Entrainment Of Water Research Articles

The role played by tropical cyclones-induced freshwater flux forcing in the upper-ocean responses: A case for Typhoon Yutu (2018)

Tropical cyclones (TCs) produce heavy precipitation and provide excessive freshwater flux (FWF) to the ocean, which alters surface salinity and upper-ocean stratification, leading to changes in ocean physical processes. At present, roles played by TC-induced FWF (FWFTC) forcing in the upper-ocean responses have received less attention than those by TC-induced wind stress (τTC) forcing. An ocean model is used to investigate the role played by FWFTC forcing in upper-ocean responses in the Pacific; several numerical experiments and observation-based analyses are performed, focusing on Typhoon Yutu (2018) . The FWFTC and τTC components are explicitly extracted from atmospheric reanalysis fields and are then taken into account or not in model sensitivity experiments. As expected, the FWFTC forcing acts to freshen the ocean surface, enhance the ocean stratification and reduce the mixing and entrainment of subsurface cold water into the mixed layer (ML), leading to potentially favoring surface warming; this FWF effect is opposite to that induced by τTC forcing (i.e., a cooling effect on sea surface temperature (SST)). The combined effects of FWFTC and τTC forcings lead to local ocean responses to Typhoon Yutu that are characterized by surface cooling and salting, which are dominated by the τTC forcing and are also modulated by the FWFTC forcing, depending on their relative magnitudes. When FWFTC forcing reaches a certain intensity and the τTC forcing is weak, however, the warming effect on SST due to FWFTC forcing can even exceed the cooling effect due to τTC forcing. When FWFTC forcing is relatively stronger than τTC forcing (e.g., TC-induced precipitation >10.37mmh−1 and τTC∼0.14Nm−2), the warming effect on SST due to FWFTC forcing can even exceed the cooling effect due to τTC forcing. Meanwhile, with the combination of τTC forcing, the FWFTC forcing can also promote SST cooling induced by the τTC forcing, which is dominated by mixing and entrainment. In this case, the FWFTC forcing increases surface heat loss from the ocean due to enhanced stratification, which contributes to τTC-induced vertical mixing and surface cooling. Further case studies are needed to thoroughly evaluate FWFTC-related effects on the upper ocean.

Projected Impacts of Antarctic Meltwater Anomalies over the Twenty-First Century

Abstract Antarctic margin and Southern Ocean surface freshening has been observed in recent decades and is projected to continue over the twenty-first century. Surface freshening due to precipitation and sea ice changes is represented in coupled climate models; however, Antarctic ice sheet/shelf meltwater contributions are not. Because Antarctic melting is projected to accelerate over the twenty-first century, this constitutes a fundamental shortcoming in present-day projections of high-latitude climate. Southern Ocean surface freshening has been shown to cause surface cooling by reducing both ocean convection and the entrainment of warm subsurface waters to the surface. Over the twenty-first century, Antarctic meltwater is expected to alter the pattern of projected surface warming as well as having other climatic effects. However, there remains considerable uncertainty in projected Antarctic meltwater amounts, and previous findings could be model dependent. Here, we use the ACCESS-ESM1.5 coupled model to investigate global climate responses to low and high Antarctic meltwater additions over the twenty-first century under a high-emissions climate scenario. Our high-meltwater simulations produce anomalous surface cooling, increased Antarctic sea ice, subsurface ocean warming, and hemispheric differences in precipitation. Our low-meltwater simulations suggest that the magnitude of surface temperature and Antarctic sea ice responses is strongly dependent on the applied meltwater amount. Taken together, these findings highlight the importance of constraining projections of Antarctic ice sheet/shelf melt to better project global surface climate changes over the twenty-first century. Significance Statement Antarctic ice sheets and shelves are melting, adding meltwater to the Southern Ocean and changing the ocean circulation. Antarctic meltwater stratifies the upper ocean, resulting in cooling of the surface Southern Ocean but warming at depth that could accelerate ice shelf melting. Coupled climate models used to project twenty-first-century climate do not represent ice sheets or shelves, neglecting important climate impacts. Here we conduct meltwater simulations with a coupled climate model and find that the magnitude of climate responses is strongly dependent on the applied meltwater amount. This highlights 1) the importance of constraining Antarctic meltwater projections to better project global climate over the twenty-first century and 2) that it is important that Antarctic meltwater be represented in future-generation coupled climate models.

