Effects Of Vertical Diffusion Research Articles

Deep inflow transport and dispersion in the Gulf of St. Lawrence revealed by a tracer release experiment

The Gulf of St. Lawrence is increasingly affected by bottom water hypoxia; however, the timescales and pathways of deep water transport remain unclear. Here, we present results from the Deep Tracer Release eXperiment (TReX Deep), during which an inert SF5CF3 tracer was released inshore of Cabot Strait at 279 m depth to investigate deep inflow transport and mixing rates. Dispersion was also assessed via neutrally-buoyant Swish floats. Our findings indicate that the tracer moves inland at 0.5 cm s−1, with an effective lateral diffusivity of 2 × 102 m2 s−1 over 1 year. Simplified 1D simulations suggest inflow water should reach the estuary head in 1.7 years, with the bulk arriving after 4.7 years. Basin-wide effective vertical diffusivity is around 10−5 m2 s−1 over 1 year; however, vertical diffusivity increases near the basin slopes, suggesting that turbulent boundary processes influence mixing. These results are compared to Lagrangian simulations in a regional 3D model to evaluate the capacity to model dispersion in the Gulf.

The Transition Layer and Remnant Transition Layer of the Western Arctic Ocean: Stratification, Vertical Diffusivity, and Pacific Summer Water Heat Fluxes

AbstractThe exchange of heat and other tracers between the ocean interior and sea ice cover depends on vertical diffusion through the thin and strongly stratified Arctic Ocean transition layer and remnant transition layer. Ice‐Tethered Profiler observations during 2004–2021 were used to characterize and investigate the active and remnant transition layers in the Beaufort Gyre region. Transition layers evolved seasonally, with the deepest, densest, and warmest transition layers observed in spring. In a composite view, the summer mixed layer shoaled over a timescale of approximately 2 weeks, leaving behind a remnant transition layer whose stratification had already begun to weaken. The stratification maximum of the remnant transition layer continued to weaken and broaden throughout the summer, including changes associated with storm events. The weakening stratification was used to estimate an effective vertical turbulent diffusivity which ranged from 10−8 to 10−6 m2/s for individual ITP records; a value of 6 × 10−7 m2/s is representative of the Beaufort Gyre region. Vertical heat fluxes through the active and remnant transition layers were estimated over 90‐day timescales. Springtime vertical heat fluxes increased from 0.3 ± 0.3 W/m2 during 2006–2011 to 1.1 ± 0.7 W/m2 during 2017–2021. Summertime vertical heat fluxes through the remnant transition layer were somewhat less, but also increased interannually. Vertical gradients of density during spring and summer, and so diffusion of density through the active and remnant transition layers, did not increase interannually. Pacific Summer Water warming has increased ocean to ice heat fluxes, which may continue to become more significant in the future.

Roles of thermal advection in setting the Southern Ocean warming pattern

AbstractThe Southern Ocean is the major region of global ocean heat gain. Most of the heat gain is concentrated in the region of the Antarctic Circumpolar Current (ACC), but SST within the ACC only exhibits a slight increase. Ocean circulation plays a fundamental role in setting the pattern of temperature response to global warming. In this study, a 1/4° Southern Ocean eddy‐permitting model configured based on the Regional Ocean Modeling System is used to study the role of ocean in setting the warming pattern, by examining the response of ocean temperature and heat budget terms to a uniform air–sea heat flux (Qnet) increase. As Qnet increases by a constant value, the SST increases, but with a smaller value within the ACC region. Consequently, more heat flux in the ACC region is induced from the atmosphere to the ocean through thermal adjustment. Heat budget analyses suggest that the surface warming induced by vertical diffusion is largely offset by the cooling effect of heat advection, especially within the ACC region; however, in the subsurface, the effect of vertical diffusion becomes slight, and heat advection dominates the temperature change and induces warming in the ACC region. These results highlight the importance of heat advection in setting the temperature response pattern to air–sea heat flux increase: by transporting more heat from the surface to the deep ocean, the SST increase within the ACC region becomes smaller, and this induces more heat flux from the atmosphere to the ocean and thus makes the ACC region become a major region of heat gain.

