Entrainment Of Water Research Articles

Sea Ice–Ocean Feedbacks in the Antarctic Shelf Seas

AbstractObserved changes in Antarctic sea ice are poorly understood, in part due to the complexity of its interactions with the atmosphere and ocean. A highly simplified, coupled sea ice–ocean mixed layer model has been developed to investigate the importance of sea ice–ocean feedbacks on the evolution of sea ice and the ocean mixed layer in two contrasting regions of the Antarctic continental shelf ocean: the Amundsen Sea, which has warm shelf waters, and the Weddell Sea, which has cold and saline shelf waters. Modeling studies where we deny the feedback response to surface air temperature perturbations show the importance of feedbacks on the mixed layer and ice cover in the Weddell Sea to be smaller than the sensitivity to surface atmospheric conditions. In the Amundsen Sea the effect of surface air temperature perturbations on the sea ice are opposed by changes in the entrainment of warm deep waters into the mixed layer. The net impact depends on the relative balance between changes in sea ice growth driven by surface perturbations and basal-driven melting. The changes in the entrainment of warm water in the Amundsen Sea were found to have a much larger impact on the ice volume than perturbations in the surface energy budget. This creates a net negative ice albedo feedback in the Amundsen Sea, reversing the sign of this typically positive feedback mechanism.

Ocean Barrier Layers in the Energy Exascale Earth System Model

AbstractOcean barrier layers (BLs) separate the mixed layer from the top of the thermocline and are able to insulate the mixed layer from entrainment of cold thermocline water. Here, we provide the first global BL assessment in E3SMv1 and two other Earth system models. Compared to observations, models reproduce the global distributions as semipermanent features in some tropical regions and seasonal features elsewhere. However, model BLs are generally too thin in tropical regions and too thick in higher latitudes. BLs' ability to insulate the ocean surface from entrainment of cold thermocline water is most apparent in the tropics. Thus, E3SMv1s BL thickness biases most affect entrainment here. Tropical BLT biases appear driven by atmosphere biases, mainly through the effect of precipitation minus evaporation on mixed layer depth. At higher latitudes BL thickness biases are dominated by thermocline depth errors related to ocean circulation and vertical mixing.

Mean velocity, spreading and entrainment characteristics of weak bubble plumes in unstratified and stationary water

In this paper we present an experimental and theoretical study of weak bubble plumes in unstratified and stationary water. We define a weak bubble plume as one that spreads slower than the linear rate of a classic plume. This work focuses on the characteristics of the mean flow in the plume, including centreline velocity, plume spreading and entrainment of ambient water. A new theory based on diffusive spreading instead of an entrainment hypothesis is used to describe the lateral spreading of the bubbles and the associated plume. The new theory is supported by the experimental data. With the measured data of liquid volume fluxes and the new theory, we conclude that the weak bubble plume is a decreasing entrainment process, with the entrainment coefficient $\unicode[STIX]{x1D6FC}$ in the weak bubble plume decreasing with height $z$, following $\unicode[STIX]{x1D6FC}\sim z^{-1/2}$, and taking on values much smaller than those in a classic bubble plume. An additional non-dimensional diffusion coefficient, $\hat{E_{t}}\sim E_{t}U_{s}^{2}/B_{0}$, is also needed to describe the evolution of the volume and kinematic momentum fluxes for the mean flow in the weak bubble plume. Here, $E_{t}$ is the effective turbulent diffusion coefficient, $U_{s}$ is the terminal rise velocity of the bubbles, and $B_{0}$ is the kinematic buoyancy flux of the source. Finally, we provide a unified framework for the mean flow characteristics, including volume flux, momentum flux and plume spreading for the classic and weak bubble plumes, which also provides insight on the transition from classic to weak bubble plume behaviour.

