The effects of KT ≠ KS in a Stommel-like model of the upper Atlantic Meridional Overturning Circulation under steady surface flux forcing

This study examines a simple 6-box model of a single pole-to-pole ocean basin. Each of a northern polar gyre, a southern polar gyre, and an equatorial gyre, consisting of north and south subtropical gyres plus the equatorial region, is represented by two boxes: a surface box receiving constant fluxes of both temperature (heat) and salt (freshwater) and a deep box. The model includes four dominant processes: surface flux forcing, horizontal meridional advection driven by Southern Ocean winds, horizontal eddy diffusion at gyre boundaries, and convection, as well as the process of vertical diffusion by small-scale processes. Provided that heat loss from the northern polar gyre is sufficiently larger than that from the southern polar gyre, a steady-state Atlantic Meridional Overturning Circulation (AMOC)-like system, i. e., one with sinking in the north polar gyre and upwelling in a weakly stratified southern polar gyre, is obtained at present values of RF ≡ βFS / αFT, the ratio of surface forcing by fluxes of temperature (T ) and salinity (S ) in the equatorial gyre. Despite the fact that vertical diffusive fluxes are much smaller than those associated with all the other processes, it is shown that implementation in this model of a simple water mass–based representation of different vertical diffusivities for T and S, the two water properties that, with pressure, determine the density of seawater, can lead to profound change in the steady-state modes of the system. With equal diffusivities, the AMOC-like mode with north polar convection shifts abruptly to a mode with equatorial convection at sufficiently large values of RF. With unequal diffusivities, this mode boundary is replaced by an intermediate region of RF values in which all three gyres are stratified. The existence and extent of this stratified regime is shown to result predominantly from the differences between vertical turbulent diffusivities of T and S in the salt fingering equatorial gyre. Existence of a stratified regime at values of RF somewhat larger that present implies a tendency towards stable stratification throughout the oceans if, under climate change, the equatorial diffusivity difference were to increase as a result of water mass changes in the subtropical gyres and/or an increase in RF as a result of increased atmospheric freshwater fluxes and/or decreased heat fluxes. This tendency towards an everywhere-stratified ocean is independent of that expected from increased freshwater addition to surface polar oceans due to ice melt

High Resolution Radiation Therapy Dosimetry in Magnetic Fields using Novel Silicon Array Dosimeters: A Pilot for MRI-linac Applications

The integration of online magnetic resonance imaging with linear accelerators presents an exciting new era for radiation therapy. A hybrid MRI-linac machine provides improved soft-tissue contrast enabling the target to be visualised at the time of treatment rather than a surrogate, as is currently common practice. With this advancement comes a number of unique challenges related to the presence of a magnetic field during treatment, one of which is the change in the dose distribution. The magnetic field’s associated force vector, the Lorentz force, influences the trajectory of charged particles within it. Dose-depositing secondary electrons are no exception. Using Monte Carlo simulations, point spread dose arrays have been modelled to demonstrate on a fundamental level how dose distributions are perturbed by magnetic fields as a function of magnetic field strength, orientation with respect to the primary beam and the density of the medium. For the case of inline magnetic fields, the point spread distribution maintains its symmetry, but lengthens and becomes more narrow laterally

Vertical eddy diffusivity in the ocean interior

Vertical turbulent transport of density (mass) in a system of stable stratification ∂p/∂z \u3c 0 (z positive upward) is often modelled by an “eddy” diffusivity Kv ≡ −ρω/(∂p/∂z), normally assumed to be constant. Recent evidence from stratified lakes, fjords and oceans suggests that Kv may be more accurately described as a decreasing function of buoyancy frequency N ≡ (–g(ρo)–1 (∂p/∂z))1/2. A main purpose of this paper is to review available estimates of Kv from a variety of stratified geophysical systems. Particular emphasis is placed upon the degree to which these estimates are dependent upon underlying models used to derive values for Kv from observable quantities. Most techniques reveal a disagreeable degree of model-dependence, frequently providing only upper bounds to the magnitude of Kv. I have coupled the functional dependence which emerges from the least model-dependent of available techniques with ensemble-averaged values of oceanic turbulent kinetic energy dissipation rate per unit mass ε as a function of N, and show that the resulting parameterization for Kv is consistent with a wide range of present oceanic data. Finally, brief re-examination of a simple vertical advection/diffusion model of thermohaline circulation illustrates possible dynamical significance of a stratification-dependent Kv