Ice Shelf Basal Melt Sensitivity to Tide‐Induced Mixing Based on the Theory of Subglacial Plumes

AbstractTidal currents are known to influence basal melting of Antarctic ice shelves through two types of mechanisms: local processes taking place within the boundary current adjacent to the ice shelf‐ocean interface and far‐field processes influencing the properties of water masses within the cavity. The separate effects of these processes are poorly understood, limiting our ability to parameterize tide‐driven ice shelf‐ocean interactions. Here we focus on the small‐scale processes within the boundary current. We apply a one‐dimensional plume model to a range of ice base geometries characteristic of Antarctic ice shelves to study the sensitivity of basal melt rates to different representations of tide‐driven turbulent mixing processes. Our simulations demonstrate that tides can either increase or decrease melt rates depending on the approach chosen to parameterize entrainment of ambient water into the turbulent plume layer, a process not yet well constrained by observations. A theoretical assessment based on an analogy with tidal bottom boundary layers suggests that tide‐driven shear at the ice shelf‐ocean interface enhances mixing through the pycnocline. Under this assumption our simulations predict a tide‐induced increase in melt and freeze rates along the base of the ice shelf, with the strongest plume path‐integrated effects for cold cavities (up to +400% in the realistic set up). An approximation is provided to account for this response in basal melt rate parameterizations that neglect the effect of tide‐induced turbulent mixing.

Comparison of structure and turbulent mixing between lateral and leading-edge river plume fronts: Microstructure observations from a T-REMUS AUV

The energetic frontal region of the tidally pulsed Connecticut River plume was sampled with the T-REMUS Autonomous Underwater Vehicle (AUV) which navigated ten cross-front transects at various depths. Synchronous and high resolution hydrographic, microstructure, velocity and backscatter observations allowed the creation of detailed views of frontal structure and turbulent mixing in the reference frame of the propagating front. The front was defined by a sharp horizontal density gradient of ∼18.5 kg m−4 across only 0.7 m in the horizontal, and strong downwelling velocities of 0.17 m/s. Three successive zones were consistently identified. The first O (10 m) wide downwelling zone in which buoyant plume water was forced downward, thereby forming a highly energetic frontal head that extended down to 6 m depth and had TKE dissipation rates (ε) of order 10−3 m2/s3. Beyond the downwelling zone to O (100 m), a transition zone was observed, characterized by substantial entrainment of ambient water. This zone had high amplitude and high frequency density anomaly (σθ) variability consistent with elevated but decreasing turbulent kinetic energy dissipation rates (ε) which ranged between 10−4 and 10−5 m2/s3. Beyond O (100 m) from the front, stable stratification, high Ri values, and low Reb values, suppressed the growth of instabilities and diminished turbulent mixing. In this stable interior region the plume base shoaled to 2 m depth and ε decreased further, from 10−5 to 10−7 m2/s3. In ambient Long Island Sound waters, ε ranged between 10−6 and 10−7 m2/s3. The cross-front distributions of σθ and ε did not change substantially as the front transformed from a lateral boundary, where plume flow was primarily in the along-front direction, to one that was at the leading edge of the plume, where plume flow was mostly in the cross-front direction. In both cases, a significant convergence was present at the front due to strong ambient tidal velocities. The observed ε values and those from four previous studies of large and mid-sized plumes were nondimensionalized using the velocity and length scales associated with the downwelling region at the front. The effective normalization suggests that local dynamics primarily control ε in the near-frontal region.