Three-dimensional dust stirring by a giant planet embedded in a protoplanetary disc

ABSTRACT The motion of solid particles embedded in gaseous protoplanetary discs is influenced by turbulent fluctuations. Consequently, the dynamics of moderately to weakly coupled solids can be distinctly different from the dynamics of the gas. Additionally, gravitational perturbations from an embedded planet can further impact the dynamics of solids. In this work, we investigate the combined effects of turbulent fluctuations and planetary dust stirring in a protoplanetary disc on three-dimensional dust morphology and on synthetic ALMA continuum observations. We carry out 3D radiative two-fluid (gas + 1-mm-dust) hydrodynamic simulations in which we explicitly model the gravitational perturbation of a Jupiter-mass planet. We derived a new momentum-conserving turbulent diffusion model that introduces a turbulent pressure to the pressureless dust fluid to capture the turbulent transport of dust. The model implicitly captures the effects of orbital oscillations and reproduces the theoretically predicted vertical settling-diffusion equilibrium. We find a Jupiter-mass planet to produce distinct and large-scale three-dimensional flow structures in the mm-sized dust, which vary strongly in space. We quantify these effects by locally measuring an effective vertical diffusivity (equivalent alpha) and find azimuthally averaged values in a range δeff ∼ 5 × 10−3–2 × 10−2 and local peaks at values of up to δeff ∼ 3 × 10−1. In synthetic ALMA continuum observations of inclined discs, we find effects of turbulent diffusion to be observable, especially at disc edges, and effects of planetary dust stirring in edge-on observations.

The spatio-temporal distribution and transport of suspended sediment in Laizhou Bay: Insights from hydrological and sedimentological investigations

Suspended sediment transport and deposition are crucial physical processes controlling the geomorphological evolution of estuaries and bays. Specially, under the context of worldwide coastal erosion, knowledge of the spatio-temporal distribution of suspended sediment concentration (SSC) and its associated sediment load have become increasingly important for bay management. However, our understanding of the mechanisms of suspended sediment dynamics continues to be hampered by the lack of high-resolution observations. Here, we present a study of the transport mechanisms and controlling factors of suspended sediment over Laizhou Bay. For this, we conducted continuous measurements of SSC, salinity, temperature, and flow velocity at nine stations throughout Laizhou Bay for one 25-h period during each of the spring and moderate tides. Based on these data, residual current, gradient Richardson numbers, and suspended sediment flux were calculated. Our results indicate that a strong current field occurs near the Yellow River mouth, corresponding to the zone with high SSC. The overall diffusion characteristics of suspended sediment are controlled by the tidal current field. Furthermore, our findings suggest that different degrees of stratification occur in the water column, which inhibit the effective vertical diffusion of suspended sediment; Higher water temperature was the main cause of stronger water column stratification of spring tide than moderate tide. Finally, our results reveal that the type of seabed sediment is an important factor controlling SSC by influencing resuspension flux. We conclude that the primary mechanisms controlling suspended sediment transport in Laizhou Bay are advection and tidal pumping, especially advection. Our research provides both a foundational reference for the sediment source-to-sink process from the Yellow River to the sea, as well as guideline implications for coastal engineering construction and channel dredging.

Effective Vertical Diffusion by Atmospheric Gravity Waves

AbstractQuantification of heat and constituent transport by gravity waves (GWs) in global models is challenging due to limited model resolutions. Current parameterization schemes suffer from oversimplification and often underestimate the transport rate. In this study, a new approach is explored to quantify the effective vertical eddy diffusion by using a high‐resolution Whole Atmosphere Community Climate Model (WACCM) simulation based on scale invariance. The WACCM simulation can partially resolve the mesoscale GW spectrum down to 250 km horizontal wavelength, and the heat flux and the effective vertical eddy diffusion by these waves are calculated directly. The effective vertical diffusion by the smaller‐scale, unresolved waves, is then deduced based on scale invariance, following the method outlined by H.‐L. Liu (2019) in quantifying GW momentum flux and forcing. The effective vertical diffusion obtained is generally larger than that obtained from parameterizations, and is comparable with that derived from observations in the mesosphere and lower thermosphere region.

