| 1. |
- Ballarotta, Maxime, et al.
(författare)
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On the glacial and interglacial thermohaline circulation and the associated transports of heat and freshwater
- 2014
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Ingår i: Ocean Science. - Gottingen : Copernicus Publications. - 1812-0784 .- 1812-0792. ; 10:6, s. 907-921
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Tidskriftsartikel (refereegranskat)abstract
- The thermohaline circulation (THC) and the oceanic heat and freshwater transports are essential for understanding the global climate system. Streamfunctions are widely used in oceanography to represent the THC and estimate the transport of heat and freshwater. In the present study, the regional and global changes of the THC, the transports of heat and freshwater and the timescale of the circulation between the Last Glacial Maximum (LGM, ≈ 21 kyr ago) and the present-day climate are explored using an Ocean General Circulation Model and streamfunctions projected in various coordinate systems. We found that the LGM tropical circulation is about 10% stronger than under modern conditions due to stronger wind stress. Consequently, the maximum tropical transport of heat is about 20% larger during the LGM. In the North Atlantic basin, the large sea-ice extent during the LGM constrains the Gulf Stream to propagate in a more zonal direction, reducing the transport of heat towards high latitudes by almost 50% and reorganising the freshwater transport. The strength of the Atlantic Meridional Overturning Circulation depends strongly on the coordinate system. It varies between 9 and 16 Sv during the LGM, and between 12 to 19 Sv for the present day. Similar to paleo-proxy reconstructions, a large intrusion of saline Antarctic Bottom Water takes place into the Northern Hemisphere basins and squeezes most of the Conveyor Belt circulation into a shallower part of the ocean. These different haline regimes between the glacial and interglacial period are illustrated by the streamfunctions in latitude–salinity coordinates and thermohaline coordinates. From these diagnostics, we found that the LGM Conveyor Belt circulation is driven by an enhanced salinity contrast between the Atlantic and the Pacific basin. The LGM abyssal circulation lifts and makes the Conveyor Belt cell deviate from the abyssal region, resulting in a ventilated upper layer above a deep stagnant layer, and an Atlantic circulation more isolated from the Pacific. An estimate of the timescale of the circulation reveals a sluggish abyssal circulation during the LGM, and a Conveyor Belt circulation that is more vigorous due to the combination of a stronger wind stress and a shortened circulation route.
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| 2. |
- Ballarotta, Maxime, 1984-, et al.
(författare)
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The vigorous large-scale ocean circulations during the Last Glacial Maximum
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The representation of the ocean thermohaline circulation (THC) under glacial and interglacial climate conditions is investigated using a new global thermohaline stream function. Consequently, the interglacial and glacial THCs are compared from two experiments based on an ocean general circulation model forced at the surface by conditions representing the present-day and the period of the Last Glacial Maximum (LGM, ≈ 21kyr ago). It is shown that the LGM THC is amplified by the salinity/density contrast between the Atlantic and the Pacific basins, as well as in the abyss due to larger salinity gradients. Even though the circuit along the Conveyor Belt loop is not drastically changed, the water mass transformations can regionally differ between the two periods. Additionally, the LGM Conveyor Belt Cell is more isolated from the abyss and its turnover time is between two and three times shorter than in the present-day simulation, suggesting vigorous large-scale circulation.
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| 3. |
- Baranizadeh, Elham, et al.
(författare)
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Implementation of state-of-the-art ternary new-particle formation scheme to the regional chemical transport model PMCAMx-UF in Europe
- 2016
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Ingår i: Geoscientific Model Development. - : Copernicus GmbH. - 1991-959X .- 1991-9603. ; 9:8, s. 2741-2754
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Tidskriftsartikel (refereegranskat)abstract
- The particle formation scheme within PMCAMx-UF, a three-dimensional chemical transport model, was updated with particle formation rates for the ternary H2SO4-NH3-H2O pathway simulated by the Atmospheric Cluster Dynamics Code (ACDC) using quantum chemical input data. The model was applied over Europe for May 2008, during which the EUCAARI-LONGREX (European Aerosol Cloud Climate and Air Quality Interactions-Long-Range Experiment) campaign was carried out, providing aircraft vertical profiles of aerosol number concentrations. The updated model reproduces the observed number concentrations of particles larger than 4 nm within 1 order of magnitude throughout the atmospheric column. This agreement is encouraging considering the fact that no semi-empirical fitting was needed to obtain realistic particle formation rates. The cloud adjustment scheme for modifying the photolysis rate profiles within PMCAMx-UF was also updated with the TUV (Tropospheric Ultraviolet and Visible) radiative-transfer model. Results show that, although the effect of the new cloud adjustment scheme on total number concentrations is small, enhanced new-particle formation is predicted near cloudy regions. This is due to the enhanced radiation above and in the vicinity of the clouds, which in turn leads to higher production of sulfuric acid. The sensitivity of the results to including emissions from natural sources is also discussed.