Eddy‐Freshwater Interaction Using Regional Ocean Modeling System in the Bay of Bengal

AbstractEddy‐freshwater interaction is studied in the north Bay of Bengal (BoB) with a high‐resolution simulation using the Regional Ocean Modeling System. Following observations, the model simulates the trapping and homogenization of river water by a cyclonic mesoscale eddy on a submonthly time scale from October to November 2015. As fresh river water is trapped in the eddy, it is characterized by strong vertical and lateral gradients in salinity. Within a few weeks, these gradients relax along with the progressive homogenization of freshwater within the eddy. A mixed‐layer salinity budget shows the importance of ageostrophic vertical advection in addition to lateral advection during the evolution of salinity within the eddy. An analysis of the eddy kinetic energy (EKE) budget in the upper ocean indicates the development of barotropic and baroclinic instabilities. The vertical profiles of EKE conversion terms reveal that the surface freshwater was involved in the evolution of baroclinic instability within the mixed layer. In addition, an eddy available potential energy (EPE) budget shows that the entrainment of the river water raises the EPE, which is due to an increase in lateral salinity gradients across the eddy during the trapping event. Subsequently, the salinity homogenization leads to a decrease in the EPE, and its rate of decay is modulated by a correlation between surface buoyancy fluxes and density anomalies. Finally, reanalysis data show similar trapping and homogenization of freshwater events across multiple years, highlighting the importance of this mechanism on the subseasonal time scale in the BoB.

Projected Atlantic overturning slow-down is to be compensated by a strengthened South Atlantic subtropical gyre

The upper limb of the Atlantic meridional overturning circulation brings shallow interocean contributions to replenish the North Atlantic export of deepwaters. It is primarily formed in the southern South Atlantic where the converging entrainment of Pacific and Indian Ocean waters meet and incorporate into the South Atlantic subtropical gyre. Here, we use Community Earth System Model 1 Large Ensemble simulation results along 1920–2100 to investigate the response of the Atlantic meridional overturning circulation upper limb and the South Atlantic subtropical gyre to future human-induced climate warming under business-as-usual greenhouse gas emissions. In terms of flow redistribution, we find that the Atlantic meridional overturning circulation upper limb weakens not because less waters are being imported from the Pacific and Indian basins — but because waters are being mostly directed to recirculate in the southwestern portion of a distorted South Atlantic subtropical gyre, turning back southward after reaching the South Atlantic western boundary.

Observational evidence of salt finger in the diurnal thermocline

Due to strong turbulent mixing, the ocean surface boundary layer region is generally not conducive to double diffusion. However, vertical microstructure profiles observations in the northeastern Arabian Sea during May 2019 imply the formation of salt fingers in the diurnal thermocline (DT) region during the daytime. In the DT layer, conditions are favorable for salt fingering: Turner angle values are between 50 and 55° with both temperature and salinity decreasing with depth; shear-driven mixing is weak with a turbulent Reynolds number of about 30. The presence of salt fingering in the DT is confirmed by the presence of staircase-like structures with step sizes larger than the Ozmidov length and by the dissipation ratio that is larger than the mixing coefficient. The unusual daytime salinity maximum in the mixed layer that supports salt fingering is primarily due to a daytime reduction in vertical entrainment of fresh water along with minor contributions from evaporation and horizontal advection and a significant contribution from detrainment processes.

Continued glacial retreat linked to changing macronutrient supply along the West Antarctic Peninsula