Theoretical study on Janus graphene oxide membrane for water transport

Graphene oxide (GO) membranes have received considerable attention owing to their outstanding water-permeation properties; however, the effect of the membrane’s microstructures (such as the distribution of oxidized and pristine regions) on the transport mechanism remains unclear. In this study, we performed molecular simulations to explore the permeation of a water-ethanol mixture using a new type of Janus GO membranes with different orientations of oxidized and pristine surfaces. The results indicate that the oxidized upper surface endows the GO membrane with considerable water-capture capability and the in-built oxidized interlayer promotes the effective vertical diffusion of water molecules. Consequently, using the optimized Janus GO membrane, infinite water selectivity and outstanding water flux (∼40.9 kg·m−2·h−1) were achieved. This study contributes to explaining the role of oxidized regions in water permeation via GO membranes and suggests that Janus GO membranes could be used as potential candidates for water-ethanol separation.

Contaminant transport in wetland flows: Different fate between the upper and bottom layers

Abstract The mechanism of contaminant transport in wetland flows plays a central role in various applications. A two-zone model is applied to study the transport process. The significantly different fate of concentrations between the upper and bottom layers is characterized. This model provides a practicable and useful tool to quickly capture the evolution of contaminant concentration, being of particular importance in engineering science. The results agree well with the numerical solutions, which reveal that the concentrations in the fast-moving zone in the upper layer and in the slow-moving zone in the bottom layer perform quite different in the transient dispersion stage. The two concentrations respectively form a leading right-skewed Gaussian curve and a tailing left-skewed Gaussian one. Under the effect of vertical diffusion at the interface of two zones, the two concentrations gradually approach the depth-averaged mean, and all evolve to asymptotically Gaussian distribution finally. The concentration variation rate between the two zones reaches about 50 % as the process falls into the asymptotic Taylor dispersion stage at dimensionless time 1.0. And it remains about 10 % even at one order larger dimensionless time 10.0. The findings of the different fate of two concentrations will offer meaningful information to study the benthon and plankton in related areas.

Isopycnal mixing by mesoscale eddies significantly impacts oceanic anthropogenic carbon uptake

AbstractAnthropogenic carbon dioxide uptake varies across Earth System Models for reasons that have remained obscure. When varied within a single model, the lateral eddy mixing coefficient ARedi produces a range of uptake similar to the modeled range. The highest uptake, resulting from a simulation with a constant ARedi of 2400 m2/s, simulates 15% more historical carbon uptake than a model with ARedi = 400 m2/s. A sudden doubling in carbon dioxide produces a 21% range in carbon uptake across the models. Two spatially dependent representations of ARedi produce uptake that lies in the middle of the range of constant values despite predicting very large values in the subtropical gyres. One‐dimensional diffusive models of the type used for integrated assessments can be fit to the simulations, with ARedi accounting for a substantial fraction of the effective vertical diffusion. Such models, however, mask significant regional changes in stratification and biological carbon storage.

Observational constraints on parameter estimates for a simple climate model

Observations of ocean temperature and ocean heat content changes are investigated in order to better estimate the primary ocean parameters in a simple climate model, namely MAGICC (Model for the Assessment of Greenhouse gas Induced Climate Change). A re-examination of the simulated 20th century ocean temperature changes has been made possible by the release of new observational data, which indicates that complex climate models and MAGICC are mixing too much heat into the deeper ocean. Goodness-of-fit testing between the simulated and observed 1960-2008 world ocean temperature change leads to a revised set of parameters for the simple climate model that include a value for the ocean effective vertical diffusivity that is nearly a quarter of that used in the IPCC Third and Fourth Assessment Report versions of the model. Testing an independent constraint based on the ratio of changes in sea surface temperature to 700 m ocean heat content changes produces similar results. The lower ocean diffusivity affects the surface temperature results and alters the best estimate for the climate sensitivity parameter. The projected temperature changes for a high-growth emissions scenario show a larger increase in temperature by 2100 even with a reduced climate sensitivity.

