frequencies, thus reducing target detail. The 2011 Bahamas Optical Turbulence Experiment (BOTEX) was conducted to investigate this impact of turbulence on underwater optical signal transmission. Investigating naturally occurring “optical turbulence” requires a platform held at depth, capable of concurrent measurements of optical impairment by turbulence, which requires a significant optical path length, as well as associated physical and optical background conditions of the ambient environment. Our novel platform consisted of a high-speed camera and optical target mounted on a 5m-long frame, along with several Nortek Vector Acoustic Doppler Velocimeter (ADV) and PME Conductivity–Temperature (CT) probes, to estimate turbulent kinetic energy and temperature variance dissipation rates experienced by the frame. Data on the background turbulence was collected with a Rockland Oceanographic Vertical Microstructure Profiler, to aid in analysis and guide error estimates of the ADV/CT measurements. This study was the first effort attempting to collect turbulence measurements on a frame designed for the investigation of the effect of density microstructure variations on optical signal transmission in the open ocean. Our results highlight the numerous challenges associated with studying this phenomenon in the dynamic oceanic environment. Here, we present the interpretation of the high-resolution velocity and temperature measurements collected on the frame and discuss the associated difficulties. Despite the numerous challenges, the investigation of the effect of microstructure on underwater optics is needed for efforts aimed at mitigating the impact of “optical turbulence” on underwater EO signal transmission and may help advance optical methods to quantify oceanic microstructure.

Turbulence and mixing by internal waves in the Celtic Sea determined from ocean glider microstructure measurements

We present a new series of data from a 9-day deployment of an ocean microstructure glider (OMG) in the Celtic Sea during the summer of 2012. The OMG has been specially adapted to measure shear microstructure and coincident density structure from which we derive the dissipation rate of turbulent kinetic energy (ε) and diapycnal diffusion rates (K). The methods employed to provide trustworthy turbulent parameters are described and data from 766 profiles of ε, temperature, salinity and density structure are presented. Surface and bottom boundary layers are intuitively controlled by wind and tidal forcing. Interior dynamics is dominated by a highly variable internal wave-field with peak vertical displacements in excess of 50m, equivalent to over a third of the water depth. Following a relatively quiescent period internal wave energy, represented by the available potential energy (APE), increases dramatically close to the spring tide flow. Rather than follow the assumed spring-neap cycle however, APE is divided into two distinct peak periods lasting only one or two days. Pycnocline ε also increases close to the spring tide period and similar to APE, is distinguishable as two distinct energetic periods, however the timing of these periods is not consistent with APE. Pycnocline mixing associated with the observed ε is shown to be responsible for the majority of the observed reduction in bottom boundary layer density suggesting that diapycnal exchange is a key mechanism in controlling or limiting exchange between the continental shelf and the deep ocean. Results confirm pycnocline turbulence to be highly variable and difficult to predict however a log-normal distribution does suggest that natural variability could be reproduced if the mean state can be accurately simulated.

A New Quasi-Horizontal Glider to Measure Biophysical Microstructure

Abstract This study describes the development of a new tethered quasi-horizontal microstructure profiler: the Turbulence Ocean Microstructure Acquisition Profiler–Glider [TurboMAP-Glider (TMG)]. It is a unique instrument, capable of measuring ocean microstructure (temperature and turbulent velocity shear), chlorophyll, and turbidity simultaneously through a quasi-horizontal perspective. Three field experiments were carried out near Joga-shima, Japan, to test the TMG flight performance, and those results as well as comparisons with a laser-based vertical profiler, TurboMAP-L (TM), are described here. The TMG was capable of flying with an angle of attack of less than 25° and was reasonably stable for up to 300 m horizontally over 100-m depth. Some new and relevant empirical results about quasi-horizontal application of high-resolution chlorophyll-a fluorescence sensors are presented. The ratio between the Thorpe length scale and the Ozmidov length scale was used as a tracer to demonstrate that most of the TMG density inversions are due to horizontal variability and not to vertical overturning. These waveform structures are probably due to the horizontal inhomogeneity of the density field and are likely caused by internal waves.

Dissipation measurements using temperature microstructure from an underwater glider

Microstructure measurements of temperature and current shear are made using an autonomous underwater glider. The glider is equipped with fast-response thermistors and airfoil shear probes, providing measurements of dissipation rate of temperature variance, χ, and of turbulent kinetic energy, ε, respectively. Furthermore, by fitting the temperature gradient variance spectra to a theoretical model, an independent measurement of ε is obtained. Both Batchelor (εB) and Kraichnan (εK) theoretical forms are used. Shear probe measurements are reported elsewhere; here, the thermistor-derived εB and εK are compared to the shear probe results, demonstrating the possibility of dissipation measurements using gliders equipped with thermistors only. A total of 152 dive and climb profiles are used, collected during a one-week mission in the Faroe Bank Channel, sampling the turbulent dense overflow plume and the ambient water above. Measurement of ε with thermistors using a glider requires careful consideration of data quality. Data are screened for glider flight properties, measurement noise, and the quality of fits to the theoretical models. Resulting dissipation rates from the two independent methods compare well for dissipation rates below 2×10−7 W kg−1. For more energetic turbulence, thermistors underestimate dissipation rates significantly, caused primarily by increased uncertainty in the time response correction. Batchelor and Kraichnan spectral models give very similar results. Concurrent measurements of ε and χ are used to compute the dissipation flux coefficient Γ (or so-called apparent mixing efficiency). A wide range of values is found, with a mode value of Γ≈0.14, in agreement with previous studies. Gliders prove to be suitable platforms for ocean microstructure measurements, complementary to existing methods.