At the West Antarctic Peninsula (WAP), continued atmospheric and oceanic warming is causing significant physical and biogeochemical changes to glaciers and the marine environment. We compare sediment sources and drivers of macronutrient distributions at two bays along the WAP during austral summer 2020, using radioactive radium and stable oxygen isotopes to trace sedimentary influences and quantify different freshwater inputs. In the Ryder Bay, where the Sheldon Glacier is marine-terminating, radium activities at the sediment-water interface indicate considerable benthic mixing. Using radium isotope activity gradients to resolve radium and macronutrient fluxes, we find buoyant meltwater proximal to the glacier drives vigorous mixing of sediment and entrainment of macronutrient deep waters, on the order of 2.0 × 105 mol d−1 for nitrate. Conversely, in the Marian Cove, where the Fourcade Glacier terminates on land, low salinities and oxygen isotopes indicate a meltwater-rich surface layer <1 m thick and rich in sediment, and strong vertical mixing to the seafloor. A continued shift to land-terminating glaciers along the WAP may have a significant impact upon nutrient and sediment supply to the euphotic zone, with impacts upon primary productivity and carbon uptake efficiency. The future of primary production, carbon uptake, and food web dynamics is therefore linked to glacier retreat dynamics in the many fjords along the WAP.

Surface ocean carbon budget in the 2017 south Georgia diatom bloom: Observations and validation of profiling biogeochemical argo floats

Estimates of the partial pressure of CO2 (pCO2) derived from biogeochemical Argo floats have the potential to improve our knowledge of the highly variable and partially observed Southern Ocean carbon sink through sampling at improved temporal and spatial resolution. Here we use the data from six biogeochemical Argo floats to characterise near-surface dissolved inorganic carbon (DIC) concentrations and fluxes at the site of an intense (Chl-a >3 mg m−3) mesoscale diatom bloom situated northwest of South Georgia. Concurrently, we provide independent analysis and validation of the methodology used by the Southern Ocean Carbon and Climate Observational and Modelling (SOCCOM) project for deriving surface pCO2 from float-based pH and oxygen measurements. We compare the float observations with co-located ship data from bottle samples over a month-long period. When compared to data sampled within 24 h and 25 km of each float profile, we find good agreement with a mean offset of −0.005 ± 0.018 (1σ) between float pH and bottle-derived pH. This translates to comparable pCO2 estimates between ship measurements and floats with a mean difference of 2.6 ± 12.8 (1σ) μatm, providing support for the use of biogeochemical Argo float data to supplement shipboard pCO2 measurements in the Southern Ocean. Based on float-derived pCO2 we calculate a sizeable local flux of CO2 of 24 ± 7 mmol C m−2 d−1 (over a 27-day period) from the atmosphere into the surface mixed layer, driven by a large air-sea pCO2 gradient and strong but variable winds. Despite the considerable air-sea flux, the local mixed layer carbon budget appears to be dominated by entrainment and detrainment of carbon-rich waters into and out of the mixed layer. However, given the large uncertainties associated with these fluxes and the significant challenges associated with closing the mixed layer budget, further research is required to refine float-based mixed layer DIC fluxes.

Antarctica Slope Front bifurcation eddy: A stationary feature influencing CO2 dynamics in the northern Antarctic Peninsula

The Southern Ocean is a key region for analyzing environmental drivers that regulate sea-air CO2 exchanges. These CO2 fluxes are influenced by several mesoscale structures, such as meanders, eddies and other mechanisms responsible for energy dissipation. Aiming to better understand sea-air CO2 dynamics in the northern Antarctica Peninsula, we investigated an anticyclonic stationary eddy located south of Clarence Island, in the eastern basin of Bransfield Strait – named the Antarctica Slope Front bifurcation (ASFb) eddy. Physical, chemical and biological data were sampled, and remote sensing measurements taken, in the region during late summer conditions in February 2020. The eddy’s core consisted of cold (0.31 °C), salty (34.38) and carbon-rich (2247 μmol kg−1) waters with dissolved oxygen depletion (337 μmol kg−1). The core retains a mixture of local surface waters with waters derived from Circumpolar Deep Water (i.e., Warm Deep Water from the Weddell Sea and modified Circumpolar Deep Water from the Bransfield Strait) and Dense Shelf Water. The ASFb eddy acts as a CO2 outgassing structure that reaches a CO2 emission to the atmosphere of ∼1.5 mmol m−2 d–1 in the eddy’s core, mostly due to enhanced dissolved inorganic carbon (DIC). The results suggest that surface variation in DIC in the eddy’s core is modulated by (i) the entrainment of CO2-rich intermediate waters at ∼500 m, (ii) low primary productivity, associated with small phytoplankton cells such as cryptophytes and green flagellates, and (iii) respiration processes caused by heterotrophic organisms (i.e., zooplankton community). By providing a comprehensive view of these physical and biogeochemical properties of this stationary eddy, our results are key to adding new insights to a better understanding of the behavior of mesoscale features influencing sea-air CO2 exchanges in polar environments.