Inter-decadal changes in stratification and double diffusion in a transatlantic section along 7.5°N

We use three transatlantic cruises (1957, 1993 and 2010) along 7.5°N to analyse inter-decadal variations of the neutral buoyancy frequency (with neutral density replacing potential density) and Turner angle. We also use Argo data from the 2003-2011 period to show that, within central and intermediate waters, the changes between the three sections are slightly greater than the seasonal and inter-annual variations, so they may be interpreted as actual inter-decadal variations. The results point to a generalized sinking of isoneutrals between 1957 and 2010, with maximum zonally-averaged values of about 100 m in the central and upper deep layers. They also reveal the occurrence of substantial changes in the intermediate and neighbouring water strata, with differential vertical sinking of isoneutrals and the transformation of their thermohaline characteristics. The neutral buoyancy frequency increased in the lower central and upper intermediate layers and decreased in the lower intermediate and upper deep layers. The distributions of Turner angle highlight a predominance of salt-fingering in the North Atlantic, except for a gravitationally doubly-stable layer located immediately below the intermediate water core; this stable layer thinned substantially between 1957 and 2010 because of the sinking of the lower intermediate isoneutrals acting together with actual water transformations from Antarctic Intermediate Waters into North Atlantic Deep Waters. We conclude that a significant portion of the water column underwent both increased vertical stratification and enhanced salt-fingering, two mechanisms with opposite effects on the effective vertical diffusion.

The Evolution of an Innovation: Variations in Medicaid Managed Care Program Extensiveness

This article utilizes a theoretical framework of policy innovation, diffusion, and reinvention to investigate the evolving nature of Medicaid managed care programs over time. By estimating two separate models, one for primary care case management (PCCM) and a second for risk-based program enrollment, this study seeks to disentangle two different paths of learning (internal and external), investigate the potential effects of vertical diffusion of policy, and examine the impact of internal state characteristics on the extent of Medicaid managed care. With respect to diffusion and learning, the data reveal that earlier adopters implement more extensive programs. The data fail to reveal much internal learning, although there is evidence of some. External impacts are clear: managed care enrollments in neighboring states and changes in the federal waiver process affect states' decisions. Other policy choices are important: states with more generous Medicaid eligibility rules implement more extensive managed care programs. Complementing other studies of Medicaid, we find that politics and economics make a difference for the extent of managed care programs; unlike other Medicaid studies, we find no effect of race and ethnicity.

Hadley Cell Dynamics in a Virtually Dry Snowball Earth Atmosphere

Abstract The Hadley cell of a virtually dry snowball Earth atmosphere under equinox insolation is studied in a comprehensive atmospheric general circulation model. In contrast to the Hadley cell of modern Earth, momentum transport by dry convection, which is modeled as vertical diffusion of momentum, is important in the upper branch of the snowball Earth Hadley cell. In the zonal momentum balance, mean meridional advection of mean absolute vorticity is not only balanced by eddies but also by vertical diffusion of zonal momentum. Vertical diffusion also contributes to the meridional momentum balance by decelerating the Hadley cell through downgradient mixing of meridional momentum between its upper and lower branches. When vertical diffusion of momentum is suppressed in the upper branch, the Hadley cell strengthens by a factor of about 2. This is in line with the effect of vertical diffusion in the meridional momentum balance but in contrast with its effect in the zonal momentum balance. Neither axisymmetric Hadley cell theories based on angular momentum conservation nor eddy-permitting Hadley cell theories that neglect vertical diffusion of momentum are applicable to the snowball Earth Hadley cell. Because the snowball Earth Hadley cell is a particular realization of a dry Hadley cell, these results show that an appropriate description of dry Hadley cells should take into account vertical transport of momentum by dry convection.

Mixing in the western equatorial Pacific and its modulation by ENSO

High vertical resolution measurements of the flow in the western equatorial Pacific reveal the vertical shear to be dominated by flow features that have a small vertical scale (O(10 m)). This is true for all the measurements we have taken over a 3 year period and differing ENSO states. Much of the measured turbulent activity is found to be associated with these small scale features, with the suggestion that mixing in the region is heavily influenced by the presence of this small scale activity. The level of mixing within and above the thermocline is strongly modulated by ENSO events, with the level of mixing being significantly greater during the observed La Niña event. Changes to the stratification above the thermocline during ENSO events play a major role in both changes to the level of turbulent activity and the effective vertical diffusion coefficient.