Fe sources and transport from the Antarctic Peninsula shelf to the southern Scotia Sea

The Antarctic Peninsula (AP) shelf is an important source of dissolved iron (Fe) to the upper ocean in the southern Scotia Sea, one of the most productive regions of the Southern Ocean. Here we present results from a four-year (2003-2006) numerical simulation using a regional coupled physical-biogeochemical model to assess the Fe sources and transport on the AP shelf and toward the southern Scotia Sea. The model was validated with a suite of data derived from in situ surveys and remote sensing. Model results indicate that sediments in the AP shelf and the South Orkney Plateau (SOP) provide the dominant source of Fe to the upper 500 m in the southern Scotia Sea. Additional Fe inputs to the region are associated with the Antarctic Circumpolar Current (ACC) and the northern limb of the Weddell Gyre, deep-ocean sediment sources, dust deposition, and icebergs. Fe on the AP shelf originates primarily from sediments on the relatively shallow inner shelf and is directly injected into the water column and subsequently transported toward Elephant Island by the confluent shelf currents. Off-shelf Fe export is primarily through entrainment of shelf waters by the ACC’s Southern Boundary frontal jet along the northern edge of the AP shelf, the Hesperides Trough, and the SOP shelf. About 70% of the off-shelf export takes place below the surface mixed layer, and is subsequently re-supplied to the euphotic zone through vertical mixing, mainly during austral fall and winter. The exported shelf-derived Fe is then advected downstream by the ACC and Weddell Gyre and spread over the southern and eastern Scotia Seas. Taken together, shelf Fe export witin top 500 m meets nearly all of the Fe demand of phytoplankton photosynthesis in the southern Scotia Sea. Waters with elevated Fe concentrations in the Scotia Sea are largely restricted to south of the Southern ACC Front.

The Effects of Water Flow Rate On The Air Entrainment Process From Vertical Plunging Water Jet With Downcomer

This study is intended to research the characteristics of bubbles produced by the phenomenon of air entrainment. This phenomenon is used in the Jameson cell, a flotation system used to separate valuable minerals from impurities. This process has proven successful in providing high separation efficiency. However, the characteristics of hydrodynamic bubbles and water entrainment have not been widely described. Research was set up in the form of a water loop consisting of a pump, flow meter, nozzle, downcomer, and water box. Data collection is done by using a digital camera and a high-speed digital video camera with a backlighting system. The resulting image is processed with software to obtain quantitative data. Results indicate that increasing the water flow rate also causes the ratio of air to water to increase (air entrainment).Air Entrainment is natural phenomenon air carried out as a result of a fluid jet stream. Mass transfer deepens penetration because a high jet speed produces bubbles and tends to force those bubbles towards the flow. Increasing the water flow rate will expand the surface roughness to produce more bubbles. Thus, it can be concluded that there are three phenomena that will affect whether or not the discharge will increase the air entrainment process: depth of bubble penetration, area of bubble dispersion, and air entrainment rate.

Experimental Analysis on the Water Entrainment of Turbidity Current over a Mobile Bed

Experimental Analysis on the Water Entrainment of Turbidity Current over a Mobile Bed

Winter Extratropical Cyclones as a Potential Driver of a Long‐Term Decline of Bacterial Production in the Sargasso Sea Near Bermuda

AbstractAnalysis of plankton data sets from the Bermuda Atlantic Time‐series Study site (31°40′N, 64°10′W) demonstrates a long‐term decreasing trend of marine heterotrophic bacterial production (BP) in winter (November–February) over the past 24 years (1989–2012). We propose winter extratropical cyclones (storms) as a potential driver of the observed BP trend. Stormy winters were characterized by significantly enhanced phytoplankton biomass and productivity as well as organic carbon pools, as storms induce deeper convective mixing and increased nitrate fluxes into upper mixed‐layer. Counter to expectation, BP was lower by 24% due to storm‐mediated entrainment of cold water into upper mixed‐layer. The frequency of storm‐impacted months has increased over time, driven by a decline of the North Atlantic Oscillation (NAO) index. Our findings suggest that a NAO‐induced southward shift of winter storm tracks contributes to the decreasing BP trend, via temperature‐dependent decoupling of bacteria from phytoplankton, ultimately impacting carbon fluxes and cycling near Bermuda.