Effects of external injection of deionized water and water with impurities on Water-assisted flares

Water-assisted flares are used on offshore platforms and occasionally at downstream oil and gas facilities to reduce soot and oxides of nitrogen (NOx) emissions and flame luminosity. Typically, water-assisted flares inject water into the flame from outside the flare stack (external injection), and the water often contains high concentrations of impurities. Different configurations of water injection were explored using a range of water flow rates, atomizer angles, and radial displacements from the burner center to understand the effects of water flow rate and location of water entrainment into the flame on emissions. A range of sodium chloride (NaCl) concentrations was also tested as a surrogate species for the impurities contained in the water used at offshore and downstream oil and gas facilities. Flow visualization analysis was carried out, and the water droplet velocity field was used to discuss their motion transitioning from being dominated by their momentum to that of the buoyancy of the combustion products. This study used atmospheric turbulent non-premixed flames on a two-inch circular burner at a constant propane flow rate of 20 standard L/min. It was discovered that the atomizer’s angular displacement plays a significant role in black carbon (BC) and NOxemissions suppression, but the atomizer’s horizontal displacement does not affect NOxemission. Water directed very close to the base of the flame ensured water was present early in the combustion process relative to other external injected configurations and reduced emissions by a greater extent compared to when the water was directed higher in the flame. The buoyancy of the flame affects the velocity of the injected water, helps to redirect its flow in the flame, and increases its mixing with fuel and air. Large amounts of water droplets pass through the flame front and evaporate within the fuel side of the flame envelope. The flame color and luminosity were affected by an increased NaCl concentration and the different configurations of the injected water. Higher NaCl concentration in the water causes an increase in BC emissions and a slight decrease in NOx emissions relative to deionized water. The smaller NaCl concentration in the water does not affect NOxand BC emissions compared to deionized water.

Modulation of the seasonal cycle of the Antarctic sea ice extent by sea ice processes and feedbacks with the ocean and the atmosphere

Abstract. The seasonal cycle of the Antarctic sea ice extent is strongly asymmetric, with a relatively slow increase after the summer minimum followed by a more rapid decrease after the winter maximum. This cycle is intimately linked to the seasonal cycle of the insolation received at the top of the atmosphere, but sea ice processes as well as the exchanges with the atmosphere and ocean may also play a role. To quantify these contributions, a series of idealized sensitivity experiments have been performed with an eddy-permitting (1/4∘) NEMO-LIM3 (Nucleus for European Modelling of the Ocean–Louvain-la-Neuve sea ice model version 3) Southern Ocean configuration, including a representation of ice shelf cavities, in which the model was either driven by an atmospheric reanalysis or coupled to the COSMO-CLM2 regional atmospheric model. In those experiments, sea ice thermodynamics and dynamics as well as the exchanges with the ocean and atmosphere are strongly perturbed. This perturbation is achieved by modifying snow and ice thermal conductivities, the vertical mixing in the ocean top layers, the effect of freshwater uptake and release upon sea ice growth and melt, ice dynamics, and surface albedo. We find that the evolution of sea ice extent during the ice advance season is largely independent of the direct effect of the perturbation and appears thus mainly controlled by initial state in summer and subsequent insolation changes. In contrast, the melting rate varies strongly between the experiments during the retreat, in particular if the surface albedo or sea ice transport are modified, demonstrating a strong contribution of those elements to the evolution of ice coverage through spring and summer. As with the advance phase, the retreat is also influenced by conditions at the beginning of the melt season in September. Atmospheric feedbacks enhance the model winter ice extent response to any of the perturbed processes, and the enhancement is strongest when the albedo is modified. The response of sea ice volume and extent to changes in entrainment of subsurface warm waters to the ocean surface is also greatly amplified by the coupling with the atmosphere.