A time series of environmental tracer data from deep meromictic Lake Lugano, Switzerland

An 11‐yr (1990–2001) time series of tritium‐helium‐3 (3H‐3He) apparent water ages as well as one sulfur hexafluoride (SF6) profile were used to study the development of the vertical mixing dynamics of the deep, meromictic northern basin of Lake Lugano. The density stratification of the water column was dominated by an increase in dissolved ions with depth, which remained approximately constant during the 1990s. The deep‐water temperature increased steadily during this period, passing a threshold above which cooling of the surface water could force convection. However, increasingly mild winter temperatures prevented the occurrence of a turnover until 2005. The maximum apparent 3H‐3He water age increased from about 16 yr in 1990 to 23 yr in 2001. The maximum apparent SF6 age in 2001 was 12.8 yr. The large difference between the apparent 3H‐3He and SF6 ages is at least partly due to nonlinear effects of mixing, causing SF6 ages to underestimate the true mean deep‐water residence time, whereas 3H‐3He ages overestimate it. The decreasing concentrations of 3H and 3He are more reliable indicators of the continuous deep‐water exchange in the lake than are the apparent ages. Budget calculations using the tracer concentrations reveal an annual renewal of the deep water below 100 m in depth by about 8% and enable the calculation of long‐term mean profiles of the effective vertical turbulent diffusivity Kz. No trend in the mixing intensity during the 1990s could be determined. The radiogenic He flux into the lake is comparable to estimates of the whole crustal degassing flux.

Vertical eddy diffusion and nutrient supply to the surface mixed layer of the Antarctic Circumpolar Current

Dispersion of the tracer sulphur hexafluoride (SF6) during the Southern Ocean Iron Enrichment Experiment (SOIREE) provided an estimate of vertical exchange at the base of the surface mixed layer (60 m) at 61°S 140°E. Budget analysis confirmed that the SF6 patch was well constrained by surface mapping, with the decline in total SF6 showing good agreement with that predicted from wind speed parameterizations. Two approaches were used to calculate the mean effective vertical diffusivity Kz from the diapycnal transfer of SF6, with complementary error function and second‐moment fits to the SF6 depth profiles indicating that Kz was less than 0.3 × 10−4 m2s−1. This result was examined using a three‐dimensional diffusion model that incorporated lateral dispersion and air‐sea exchange losses, which confirmed that vertical shear and subpycnocline dispersion did not influence the Kz estimate. Current shear at the base of the mixed layer was generated by wind‐driven inertial oscillation, with a decrease in wind speed and increasing stratification in the latter half of the experiment reducing diapycnal transfer of SF6. A compilation was used to examine the potential of both N (Brunt‐Väisäla frequency) and Ri (gradient Richardson number) on the basis of parameterizations of Kz in the seasonal pycnocline. Application of Kz to nutrient gradients in the seasonal pycnocline suggests that vertical diffusion represents a significant pathway for silicic acid supply in late summer. Furthermore, use of the mean effective Kz (0.11 ± 0.2 × 10−4 m2s−1) indicates that vertical diffusion supplies a large proportion of the iron required for new production in this region.

Estimation of Effective Vertical Diffusivity in Turbulent Exchange Flows by Numerical Experiments

The importance of exchange flows through channels or straits, which are interconnecting water bodies of different densities, lies in the fact that water mass characteristics in sub-basins are determined by these flows, extend of which depends on the exchange rate between the interconnected water bodies. The knowledge of vertical diffusivity, κ, along a channel/strait is therefore crucial as this gives us an idea about the amount of heat, mass and dissolved solids transported across it and provide parameterisations for circulation models. From comparison with numerical results of an exchange through a channel at various values for Gr, Pr and aspect ratio, we infer the effective value for κ as it appears in the Grashof number.

A quasi‐one‐dimensional coupled climate‐carbon cycle model: 2. The carbon cycle component