Upper-Ocean Response to the Super Tropical Cyclone Phailin (2013) over the Freshwater Region of the Bay of Bengal

AbstractThis study investigates the impact of salinity stratification on the upper-ocean response to a category 5 tropical cyclone, Phailin, that crossed the northern Bay of Bengal (BOB) from 8 to 13 October 2013. A drastic increase of up to 5.0 psu in sea surface salinity (SSS) was observed after Phailin’s passage, whereas a weak drop of below 0.5°C was observed in sea surface temperature (SST). Rightward biases were apparent in surface current and SSS but not evident in SST. Phailin-induced SST variations can be divided into the warming and cooling stages, corresponding to the existence of the thick barrier layer (BL) and temperature inversion before and erosion after Phailin’s passage, respectively. During the warming stage, SST increased due to strong entrainment of warmer water from the BL, which overcame the cooling induced by surface heat fluxes and horizontal advection. During the cooling stage, the entrainment and upwelling dominated the SST decrease. The preexistence of the BL, which reduced entrainment cooling by ~1.09°C day−1, significantly weakened the overall Phailin-induced SST cooling. The Hybrid Coordinate Ocean Model (HYCOM) experiments confirm the crucial roles of entrainment and upwelling in the Phailin-induced dramatic SSS increase and weak SST decrease. Analyses of upper-ocean stratification associated with 16 super TCs that occurred in the BOB during 1980–2015 show that intensifications of 13 TCs were associated with a thick isothermal layer, and 5 out of the 13 were associated with a thick BL. The calculation of TC intensity with and without considering subsurface temperature demonstrates the importance of large upper-ocean heat storage in TC growth.

Potential erosion capacity of gravity currents created by changing initial conditions

Abstract. We investigate to what extent the initial conditions (in terms of buoyancy and geometry) of saline gravity currents flowing over a horizontal bottom influence their runout and entrainment capacity. In particular, to what extent the effect of the introduction of an inclined channel reach, just upstream from the lock gate, influences the hydrodynamics of gravity currents and consequently its potential erosion capacity is still an open question. The investigation presented herein focuses on the unknown effects of an inclined lock on the geometry of the current, on the streamwise velocity, on bed shear stress, and on the mechanisms of entrainment and mass exchange. Gravity currents were reproduced in the laboratory through the lock-exchange technique, and systematic tests were performed with different initial densities, combined with five initial volumes of release on horizontal and sloped locks. The inclination of the upstream reach of the channel (the lock) was varied from 0 % to 16 %, while the lock length was reduced by up to 1∕4 of the initial reference case. We observed that the shape of the current is modified due to the enhanced entrainment of ambient water, which is the region of the current in which this happens most. A counterintuitive relation between slope and mean streamwise velocity was found, supporting previous findings that hypothesized that gravity currents flowing down small slopes experience an initial acceleration followed by a deceleration. For the steepest slope tested, two opposite mechanisms of mass exchange are identified and discussed, i.e., the current entrainment of water from the upper surface due to the enhanced friction at the interface and the head feeding by a rear-fed current. The bed shear stress and the corresponding potential erosion capacity are discussed, giving insights into the geomorphological implications of natural gravity currents caused in different topographic settings.

Numerical investigation of terminal residual thickness of the water film and surface flow in the entrainment region in Marangoni drying

Marangoni drying is widely employed in integrated circuit manufacturing to remove water from the wafer surface. The organic vapor blown at the meniscus induces a strong Marangoni effect, which strips the entrained water film induced by the wafer’s withdrawal from a deionization water bath, and realizes ultra-clean drying of the wafer. The present work implements a numerical investigation of the terminal residual film and free surface flow in the water entrainment region during Marangoni drying based on a trans-scale model containing the thin water film, meniscus, and bulk region. The results show that the terminal thickness of the water film is 30 nm, which approximately agrees with the previous experimental and numerical results. In addition, the stagnation point moves away from the thin film region compared with that of wafer withdrawn in the absence of the Marangoni effect. The magnitude of free surface velocity in the thin film region after drying depends only on the withdrawal velocity, and is irrelevant to the Marangoni number. The analyses of the entrained water film thinning and surface flow provide useful guidance about controlling the drying process.