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| 4. |
- de Lavergne, C., et al.
(författare)
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Toward global maps of internal tide energy sinks
- 2019
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Ingår i: Ocean Modelling. - : Elsevier BV. - 1463-5003 .- 1463-5011. ; 137, s. 52-75
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Tidskriftsartikel (refereegranskat)abstract
- Internal tides power much of the observed small-scale turbulence in the ocean interior. To represent mixing induced by this turbulence in ocean climate models, the cascade of internal tide energy to dissipation scales must be understood and mapped. Here, we present a framework for estimating the geography of internal tide energy sinks. The mapping relies on the following ingredients: (i) a global observational climatology of stratification; (ii) maps of the generation of M-2, S-2 and K-1 internal tides decomposed into vertical normal modes; (iii) simplified representations of the dissipation of low-mode internal tides due to wave-wave interactions, scattering by small-scale topography, interaction with critical slopes and shoaling; (iv) Lagrangian tracking of low-mode energy beams through observed stratification, including refraction and reflection. We thus obtain a global map of the column-integrated energy dissipation for each of the four considered dissipative processes, each of the three tidal constituents and each of the first five modes. Modes >= 6 are inferred to dissipate within the local water column at the employed half-degree horizontal resolution. Combining all processes, modes and constituents, we construct a map of the total internal tide energy dissipation, which compares well with observational inferences of internal wave energy dissipation. This result suggests that tides largely shape observed spatial contrasts of dissipation, and that the framework has potential in improving understanding and modelling of ocean mixing. However, sensitivity to poorly constrained parameters and simplifying assumptions entering the parameterized energy sinks calls for additional investigation. The attenuation of low-mode internal tides by wave-wave interactions needs particular attention.
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| 5. |
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| 6. |
- Falahat, Saeed, et al.
(författare)
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Comparison of calculated energy flux of internal tides with microstructure measurements
- 2014
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 66, s. 23240-
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Tidskriftsartikel (refereegranskat)abstract
- Vertical mixing caused by breaking of internal tides plays a major role in maintaining the deep-ocean stratification. This study compares observations of dissipation from microstructure measurements to calculations of the vertical energy flux from barotropic to internal tides, taking into account the temporal variation due to the spring-neap tidal cycle. The dissipation data originate from two surveys in the Brazil Basin Tracer Release Experiment (BBTRE), and one over the LArval Dispersal along the Deep East Pacific Rise (LADDER3), supplemented with a few stations above the North-Atlantic Ridge (GRAVILUCK) and in the western Pacific (IZU). A good correlation is found between logarithmic values of energy flux and local dissipation in BBTRE, suggesting that the theory is able to predict energy fluxes. For the LADDER3, the local dissipation is much smaller than the calculated energy flux, which is very likely due to the different topographic features of BBTRE and LADDER3. The East Pacific Rise consists of a few isolated seamounts, so that most of the internal wave energy can radiate away from the generation site, whereas the Brazil Basin is characterised by extended rough bathymetry, leading to a more local dissipation. The results from all four field surveys support the general conclusion that the fraction of the internal-tide energy flux that is dissipated locally is very different in different regions.
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| 7. |
- Falahat, Saeed, et al.