Multi-component avalanches for rock- and ice-falls to potential debris flow transition modelling

Rock/ice avalanches are complex gravitational flows involving three important physicalprocesses: entrainment, phase-changes, and flow transitions. The basic complexity of modelling rock/ice avalanches lies in the fact that the flow is a mixture of rock, ice, water, snow, soil sediments. The interaction of these components can lead to flow transitions which is difficult to model with simplified flow rheologies. For example, a flow initially composed of rocks and ice can transition into a long-runout debris flow, dependent on the entrainment of water, water-saturated sediments and/or snow. The dynamic behaviour of rock/ice avalanches is therefore highly difficult to predict because the flow mixture is dependent on the hydrological and geomorphological properties along the avalanche track. These properties can vary from year to year but even from season to season. The problem is intensified by the fact that frictional shearing can produce enough heat energy to melt ice and snow, leading to additional water in the flow. In this contribution, we present a new RAMMS module specifically designed to simulate single- and multicomponent avalanches of rock, ice, water, snow, and ice. The rheology of the flow is treated by using the concept of component activation energy. The model includes both snow and sediment/water/ice entrainment modules. A unique feature of the model is that it tracks the temperature of the rock, ice and water phases and therefore can treat phase changes. We apply the model on the Chamoli case-study to highlight the potential and present limitations of the model..

Haline Сontrol of Unusually Deep Winter Mixing in the Gulf of Maine Investigated Using a Regional Data‐Assimilative Model

AbstractAn unusually large positive salinity anomaly was observed across the eastern Gulf of Maine in winter 2017–2018. Buoy measurements in Jordan Basin found this anomaly extended down to at least 100 m, the deepest mixing observed in the past 19 years. Similarly, this is the strongest positive regional salt anomaly ever observed in sea surface salinity (SSS) satellite observations. To determine the source waters driving this event and to diagnose the relative importance of forcing processes, passive tracer adjoint sensitivity experiments are performed using a data assimilating version of the Regional Ocean Modeling System. The model suggests that northeastward Scotian Shelf wind anomalies is one factor in the dramatic decrease in freshwater transport to Jordan Basin, which leads to an early winter upper water column salinity surplus. This salinity change weakens the normally haline‐controlled vertical stratification across the eastern Gulf. Modeled upper ocean density and vertical diffusivity from 2007 to 2021 both show a maximum in January 2018. Winter 2017–2018 is the only period where the enhanced winter mixing extends below 100 m. Thus, anomalous vertical entrainment of saltier subsurface Gulf water is the major factor driving the extreme positive satellite‐observed SSS anomaly across the eastern Gulf including Jordan Basin. Other factors, including a modest increase in wind‐forced slope water transport, and positive fall 2017 salinity anomalies on the Scotian Shelf and Slope Sea, appear to play lesser roles in the observed salinification. The adjoint sensitivity analysis demonstrates its utility for back tracing transport pathways for periods of several months.

Effect of different mass proportions of fine slime on the flotation performance of coal

ABSTRACT The effect of mass proportions of fine slime on the flotation performance of coal was investigated in the present study. Generally, the presence of fine slime showed a negative effect on flotation, i.e. as the mass proportion of fine slime increased, both flotation yield and kinetics decreased, while ash content increased, which was attributed to slime coating and water entrainment. The sedimentation tests, adsorption amount measurements, and Cryo-SEM technique directly indicated the incidence of slime coating, which became more significant with the increase of slime mass proportion. Except for the higher collision efficiency between slime and coal particles, the increased pulp viscosity may also be the reason for the worse slime coating as scrubbing effect of turbulence was weakened. The froth drainage tests showed the occurrence of water entrainment was also correlated to slime mass ratio. However, the water recovery and froth stability showed an increase trend firstly and then a decrease. It may be attributed that high mass proportion of slime increased the pulp viscosity, leading to poor gas dispersion and froth stability.