Constraining Evaporation Rates Based on Large‐Scale Sea Surface Transects of Salinity or Isotopic Compositions

AbstractA Lagrangian model is constructed for a surface column of initial height h(0) that propagates at an average speed u and is subject to excess (i.e., net) evaporation of q m/year. It is shown that these parameters combine to form an evaporation length, L = uh(0)/q, which provides an estimate for the distance the column must travel before evaporating completely. While these changes in the surface water level due to evaporation are compensated by entrainment of water into the overall column, the changes in either near‐surface salinity or isotopic compositions are retained and can be measured. Observations of surface salinity and isotopic compositions of δ18O and δD along 1,000‐ to 3,500‐km long transects are used to estimate values of L in the Red Sea, Mediterranean Sea, Indian Ocean, and Gulf Stream. The variations of salinity, δ18O and δD in all four basins are linear. As anticipated, the estimated value of L is smallest in the slowly moving and arid Red Sea and is greatest in the fast‐moving Gulf Stream.

Near‐Field Model for a High‐Momentum Negatively Buoyant Line Source Within a Three‐Dimensional Hydrostatic Lake Model

AbstractThis work is motivated by a case study in which cooling water effluent is discharged from a line source diffuser onto the shallow southern shelf of a large lake. The effluent is discharged with high momentum to the north, and the plume influences the surrounding flow for hundreds of meters before becoming a passive tracer. We plan to use a three‐dimensional hydrodynamic model to examine the impact of this effluent on water residence time within the southern shelf, but in order to accomplish this task, we require a method for incorporating the effluent‐driven flow into the hydrodynamic model, which by default does not predict the correct rate of spreading of the plume or the correct rate of lateral entrainment of ambient water. In this paper we develop a method for incorporating the effects of high‐momentum line source discharges into any three‐dimensional or two‐dimensional (depth‐averaged) hydrodynamic model that employs a horizontal eddy viscosity and diffusivity. Our approach is to modify horizontal eddy viscosity and diffusivity to enforce the correct rate of lateral entrainment of the plume. We validate the new method using three test cases relevant to our case study: a neutrally buoyant jet, a cylindrical gravity current, and a negatively buoyant jet.

Centrifuge Study on the Submarine Mudflows

Submarine mudflow is one of the most serious geohazards. A new Mini-drum Centrifuge apparatus was developed for use. A series of experiments were performed to investigate the flow behaviour of submarine mud, based on the following aspects: (i) side view; (ii) change in basal pore pressure; (iii) change in water content and (iv) change in the Reynolds number. In this series of centrifuge tests in the subaqueous environment, the submarine mudflow transformed from a highly concentrated mudflow to a turbulent current at a water content of 113% to 121%. Accordingly, the Reynolds number Re, as the threshold of turbulent flow was about 1,150–2,250. Based on the water content measured before and after each test, the water entrainment during the flowing process was minimal for laminar flows, but large for turbulent flows. In addition, the evolution of basal pore pressure was found to be closely related to the regime of submarine mudflow: laminar or turbulent.

A TWO-LAYER NON-HYDROSTATIC LANDSLIDE MODEL FOR TSUNAMI GENERATION ON IRREGULAR BATHYMETRY

In this study, a two-layer landslide model is presented for investigating submarine landslides and generated waves that propagate over irregular bathymetry. The landslide is described as either a mudflow or a fully saturated granular flow, which are distinguished by using different rheological closure based on physical principles. Depth-averaged governing equations for the landslide are derived in a regular Cartesian coordinate system, and take into account the effect of vertical acceleration and interface traction from the upper-layer water. In addition, sediment erosion from basal boundary and water entrainment are also considered. Tsunami waves generated by the landslide are simulated by the three-dimensional non-hydrostatic wave model NHWAVE (Ma et al., 2012). The governing equations for both the lower-layer slide and the upper-layer water body are solved using a Godunov-type finite volume TVD scheme in space and a Strong Stability-Preserving (SSP) Runge- Kutta scheme in time.