Multiple thermoclines are barriers to vertical exchange in the subarctic Pacific during SUPER, May 1984

As part of the Subarctic Pacific Ecosystem Research program, we made observations of upper ocean physical and biological properties at 50N; 145W during 12–21 May 1984 from a drifting buoy, instrumented with a thermistor chain and meteorological sensors; a CTD/rosette bottle profiler; a shipboard solar radiometer; and a microstructure profiler equipped with a fast response thermistor and two airfoil velocity probes. At that time, the ocean above the seasonal thermocline was divided by a shallow thermocline step (∼0.5°C) into two layers with different turbulence characteristics and dynamics. The surface layer thermal structure (even in wind speeds up to 14 m s–1) underwent a clear diurnal cycle down to at least 20 m, but the rate of dissipation of turbulent kinetic energy ε did not display day/night differences and was negatively or not correlated with the buoyancy frequency N. Below the shallow thermocline step, ε and N covaried, both reaching maximum values in the permanent pycnocline at 80–90 m. Bio-optical properties of the phytoplankton showed different responses to the different physical environments in the two layers. The initial slope of the relationship between photosynthetic rate and irradiance differed significantly between the two layers; and the phytoplankton in the surface layer displayed strong midday inhibition of fluorescence yield down to 30 m. On the one calm day, both the diurnal thermal signal and the fluorescence inhibition were confined to the top few meters, indicating that the deeper penetration on other days was due to near-surface effects being redistributed throughout the upper layer by wind mixing. The turbulence within the permanent pycnocline appeared to be anisotropic down to viscous scales, effectively eliminating vertical turbulent exchange. Such anisotropy in highly stable layers may favor persistence of “microzones” of enriched nutrients but it precludes calculation from microstructure measurements of accurate estimates of the vertical coefficient of turbulent diffusion Kz, required to estimate the vertical flux of dissolved nitrate through the permanent pycnocline

Langmuir supercells on the middle shelf of the South Atlantic Bight: 1. Cell structure

Langmuir circulation cells extending through entire water columns exert a profound influence on sediment transport on shallow shelves, hence their designation as Langmuir Supercells (LSs) upon discovery in 2004 in water of 15 m depth at the Long-Term Ecosystem Observatory (LEO) off New Jersey (United States). Until now, similar high-frequency, full water column measurements of turbulent velocities and density fields have not been reported from significantly deeper continental shelf environments. Such deeper measurements are needed to determine whether LSs exist to influence sediment transport outside the inner shelf. In this article, that deficiency is addressed, using measurements from a midshelf location in the South Atlantic Bight. These data indicate that LSs form during high wind and wave forcing in water of 26 m depth and are thus capable of affecting sediment transport over more than about half of the area of this wide, shallow shelf. Relative to those at LEO, the LSs reported here are less organized and more temporally variable despite similar magnitude forcing. Possible causes of cell weakness and variability are considered

Measuring turbulent large-eddy structures with an ADCP. Part 2. Horizontal velocity variance