A quasi‐one‐dimensional, coupled climate‐carbon cycle model is presented which consists of two polar domains and one nonpolar domain. The model simulates the distribution of dissolved inorganic carbon (DIC), alkalinity, phosphate, dissolved oxygen, and temperature and contains a biological pump with production of organic tissue, calcite, and arogonite. Bottom water is conditioned in one polar domain through interaction with the atmosphere and convective mixing and is injected into the lower portion of the nonpolar domain. Intermediate water flows into the other polar domain and upwells. Successful simultaneous simulation of the observed distribution of all the tracers (including isotope ratios) requires (1) an upwelling velocity in the nonpolar domain that peaks around 2 m yr−1 at a depth of 1 km, with a gradual decrease above and below this depth; (2) effective vertical diffusion coefficients for temperature and other tracers that are different in the upper 0.5 km; and (3) a carbonate carbon to organic carbon production ratio of only 0.09. These requirements are consistent with physical considerations and/or observational evidence. In particular, observational data combined with a consideration of mixing along isopycnal surfaces and model results both indicate that the effective vertical diffusion coefficient in the upper ocean should be smallest for temperature and largest for oxygen, with the values for alkalinity and phosphate modestly smaller than for DIC. The model parameters obtained by tuning the model to preindustrial tracer distributions also provide the best (and generally excellent) fit to observed transient isotope changes. Interactions between alkalinity and DIC modulate the effect on steady state atmospheric pCO2 of changes in the model parameters. However, the model uptake of anthropogenic CO2 and the computed atmospheric CO2 variation to year 2200 are remarkably insensitive to the choice of model mixing parameters, given that these are assumed to be constant during a given simulation. Finally, the sensitivity of the model atmospheric pCO2 to changes in the temperature of warm ocean surface matches that obtained by three‐dimensional ocean carbon cycle models.

A quasi‐one‐dimensional coupled climate‐change cycle model: 1. Description and behavior of the climate component

A quasi‐one‐dimensional, coupled climate‐carbon cycle model is presented, which consists of two polar domains and one nonpolar domain. The model simulates the distribution of dissolved inorganic carbon (DIC), alkalinity, phosphate, dissolved oxygen, and temperature and contains a biological pump with production of organic tissue, calcite, and arogonite. Bottom water is conditioned in one polar domain through interaction with the atmosphere and convective mixing and is injected into the lower portion of the nonpolar domain. Bottom water formed in the downwelling polar domain largely upwells in the nonpolar domain, although a portion upwells from intermediate depth into the other polar domain. In this paper the climate component of the coupled model and its behavior are described, while the carbon cycle component is documented by Harvey [this issue], hereafter referred to as part 2. We develop a simple physical basis for determining the relative magnitudes of the effective vertical diffusion coefficient (kv) for different tracers in a one‐dimensional (1‐D) model and find that kv is smallest for temperature, intermediate for carbon, and largest for dissolved oxygen. We deduce a substantially smaller kv for temperature in the upper ocean than previously used in 1‐D models (∼0.2 cm2 s−2 rather than 0.6–1.0 cm2 s−1) and a smaller peak upwelling velocity (2 m yr−1 rather than 4 m yr−1). The explicit representation of convective mixing has a significant effect on the model surface temperature transient response and sea level rise when the intensity of the thermohaline changes. As a result, the transient temperature response and sea level rise obtained here when the thermohaline circulation intensity decreases is significantly different from that of the classical 1‐D upwelling‐diffusion model.

A Seasonally Forced Ocean–Atmosphere Model for Paleoclimate Studies

Seasonal forcing is applied to an idealized model of the ocean‐atmosphere system by prescribing monthly values of solar insolation at the top of the atmosphere and wind stress at the ocean surface. In addition, meridional near-surface wind velocities are applied for the advection term in the parameterization of the atmospheric moisture transport. The simulated seasonal cycle is compared with observations and reanalysis climatologies. It is found that the model can reasonably well simulate the present-day seasonal cycle. Largest model errors are found in the performance of the hydrological cycle. The sensitivity of the thermohaline circulation is examined with respect to seasonal versus annual-mean forcing. It is shown that meridional overturning is increased (20%) if seasonal forcing is applied instead of annual-mean forcing. Both seasonality in wind stress and insolation forcing contribute to the increased overturning. Two stable equilibria, one with deep water formation in the North Atlantic and one without, exist irrespective of seasonal or annual-mean forcing. However, lower sensitivity of the thermohaline circulation to meltwater input into the North Atlantic results if seasonal forcing is applied. It is shown that a large part of this difference is due to an increased effective vertical heat diffusion in the seasonally forced model. Vertical mixing is enhanced by the wind-induced seasonality in meridional overturning. A quantitative estimate of the difference in effective vertical eddy diffusivities between the seasonally and the annually forced model versions is given.