(författare)
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Global Calculation of Tidal Energy Conversion into Vertical Normal Modes
- 2014
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Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 44:12, s. 3225-3244
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Tidskriftsartikel (refereegranskat)abstract
- A direct calculation of the tidal generation of internal waves over the global ocean is presented. The calculation is based on a semianalytical model, assuming that the internal tide characteristic slope exceeds the bathymetric slope (subcritical slope) and the bathymetric height is small relative to the vertical scale of the wave, as well as that the horizontal tidal excursion is smaller than the horizontal topographic scale. The calculation is performed for the M-2 tidal constituent. In contrast to previous similar computations, the internal tide is projected onto vertical eigenmodes, which gives two advantages. First, the vertical density profile and the finite ocean depth are taken into account in a fully consistent way, in contrast to earlier work based on the WKB approximation. Nevertheless, the WKB-based total global conversion follows closely that obtained using the eigenmode decomposition in each of the latitudinal and vertical distributions. Second, the information about the distribution of the conversion energy over different vertical modes is valuable, since the lowest modes can propagate over long distances, while high modes are more likely to dissipate locally, near the generation site. It is found that the difference between the vertical distributions of the tidal conversion into the vertical modes is smaller for the case of very deep ocean than the shallow-ocean depth. The results of the present work pave the way for future work on the vertical and horizontal distribution of the mixing caused by internal tides.
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| 8. |
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| 9. |
- Falahat, Saeed, et al.
(författare)
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On the Generation of Bottom - Trapped Internal Tides
- 2015
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Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 45:2, s. 526-545
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of the barotropic tide with bottom topography when the tidal frequency omega is smaller than the Coriolis frequency f is examined. The resulting waves are called bottom-trapped internal tides. The energy density associated with these waves is computed using linear wave theory and vertical normal-mode decomposition in an ocean of finite depth. The global calculation of the modal energy density is performed for the semidiurnal M-2 tidal constituent and the two major diurnal tidal constituents K-1 and O-1. An observationally based decay time scale of 3 days is then used to transform the energy density to energy flux in units of watts per square meter. The globally integrated energy fluxes are found to be 1.99 and 1.43GW for the K-1 and O-1 tidal constituents, respectively. For the M-2 tidal constituent, it is found to be 1.15 GW. The Pacific Ocean is found to be the most energetic basin for the bottom-trapped diurnal tides. Two regional estimates of the bottom-trapped energy flux are given for the Kuril Islands and the Arctic Ocean, in which the bottom-trapped waves play a role for the tidally induced vertical mixing. The results of this study can be incorporated into ocean general circulation models and coupled climate models to improve the parameterization of the vertical mixing induced by breaking of the internal tides.
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| 10. |
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| 11. |
- Falahat, Saeed, 1981-
(författare)
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Tidally generated internal waves
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the internal tide in the deep ocean, which is generated by the barotropic tide flowing over the bottom topography. The energy flux from the barotropic tide to the internal-wave field at the bottom is calculated using a method based on linear-wave theory and the traditional WKB approximation valid for a slowing varying vertical stratification. The global distribution of the baroclinic plus barotropic tidal velocities near the bottom is thus obtained, which is then used to analyzed the deep-sea sediment resuspension. The calculated energy flux of the internal tide is then compared with the energy dissipation rate obtained from different data sets of microstructure measurements conducted in several regions of the world ocean. A good correlation is generally found between the model estimates and observations, giving us some confidence that the theory reasonably well predict internal tide generation. It is also found that the ratio of the averaged energy dissipation rate to the averaged energy flux is very different in different regions. A direct global calculation of the energy flux is done by projecting the internal tides onto vertical eigenmodes, so that the vertical density profile and the finite ocean depth are taken into account in a fully consistent way. The results of the modal energy flux is important for understanding the pathway from generation to dissipation of the internal tides, since the low-mode internal tides are less affected by local nonlinear processes responsible for degrading their energy to small-scale mixing. The agreement between this detailed method and the WKB-based method is found to be high, while this methods provides new information on the vertical mode distribution of internal tide generation.Finally, the bottom-trapped internal tides, which are generated when the tidal frequency is smaller than the Coriolis frequency, is examined. The energy density associated with these waves is computed using linear wave theory and vertical normal-mode decomposition. An emphasis is placed on the bottom-trapped internal tides in the Arctic Ocean, as yet, there is a lack of the comprehensive understanding of the mixing processes in this basin. Through the development of new methods to estimate internal tide generation, this thesis provides a valuable information to the problem of the better understanding of tidal mixing in the deep ocean and its role on the large-scale ocean circulation, with a possible applications to the improvement of ocean general circulation model.