This paper considers the degree of accuracy with which observations from an acoustic Doppler current profiler (ADCP) can determine turbulent horizontal velocity variance. As in a previous paper addressing turbulent vertical velocity variance, we use a combination of techniques, deriving response functions from simple theory and from oceanic observations taken with a VADCP (an ADCP with an additional vertical (V) beam) in two different oceanic turbulent flows, Langmuir supercells (LSC) and unstable convection. In the case of LSC, we also determine response by sampling available Large-Eddy Simulations (LES) with specified beam geometry. In contrast with the previous investigation, where a direct measurement of vertical velocity variance was available from the vertical beam of the VADCP, we lack direct measurements of horizontal velocity variances. Thus the observational response reported here for horizontal variance is an estimate, taken as the ratio of first-order to the (assumed more accurate) second-order variance estimates made possible for the first time by the presence of a vertical beam.The theoretical response function is used to illustrate effects on response of horizontal scale, vertical/horizontal anisotropy and possible quasi-coherent phase structure of the large eddies of the turbulent field, and to predict the impact of changing θ, the angle of slant beams from vertical. Observational estimates show that convective turbulence is characterized by near-unity response throughout the water column for both horizontal velocity variances, in agreement with theoretical prediction. For Langmuir supercells, theoretical responses correctly predict qualitative behavior of the LES-derived response functions, specifically overestimation in the lower part of the water column shifting to underestimation toward the surface. LES-derived responses for different values of θ are also in agreement with theory: both approaches suggest that θ = 30° provides more accurate measurement of horizontal turbulent velocity variance than does θ = 20°, the present commercial standard.For all examined cases of unstable convection and most (normal) LSC cases, observationally estimated response functions generally agree with theoretical (and, where available, LES) predictions. However in a few (abnormal) LSC cases, record-averaged second-order variances are clearly underestimated (most obviously when they are actually negative). We have been unable to assign a cause to this underestimation and advise against use of second-order horizontal velocity variances until this unpredictable effect is understood. Normal LSC cases exhibit overestimation of horizontal variance by a maximum factor of 1.5 (observational estimates) to 3 (LES estimates), a degree of accuracy comparable to that associated with microscale-based estimates of turbulent large-eddy quantities. We suggest ways in which the parameters needed for theoretical prediction of the response function for horizontal velocity can be estimated directly from VADCP measurements

Anatomy of a Langmuir supercell event

Langmuir supercells (LS), which are Langmuir circulations (LC) extending over full water column depth during storms and revealed by high water column backscatter from surface-origin microbubbles and bottom-origin sediment, were discovered in 2003 during several months of measurements in 15 m of water near the coast of New Jersey. Both the structures themselves and the specific forcing conditions under which they occur have been documented elsewhere. This paper provides an account of the broader oceanographic setting of supercell events, focusing on conditions at the start and end. The start of events is associated with the presence of surface waves of intermediate type that “feel bottom” with amplitudes sufficiently large to resuspend sediment and achievement of three conditions for full-depth LC: an unstratified water column, La \u3c ∼0.3 and |Ra| \u3c 105, where Ra and La are dimensionless parameters derived from scaling of the wave-averaged momentum equation. Event cessation is associated with failure of one of the latter two conditions or the reappearance of stratification. There is no fixed order in which conditions necessary for full-depth LC are met or fail. Comparison with data from a deeper site off Georgia suggests that coherent full-depth Langmuir circulations will not generally be observed in unstratified water columns much deeper than 25–30 m, a depth determined primarily by the wavelength of surface waves generated by typical storms. We also document two features of LC acting in the surface layer of the stratified water column that existed prior to onset of the prototype LS event. First, LC confined to the surface layer generated first mode internal waves with frequency that of the stratified interior. Secondly, active surface layer LC did not act efficiently as direct agents of mixed layer deepening, which occurred primarily in two separate episodes of Richardson number lowered by increased shear. Instead, as a result of quasi-organized structure and enhanced vertical penetration relative to stress-driven turbulence, the primary role of LC may be to increase efficiency of momentum transfer to the surface layer, enhancing surface layer acceleration and contributing to onset of the shear instability that does deepen the surface layer

Measuring turbulent large-eddy structures with an ADCP. 1. Vertical velocity variance