Wintertime marine extreme temperature events modulate phytoplankton blooms in the North Pacific through subtropical mode water

Marine extreme temperature events (METs), including marine heatwaves (MHWs) and cold spells, have recently gained much attention owing to their vital influence on the marine ecosystem and social economy. Since METs can alter the upper ocean stratification and wintertime convective mixing in the northwestern North Pacific subtropical gyre (NPSG), their activities may modulate phytoplankton blooms by regulating entrainment of the subtropical mode water (STMW) with high NO. Furthermore, because STMW formed in the previous winter reemerges east of its formation site in the following winter, the METs activities imprinted in STMW affect phytoplankton blooms remote from its formation site. Here, we examined the relationship between the MET activities, STMW volume, and phytoplankton blooms using satellite observations and a data-assimilative coupled physical-biogeochemical model dataset. MET activities appearing in the STMW formation region during winter regulate the formation of STMW and the supply of NO from the subsurface, with the latter controlling the spring/autumn blooms in that region under NO-limited conditions. Subsequently, this water mass is transported eastward in the subsurface within the northern flank of the NPSG before reemerging east of the STMW formation site the following spring. This process results in a negative lag-correlation between MET activities and surface chlorophyll in the reemergence region; for example, MHWs in winter at the STMW formation site tend to lower the surface chlorophyll concentration one year later in the reemergence region. Our study suggests that the oceanic processes allow one year of predictability of the marine ecosystems by monitoring METs in the STMW formation site.

Mesoscale eddies and the impact of coastal iron supply on primary production in the South Pacific Subtropical Front

Subtropical and subantarctic waters either side of the southern hemisphere Subtropical Front are considered iron-limited, suggesting production within the front is dependent on a supply of iron from atmospheric deposition, zonal advection of coastal water, or upwelling. We present the results from a one-day biogeochemical survey in Subtropical Water east of the North Island, New Zealand, in a region where mesoscale cyclonic and anticyclonic eddies entrain chlorophyll in filaments around the eddies. There was no significant relationship between upper mixed layer chlorophyll and any physical or macronutrient quantity. However, chlorophyll was significantly positively correlated with dissolved iron. A simple model suggests that while vertical entrainment of iron into the upper mixed layer occurred, most of the dissolved iron in the eddy was due to entrainment of high-iron coastal water into low-iron offshore Subtropical Water, and that this iron supports primary production in otherwise iron-deficient water. We suggest that a significant component of the total primary production within the Subtropical Front may be determined by mesoscale eddy induced lateral advection of iron.

Investigation of Ocean Sub-Surface Processes in Tropical Cyclone Phailin Using a Coupled Modeling Framework: Sensitivity to Ocean Conditions

The present study is aimed to investigate sub-surface ocean processes and their contribution to the intensification of a tropical cyclone (TC) from a coupled-modeling perspective. The Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) model was employed to simulate TC Phailin, which originated over the Bay of Bengal and made landfall on the eastern coast of India in October 2013. Three sub-surface ocean condition datasets—viz., (a) the European Centre for Medium-Range Weather Forecast (ECMWF) Ocean Reanalysis, (b) the Climate Forecast System Version 2 (CFSV2) Operational Analysis, and (c) the Hybrid Coordinate Ocean Model (HYCOM) Reanalysis datasets—were used for the initial and boundary conditions for the oceanic component of the coupled model in three different simulations of TC Phailin. All the simulations showed a delay in intensification compared to the observation, and ECMWF simulated the most intensified TC. CFSV2 simulated a deeper mixed layer (ML) and higher mixing, which hindered the intensification. Furthermore, higher entrainment of cold water in the ML led to cold water reaching the surface and, consequently, decreased sea surface temperature, which acted as negative feedback in the intensification of the storm in the cases of CFSV2 and HYCOM. ECMWF realistically simulated the interactions of the TC with a cold-core eddy before landfall. A sudden increase in ML heat content, the addition of heat in the ML due to entrainment, and the prevention of cold water reaching the surface were indicative of the breaking of the barrier layer (BL) in ECMWF, which was further corroborated by the spatial distribution of BL thickness in the simulation. This acted as positive feedback in the intensification of the TC. The findings of this study strongly suggest that not only the incorporation of physical oceanic sub-surface processes in the modeling of TCs but also the proper representation of prevailing mesoscale features and ocean sub-surface temperature, salinity, and current profiles in datasets is essential for realistic simulations of TCs.