Microstructure Measurements from an Underwater Glider in the Turbulent Faroe Bank Channel Overflow

AbstractMeasurements of ocean microstructure are made in the turbulent Faroe Bank Channel overflow using a turbulence instrument attached to an underwater glider. Dissipation rate of turbulent kinetic energy ε is measured using airfoil shear probes. A comparison is made between 152 profiles from dive and climb cycles of the glider during a 1-week mission in June 2012 and 90 profiles collected from the ship using a vertical microstructure profiler (VMP). Approximately one-half of the profiles are collocated. For 96% of the dataset, measurements are of high quality with no systematic differences between dives and climbs. The noise level is less than 5 × 10−11 W kg−1, comparable to the best microstructure profilers. The shear probe data are contaminated and unreliable at the turning depth of the glider and for U/ut < 20, where U is the flow past the sensor, ut = (ε/N)1/2 is an estimate of the turbulent velocity scale, and N is the buoyancy frequency. Averaged profiles of ε from the VMP and the glider agree to better than a factor of 2 in the turbulent bottom layer of the overflow plume, and beneath the stratified and sheared plume–ambient interface. The glider average values are approximately a factor of 3 and 9 times larger than the VMP values in the layers defined by the isotherms 3°–6° and 6°–9°C, respectively, corresponding to the upper part of the interface and above. The discrepancy is attributed to a different sampling scheme and the intermittency of turbulence. The glider offers a noise-free platform suitable for ocean microstructure measurements.

Small-scale and lateral intermittency of oceanic microstructure in the pycnocline

The intermittency correction factor μc for the Obukhov–Corrsin ‘ − 5/3’ inertial-convective subrange spectral law is estimated using conductivity (temperature) fluctuation measurements conducted within the microstructure patches of oceanic pycnocline in Monterey Bay. The value of μc was found to be in the range of 0.46–0.51, depending on the accuracy of calculation, which is applicable to stratified, low-Reynolds-number oceanic turbulence. The intermittency factor μ Mc for mesoscale (up to 1 km) lateral variations of scalar dissipation is estimated as 0.43 ± 0.02. The breakdown mechanisms of small-scale locally isotropic and mesoscale non-isotropic (lateral) turbulent temperature fluctuations in oceanic turbulence are discussed in the light of the above observations.

Shear-induced mixing in geophysical flows: does the route to turbulence matter to its efficiency?

AbstractMotivated by the importance of diapycnal mixing parameterizations in large-scale ocean general circulation models, we provide a detailed analysis of high-Reynolds-number mixing in density stratified shear flows which constitute an archetypical example of the small-scale physical processes occurring in the oceanic interior that control turbulent diffusion. Our focus is upon the issue as to whether the route to fully developed turbulence in the stratified mixing layer is in any significant way determinant of diapycnal mixing efficiency as represented by an effective turbulent diffusivity. We characterize different routes to fully developed turbulence by the nature of the secondary instabilities through which a primary Kelvin–Helmholtz billow executes the transition to this state. We then demonstrate that different mechanisms of turbulence transition characterized in these different transition mechanisms lead to considerably different values for the efficiency of diapycnal mixing and also for the effective vertical flux of buoyancy. We show that the widely employed value of 0.15–0.2 for the efficiency of mixing in shear-induced stratified turbulence based upon both laboratory measurements and similarly low-Reynolds-number numerical simulations may be too low for the high-Reynolds-number regime characteristic of geophysical flows. Our results show that the mixing efficiency tends to a value of approximately $1/ 3$ for sufficiently large Reynolds number at an intermediate value of 0.12 for the Richardson number. This is in agreement with a theoretical predictions of Caulfield, Tang and Plasting (J. Fluid Mech., vol. 498, 2004, pp. 315–332) for the asymptotic value of mixing efficiency in stratified Couette flows. In the high-Reynolds-number regime, mixing efficiency is shown to vary over a considerable range during the course of a particular shear-induced mixing event. We explain this variation on the basis of a detailed examination of the underlying dynamics. Since values in the range 0.15–0.2 for mixing efficiency have been extensively employed to infer an effective diffusivity from ocean microstructure measurements and also in energy balance analyses of the requirements of the global ocean circulation, our findings have potentially important implications for large-scale ocean modelling. We also quantify the errors introduced by employing the Osborn (J. Phys. Oceanogr., vol. 10, 1980, pp. 83–89) formula along with an efficiency of 0.15 to infer values for effective diffusivity, and explain the logical underpinnings of this conclusion. One of the more important aspects of this work from the perspective of our theoretical understanding of stratified turbulence is the demonstration that the inverse cascade of energy, which is facilitated by the vortex-merging process that is typical of laboratory experiments and of the low-Reynolds-number simulations of shear flow evolution, is strongly suppressed by increase of the Reynolds number to values typical of geophysical flows. Based on this finding, the application of results based on low-Reynolds-number (numerical or laboratory) experiments to high-Reynolds-number geophysical shear flows needs to be reconsidered.

Observations of Thermohaline Convection adjacent to Brunt Ice Shelf

Abstract Observations were made of ocean microstructure and horizontal currents adjacent to Brunt Ice Shelf in the southeastern Weddell Sea. Periods of in situ supercooled water extending as deep as 65 m were associated with ice nucleation and frazil formation at depth. Ascending ice crystals due to convection lead to increased dissipation rates. The main outflow of potentially supercooled water from deep beneath ice shelf is suggested to be in the deep channel northeast of the measurement site. Because this water is advected southward along the front, it becomes in situ supercooled, leading to suspended ice formation, thermohaline convection, and enhanced dissipation.