Role of different types of clay in the floatability of coal: Induction time and bubble-particle attachment kinetics analysis

The presence of clay minerals presents a great challenge to fine coal and mineral flotation due to the well-known phenomena of mechanical coating and water entrainment. In this paper, we study the effect of two different types of clay, kaolinite and montmorillonite, on the floatability of coal in de-ionized water (without flotation reagents) based on induction time and bubble-particle attachment kinetics. The bubble-particle attachment angle (BPAA) as a function of time was used as the kinetics raw data. We demonstrated how the presence of montmorillonite was detrimental to the floatability of coal in de-ionized water. This was not the case with kaolinite. The induction time and BPAA kinetics of coal particles only changed slightly after conditioning together with kaolinite. In contrast, the induction time increased considerably from 8.73 ms to 83.25 ms when montmorillonite was added. In addition, both the maximum BPAA and BPAA kinetics constant decreased, and hydrophilic montmorillonite coating occurred during the conditioning stage. These hydrophilic sites on the coal surface prevent the thinning-rupture of the water film between bubble and coal particles and thus decrease the floatability and recovery.

An innovative booster for dynamic installation of OMNI-Max anchors in clay: Physical modelling

An innovative booster concept is proposed to increase both the kinetic and potential energies and hence to increase the penetration depth of the OMNI-Max anchor within the clayey soil with high strength gradient. This paper carried out 1g model tests to investigate the working efficiency of the booster on the anchor penetration depth both in normally consolidated (NC) and lightly over-consolidated (LOC) soils. The similarity relationships of the anchor dynamic penetration within the soil were first deduced, and then a series of dynamic penetration tests were performed to investigate the parameters that influence the anchor penetration depth. The parameters include the soil strength characterisation, impact velocity, booster weight, inclined impacting angle, and water drag and entrainment effects. The anchor dynamic penetration process within the soil was measured by a micro-electromechanical systems (MEMS) accelerometer, through which the soil strain-rate effect and the hydrodynamic aspects including the water drag force and potential water entrainment along the anchor-soil interface were analysed. Finally, energy-based and force-based prediction models were put forward to estimate the penetration depths of OMNI-Max anchors and hybrid anchors (i.e. an OMNI-Max anchor with a booster).

Surface-wave velocity measurements of shear stiffness of moving earthflows

Earthflows are a flow-like movement of plastic clayey soils characterized by long periods of slow motion (at rates averaging a few meters per year or less) alternated with short periods of rapid surges at high velocity (up to meters per hour). During rapid surges, most earthflows move over a long distance with a fluid-like behavior. Although the generation of flow-type failures is an important issue for hazard assessment, our knowledge is limited by the difficulty of monitoring the process in the field. This has led to different explanations for rapid earthflows including high pore–pressure generation along the basal slip surface, pervasive shearing, or material fluidization. One key question is whether or not earthflows can fluidize through remolding and water entrainment. If this occurs, the material can change from plastic to fluid as the soil moisture increases, causing the landslide to move as a viscous flow; if not, the material remains in a plastic state and, as suggested by many authors, the flow-like morphology shown by earthflows would result by distributed internal shears rather than real mass flow. In this study, we provide the first answer to this question by measuring the shear stiffness of four large active earthflows in the Northern Apennines of Italy. Shear stiffness was measured using two geophysical techniques, the multichannel analysis of surface waves (MASW) and the passive refraction microtremors (ReMi). Measurements were carried out just a few days after the mobilization of the landslides and repeated in the following 2–3 years to evaluate the change of elastic properties with time. Field data show that soon after the mobilization, earthflows are characterized by very low values of shear stiffness (about 5–15 MPa), typical of soft clay soils with the high-void ratio. Shear stiffness then increases 4–5 times in the following months (up to 40–60 MPa) as the earthflows slow down and the material consolidates. These data indicate that during a rapid movement, earthflows undergo a dramatic increase of porosity and water content that probably drive the transition from a solid to a fluid-like state.

Limited influence of basalt weathering inputs on the seawater neodymium isotope composition of the northern Iceland Basin