Extreme erosion by submarine slides

Abstract Submarine slides (including slides, slumps, and debris flows) pose major geohazards by triggering tsunami and damaging essential submarine infrastructure. Slide volume, a key parameter in hazard assessments, can increase markedly through substrate and/or water entrainment. However, the erosive potential of slides is uncertain. We quantified slide erosivity by determining the ratio of deposited (Vd) to initially evacuated (Ve) sediment volumes; i.e., slides that gain volume through erosion have a Vd/Ve ratio &amp;gt;1. We applied this method to the Gorgon slide, a large (500 km3), seismically imaged slide offshore northwestern Australia, and reviewed Vd/Ve ratios for 11 other large slides worldwide. Nine of the 11 slides have Vd/Ve &amp;gt; 1 (median value = 2), showing emplaced volumes increased after initial failure. The Gorgon slide is the most erosive slide currently documented (Vd/Ve = 13), possibly reflecting its passage across a highly erodible carbonate ooze substrate. Our new approach to quantifying erosion is important for hazard assessments given substrate-flow interactions control slide speed and runout distance. The variations in slide volume also have important implications for submarine infrastructure impact assessments, including more robust tsunami modeling.

Tracing Unconfined Nearfield Spreading of a River Plume Interflow in a Large Lake (Lake Geneva): Hydrodynamics, Suspended Particulate Matter, and Associated Fluxes

River inflows have a major influence on lake water quality due to their input of sediments, nutrients and contaminants. After leaving the river channel, river waters form a plume and interact with ambient lake waters. Strong hydrodynamic changes take place in the nearfield. To determine the nearfield hydro-sedimentary dynamics of the negatively-buoyant Rhône River plume in thermally stratified Lake Geneva, field campaigns were carried out at high and intermediate river discharge. High-resolution full-depth Acoustic Doppler Current Profiler (ADCP) transects were taken at 400, 800, 1,200, and 1,500 m from the river mouth, combined with profiles of temperature, turbidity and particle size distribution. These measurements provided, for the first time in a lake, detailed velocity fields across the full-plume cross-section in each transect. Furthermore, the unique combination of measurement techniques allowed the quantification of entrainment, Suspended Particulate Matter (SPM) concentrations and fluxes. They revealed that the negatively buoyant river inflow intruded into the metalimnion as a laterally unconfined interflow and continued flowing straight out in the streamwise direction, since currents in the lake were weak. At the same time, it mainly spread laterally due to entrainment of ambient water. The size of the interflow core and its velocity progressively decreased with distance from the mouth, as did SPM concentrations and volumes of particles (by a factor of 2–3 within 1,500 m), due to sediment settling and entrainment. The interflow momentum flux remained constant along the pathway, but interflow cross sections and discharge increased 2–3 times (within 1,500 m). The entrainment coefficient was &amp;gt;2 at 400 m and further increased non-linearly along the pathway. These values are much higher than those reported for laterally confined laboratory studies. The particle size distribution in the interflow was dominated by fine particles (&amp;lt;32 μm), which were transported up to 1,500 m from the mouth and most likely beyond, whereas larger particles (&amp;gt;62 μm) almost completely settled out before reaching that distance. The above processes occurred independent of river discharge; their intensity, however, changed with discharge.