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| 12. |
- Hordoir, Robinson, et al.
(författare)
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Nemo-Nordic 1.0 : a NEMO-based ocean model for the Baltic and North seas - research and operational applications
- 2019
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Ingår i: Geoscientific Model Development. - : Copernicus GmbH. - 1991-959X .- 1991-9603. ; 12:1, s. 363-386
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Tidskriftsartikel (refereegranskat)abstract
- We present Nemo-Nordic, a Baltic and North Sea model based on the NEMO ocean engine. Surrounded by highly industrialized countries, the Baltic and North seas and their assets associated with shipping, fishing and tourism are vulnerable to anthropogenic pressure and climate change. Ocean models providing reliable forecasts and enabling climatic studies are important tools for the shipping infrastructure and to get a better understanding of the effects of climate change on the marine ecosystems. Nemo-Nordic is intended to be a tool for both short-term and long-term simulations and to be used for ocean forecasting as well as process and climatic studies. Here, the scientific and technical choices within Nemo-Nordic are introduced, and the reasons behind the design of the model and its domain and the inclusion of the two seas are explained. The model's ability to represent barotropic and baroclinic dynamics, as well as the vertical structure of the water column, is presented. Biases are shown and discussed. The short-term capabilities of the model are presented, especially its capabilities to represent sea level on an hourly timescale with a high degree of accuracy. We also show that the model can represent longer timescales, with a focus on the major Baltic inflows and the variability in deep-water salinity in the Baltic Sea.
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| 13. |
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| 14. |
- Melet, Angélique, et al.
(författare)
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Internal tide generation by abyssal hills using analytical theory
- 2013
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Ingår i: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 118:11, s. 6303-6318
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Tidskriftsartikel (refereegranskat)abstract
- Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products barely resolve scales smaller than ∼10 km in the deep ocean. On these scales abyssal hills dominate ocean floor roughness. The impact of abyssal hill roughness on internal-tide generation is evaluated in this study. The conversion of M2 barotropic to baroclinic tidal energy is calculated based on linear wave theory both in real and spectral space using the Shuttle Radar Topography Mission SRTM30_PLUS bathymetric product at 1/120° resolution with and without the addition of synthetic abyssal hill roughness. Internal tide generation by abyssal hills integrates to 0.1 TW globally or 0.03 TW when the energy flux is empirically corrected for supercritical slope (i.e., ∼10% of the energy flux due to larger topographic scales resolved in standard products in both cases). The abyssal hill driven energy conversion is dominated by mid-ocean ridges, where abyssal hill roughness is large. Focusing on two regions located over the Mid-Atlantic Ridge and the East Pacific Rise, it is shown that regionally linear theory predicts an increase of the energy flux due to abyssal hills of up to 100% or 60% when an empirical correction for supercritical slopes is attempted. Therefore, abyssal hills, unresolved in state of the art topographic products, can have a strong impact on internal tide generation, especially over mid-ocean ridges.
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| 15. |
- Turnewitsch, Robert, et al.