Radiogenic neodymium (Nd) isotopes have been widely used as a proxy for tracing present and past water masses and ocean circulation, yet relatively few data exist for seawater from the important deep water formation area around Iceland. We have analyzed the dissolved seawater Nd isotope compositions (expressed as ƐNd) of 71 seawater samples, as well as Nd concentrations [Nd] of 38 seawater samples, collected at full water column profiles from 18 stations in the shelf area off the southern coast of Iceland. The goal of this work was to determine to what extent weathering inputs from Icelandic basalts, which are characterized by a distinctly radiogenic ƐNd signature within the North Atlantic, contribute to the Nd isotope and concentration signatures of water masses in the northern Iceland Basin.Radiogenic ƐNd values of up to −3.5 and elevated concentrations of up to 21 pmol/kg compared to nearby open ocean sites were found in surface waters at shallow sites closest to shore and to river mouths of Iceland. This documents partial dissolution of highly radiogenic basaltic particles, which are transported northwards by the coastal currents. A comparable signal is not observed, however, in offshore surface waters likely as a result of the advection of surface currents mainly directed onshore, thus isolating these sites from Icelandic weathering contributions. The dominance of Subpolar Mode Waters and Intermediate Water unaffected by Icelandic contributions in the offshore study area is supported by unradiogenic ƐNd signatures between −15 and −12.In agreement with hydrographic data, highly radiogenic bottom waters at one site on the Iceland-Faroe Ridge (ƐNd = −7.5) reveal the presence of almost pure Iceland Scotland Overflow Water (ISOW) near its formation site further to the east. In bottom waters of all deeper offshore sites, the combination of depleted Nd concentrations and similar ƐNd values (averaging at ≃−11.75 for the R/V Poseidon data and ≃−11 for the R/V Thalassa data) confirms the rapid entrainment of Atlantic mid-depth and deep waters into the overflow waters, which is accompanied by near bottom Nd removal via particle scavenging. Overall, our findings demonstrate that at present, apart from the radiogenic isotope signature of ISOW itself, the direct contribution of radiogenic Nd originating from weathering of Iceland basalts to the water column of the Iceland Basin is limited. This supports the reliable application of ƐNd values to trace changes in the mixing of open North Atlantic water masses (including ISOW).This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Frontal Subduction of the Mid‐Atlantic Bight Shelf Water at the Onshore Edge of a Warm‐Core Ring

AbstractThis work studies the subduction of the shelf water along the onshore edge of a warm‐core ring that impinges on the edge of the Mid‐Atlantic Bight continental shelf. The dynamical analysis is based on observations by satellites and from the Ocean Observatories Initiative Pioneer Array observatory as well as idealized numerical model simulations. They together show that frontogenesis‐induced submesoscale frontal subduction with order‐one Rossby and Froude numbers occurs on the onshore edge of the ring. The subduction flow results from the onshore migration of the warm‐core ring that intensifies the density front on the interface of the ring and shelf waters. The subduction is a part of the cross‐front secondary circulation trying to relax the intensifying density front. The dramatically different physical and biogeochemical properties of the ring and shelf waters provide a great opportunity to visualize the subduction phenomenon. Entrained by the ring‐edge current, the subducted shelf water is subsequently transported offshore below a surface layer of ring water and alongside of the surface‐visible shelf‐water streamer. It explains the historical observations of isolated subsurface packets of shelf water along the ring periphery in the slope sea. Model‐based estimate suggests that this type of subduction‐associated subsurface cross‐shelfbreak transport of the shelf water could be substantial relative to other major forms of shelfbreak water exchange. This study also proposes that outward spreading of the ring‐edge front by the frontal subduction may facilitate entrainment of the shelf water by the ring‐edge current and enhances the shelf‐water streamer transport at the shelf edge.

Observations of the Intermediate Water Exchange Between the South China Sea and the Pacific Ocean Deduced From Transient Tracer Measurements

AbstractThe South China Sea (SCS) is the largest semienclosed marginal sea in the western Pacific (WP) and connects to the west Pacific through the Luzon Strait (LU). In this study, we use the observation of transient tracer chlorofluorocarbon‐12 (CFC‐12) to calculate the ventilation time scales of the SCS, LU, and WP. The CFC‐12 and oxygen data are used together to identify the sandwiched structure vertically of the flows across the LU. The CFC‐12 and oxygen distributions reveal a pronounced decrease westward across the LU and a slight decrease southward in the transport of the SCS. The mean age gradient of the salinity minimum (Smin) water between the WP and the northern SCS could be a consequence of intensive mixing and entrainment of the inflow water from the WP. An expected difference in age between the LU and SSCS is verified to reflect the transit time for the given water layers in the SCS. Thus, a mean transit time of 77 ± 20 years is estimated for the intermediate water in the SCS interior.