Two different turbulent flows, Langmuir supercells and unstable convection, have been sampled with a VADCP, an acoustic Doppler current profiler (ADCP) with an additional vertical (V) beam. Direct measurements of the profile of vertical velocity variance provided by the vertical beam are used to calculate observational response functions for algorithms used to derive vertical velocity from the 4 beams of a standard ADCP. A theoretical response function derived for the vertical velocity estimate from a single pair of opposed slant beams illustrates the importance of large-scale quasi-coherent flow structures, as well as effects of different angles of slant beams from vertical. Different large-eddy characteristics for Langmuir supercells and unstable convection yield different theoretical response: however in both cases, the theoretical response agrees qualitatively with that derived from observations. For Langmuir super-cells, there is additional agreement with numerical response functions generated by using the geometry of a VADCP to sample three-dimensional flow fields available from large eddy simulations (LES). The results from all three approaches show that there can be significant error in vertical velocity inferred from slant beam velocities. The error may be either over- or under-estimation, depending upon (usually unknown) features of the large eddies of the turbulent field, such as vertical/horizontal anisotropy, phase coherence, and orientation of horizontally anisotropic turbulent structures relative to the instrument. Given only a standard ADCP, the “best” estimate of vertical velocity variance is not the usual 4-beam estimate, but the larger of the two pair estimates

The Transcriptome of Human Endometrial Mesenchymal Stem Cells Under TGFβR Inhibition Reveals Improved Potential for Cell-Based Therapies

Mesenchymal stem/stromal cells (MSCs) are multipotent cells with favorable properties for cell therapies and regenerative medicine. Human endometrium harbors a small population of perivascular, clonogenic MSCs (eMSCs) identified by the SUSD2 marker. As for other MSCs, eMSCs require extensive in vitro expansion to generate clinically relevant numbers of cells, resulting in spontaneous differentiation, replicative senescence and cell death, decreasing therapeutic potency. We previously demonstrated that A83-01, a TGF-β receptor inhibitor, maintained eMSC clonogenicity, promoted proliferation, prevented apoptosis and maintained MSC function in vitro. Here we compare the transcriptome of passaged eMSCs from six women cultured with and without A83-01 for 7 days. We identified 1206 differentially expressed genes (DEG) using a false discovery rate cut-off at 0.01 and fold change >2. Significant enrichment of genes involved in anti-inflammatory responses, angiogenesis, cell migration and proliferation, and collagen fibril and extracellular matrix organization were revealed. TGF-β, Wnt and Akt signaling pathways were decreased. Anti-fibrotic and anti-apoptotic genes were induced, and fibroblast proliferation and myofibroblast related genes were downregulated. We found increased MSC potency genes (TWIST1, TWIST2, JAG1, LIFR, and SLIT2) validating the enhanced potency of A83-01-treated eMSCs, and importantly no pluripotency gene expression. We also identified eMSCs’ potential for secreting exosomes, possibly explaining their paracrine properties. Angiogenic and cytokine protein arrays confirmed the angiogenic, anti-fibrotic and immunomodulatory phenotype of A83-01-treated eMSCs, and increased angiogenic activity was functionally demonstrated in vitro. eMSCs culture expanded with A83-01 have enhanced clinically relevant properties, suggesting their potential for cell-therapies and regenerative medicine applications

Forcing and Dynamics of Seafloor-Water Column Exchange on a Broad Continental Shelf

Relict sediments of elevated permeability characterize the majority of continental shelves globally (Emery, 1968). In these settings, interactions between benthic boundary layer (BBL) flows and seabed topography generate pressure fluctuations that drive advective and dispersive porewater transport, dramatically increasing the magnitude and variability of porewater solute and particulate exchange across the sediment-water interface (Huettel et al., 1996; Huettel and Rusch, 2000). On broad shallow shelves with a relatively large area-to-volume ratio, the seafloor’s role is magnified. Energetic events may reorganize bedforms across a significant fraction of the shelf, leading to altered exchange dynamics that may persist long after the organizing event. Ecosystem-based management of both resources and environmental status requires improved fundamental understanding of dynamic benthic exchange processes. Scattered, short-time-scale observations are unlikely to capture the full spectrum of events that affect sediment-water exchanges; a persistent observational presence on the seafloor is required