(författare)
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Deep-sea fluid and sediment dynamics-Influence of hill- to seamount-scale seafloor topography
- 2013
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Ingår i: Earth-Science Reviews. - : Elsevier BV. - 0012-8252 .- 1872-6828. ; 127, s. 203-241
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Tidskriftsartikel (refereegranskat)abstract
- Deep-sea sediments play a central role in a wide range of subject areas. A number of important controls on the formation of sedimentary deposits have been studied. However, to date, the impact of submarine landscape geometry as a possible control has received comparatively little attention. This seems to be particularly true for intermediate-scale topographic features such as abyssal hills, knolls and seamounts that can be found in many regions of the global seafloor: recent estimates suggest that in the deep open oceans, away from continental margins, there might be as many as similar to 25 x 10(6) abyssal hills, knolls and seamounts. Despite this large number very little is known about how they influence environmental complexity and patchiness, biogeochemical fluxes and the formation of sedimentary records. This paper reviews the currently known types of fluid-flow interactions with abyssal hills, knolls and seamounts that could potentially influence the way sediments are formed. The main types of relevant flow components are: quasi-steady to eddying background flow; internal lee and near-inertial waves; barotropic and baroclinic tides; and seamount-trapped waves. Previous studies looking into systematic links between fluid dynamics and sediments at hills, knolls and seamounts are reviewed. Finally, a case study is presented which aims to combine our current knowledge and investigate whether a given combination of recent fluid-flow components leaves a detectable imprint in the recent sediments on and around a short seamount. The main conclusions and implications are as follows. (1) Topographically generated flow-field geometries that are composed of a number of different prevailing fluid-flow components can be reflected and detected in properties of the underlying sediments. (2) Tidal and other higher-frequency (lee-wave, near-inertial) components of deep-ocean currents can be essential for locally driving total current velocities across threshold values for non-deposition/erosion/resuspension of freshly deposited deep-sea sediments. Moreover, there is evidence suggesting that not only maximum current speeds but also intensities of higher-frequency (tidal and/or (near-)inertial) current-direction variability might control sediment dynamics and sediment formation. This relativises the view that current speed is the main, or even only, controlling factor for sediment dynamics and sediment formation. (3) When it comes to the reconstruction of paleo-flows, these findings imply that certain sedimentary records may well reveal more about variability in the higher-frequency flow components than about variability in the basin-scale net flow component that often is the focus of paleoceanographic studies. (4) Single-core paleo-records from hill-, seamount- or similarly controlled sediment deposits may be biased due to the asymmetry of flow fields around these topographic features. To arrive at unbiased paleo-records for non-fluid-dynamic parameters, the influence of the flow-field geometry would have to be removed from the record first (5) It seems the mechanistic understanding of hill- and seamount-related flow/topography interactions and their links to sediment dynamics is approaching a level that may (a) facilitate improved interpretation of topographically controlled sedimentary paleo-records, (b) help fill in the knowledge gap that exists for functional deep-sea biodiversity at intermediate space scales, and (c) improve predictive capabilities for exploration of economically relevant iron-manganese (Fe-Mn) crusts on seamounts.
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| 16. |
- Turnewitsch, Robert, et al.
(författare)
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Recent sediment dynamics in hadal trenches : Evidence for the influence of higher-frequency (tidal, near-inertial) fluid dynamics
- 2014
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Ingår i: Deep Sea Research Part I. - : Elsevier BV. - 0967-0637 .- 1879-0119. ; 90, s. 125-138
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Tidskriftsartikel (refereegranskat)abstract
- In addition to high hydrostatic pressure, scarcity of food is viewed as a factor that limits the abundance and activity of heterotrophic organisms at great ocean depths, including hadal trenches. Supply of nutritious food largely relies on the flux of organic-rich particulate matter from the surface ocean. It has been speculated that the shape of hadal trenches helps to 'funnel' particulate matter into the deeper parts of the trench, leading to sediment 'focussing' and improved benthic food supply. Here we investigate for five Northwest Pacific trenches the efficiency of sediment focussing by evaluating ratios of measured (sediment-derived) and expected (water-column-derived) sedimentary inventories of the naturally occurring and radioactive particulate-matter tracer Pb-210(xs). The sites comprise a broad range of surface-ocean productivity and physical-oceanographic regimes. Across the five trench-axis settings the inventory ratio varies between 0.5 and 4.1, with four trench-axis settings having ratios > 1 (sediment focussing) and one trench-axis setting a ratio < 1 (sediment winnowing). Although the fluid- and sediment-dynamical forcing behind sediment focussing remains unclear, this study finds evidence for another mechanism that is superimposed on, and counteracts, the focussing mechanism. This superimposed mechanism is related to higher-frequency (tidal, near-inertial) fluid dynamics. In particular, there is evidence for a strong and negative relation between the intensity of propagating internal tides and the extent of sediment focussing in the trench-axis. The relation can be approximated by a power function and the most intense drop in sediment focussing already occurs at moderate internal-tide intensities. This suggests that propagating internal tides may have a subtle but significant influence on particulate-matter dynamics and food supply in hadal trenches in particular, but possibly also in the deep seas in general. A mechanism for the influence of internal tides on sediment dynamics is proposed.
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