• NEANIAS Underwater Research Community
• 010504 meteorology & atmospheric sciences close

We report the first direct turbulence observations in the Samoan Passage (SP), a 40 km wide notch in the South Pacific bathymetry through which flows most of the water supplying the North Pacific abyssal circulation. The observed turbulence is 1000 to 10,000 times typical abyssal levels —strong enough to completely mix away the densest water entering the passage—confirming inferences from previous coarser temperature and salinity sections. Accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence. Specifically, the flow accelerates substantially at the primary sill within the passage, reaching speeds as great as 0.55 m s−1. A strong hydraulic response is seen, with layers first rising to clear the sill and then plunging hundreds of meters downward. Turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill. In addition to the primary sill, other locations along the multiple interconnected channels through the Samoan Passage also have an effect on the mixing of the dense water. In fact, quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers, suggesting that abyssal mixing depends sensitively on bathymetric details on small scales.

Subduction of a narrow slab of oceanic lithosphere beneath a tightly curved orogenic arc requires the presence of at least one lithospheric scale tear fault. While the Calabrian subduction beneath southern Italy is considered to be the type example of this geodynamic setting, the geometry, kinematics and surface expression of the associated lateral, slab tear fault offshore eastern Sicily remain controversial. Results from a new marine geophysical survey conducted in the Ionian Sea, using high-resolution bathymetry and seismic profiling reveal active faulting at the seafloor within a 140 km long, two-branched fault system near Alfeo Seamount. The previously unidentified 60 km long NW trending North Alfeo Fault system shows primarily strike-slip kinematics as indicated by the morphology and steep-dipping transpressional and transtensional faults. Available earthquake focal mechanisms indicate dextral strike-slip motion along this fault segment. The 80 km long SSE trending South Alfeo fault system is expressed by one or two steeply dipping normal faults, bounding the western side of a 500+ m thick, 5 km wide, elongate, syntectonic Plio-Quaternary sedimentary basin. Both branches of the fault system are mechanically capable of generating magnitude 6–7 earthquakes like those that struck eastern Sicily in 1169, 1542, and 1693. peer-reviewed

Remote sensing of wave breaker patterns, clearly visible as high‐intensity bands in time exposure video images, has become a powerful tool to obtain large‐scale (kilometers) and long‐term (years) time series of nearshore sandbar position. However, intensity‐based bar crest positions xi differ from directly measured positions xb by a time‐varying distance Δx, which is of O(10 m) and depends on the offshore wave height H0, the water level η0, and the bathymetry itself. The effect of these parameters on Δx was investigated from simultaneous video observations and bathymetric surveys, obtained in the double‐barred system at Egmond aan Zee, Netherlands, and from wave model predictions, assuming that the roller energy represents image intensity. When the wave field over a bar was predicted to be nonsaturated, xi was observed and predicted to move offshore as either η0 decreased or H0 increased. Under saturated conditions, Δx only responded to changes in η0. Additional model investigations showed that an increase in outer bar crest depth, similar to that observed during interannual bar behavior, significantly reduced the Δx variability at the outer bar and increased the Δx variability at the inner bar. Implications of our observational and model findings for studying sandbar position from video imagery are outlined.

Abstract The topographic steering of the baroclinic western branch of the Norwegian Atlantic Current (NwAC) determines the extent of Atlantic Water and location of the Arctic Front in the Nordic Seas. In this paper the geographical spread of hydrographic measurements at the Ocean Weather Station M (OWSM, 66 ∘ N 2 ∘ E ) is utilized to create mean sections across the Voring Plateau Escarpment in the Norwegian Sea. In concert with a theoretical framework involving the impact of low pressure systems on frontal jets over steep bathymetry, the behaviour of the front-current system at this location is described. It is shown that the halocline and thermocline are sloped from about 200 m in the west and down to 400 m in the east over 40 km centred on the station, indicating that the western branch of the NwAC is located here. The horizontal gradients introduced by this slope are 2 ∘ C and 0.1 for salinity. The frontal slope is not seen to change its inclination on seasonal, multi-annual, nor decadal timescales, indicating that the dynamic control of this frontal slope does not change appreciably. Further supported by the theoretical framework it is shown that the subsurface part of this front and the associated western branch of the NwAC is strongly locked by topography along the Voring Plateau also on short timescales. From large scale bathymetry it is also shown how this kind of frontal locking can be expected over most of the ridges and continental slopes in the Nordic Seas.

The flank failure and collapse of Anak Krakatau on December 22nd, 2018 triggered a destructive tsunami. Whether the prior activity of the volcano led to this collapse, or it was triggered by another means, remains a challenge to understand. This study seeks to investigate the recent volcano submarine mass-landslide deposit and emplacement processes, including the seafloor morphology of the flank collapse and the landslide deposit extent. Bathymetry and sparker seismic data were used during this study. Bathymetry data collected in August, 2019 shows the run-out area and the seafloor landslide deposit morphology. Bathymetry data acquired in May, 2017, is used as the base limit of the collapse to estimate the volume of the flank collapse. Comparisons between seismic data acquired in 2017 and 2019 provide an insight into the landslide emplacement processes, the deposit sequence, and structure below the seafloor. From these results we highlight two areas of the submarine-mass landslide deposit, one proximal to Anak Krakatau island (∼1.6 km) and one distal (∼1.4 km). The resulting analysis suggests that the submarine-mass landslide deposit might be produced by a frontally compressional, faulted, landslide, triggered by the critical stability slope, and due to the recent volcanic activity. Blocky seabed features clearly lie to the southwest of Anak Krakatau, and may represent the collapse blocks of the landslide. The seismic analysis of the data acquired in August, 2019 reveals that the blocky facies extends to ∼1.62 km in the width around Anak Krakatau, and the block thicknesses vary up to 70.4 m. The marine data provides a new insight into the landslide run out and extent, together with the landslide deposit morphology and structure that are not available from satellite imagery or subaerial surveys. We conclude that the landslide run out area southwest of the recent collapse, is ∼7.02 ± 0.21 km2.

Abstract Effective spatial management planning for sedentary fishery resources requires accurate spatial distribution mapping of a focal stock including both fished and associated unfished populations. This study focuses on (sub-)adult razor clams (Solen gordonis), with shell lengths ≥25 mm, in Sasebo Bay and Omura Bay, northwestern Kyushu, Japan. They are commercially harvested, but little is known about their spatial distribution pattern. The objective was to estimate the spatial distribution and configuration of clam beds existing within an area of ca. 12 km2. For this, clam and sediment sampling by scuba-equipped divers, acoustic bathymetric surveys using a recreational-grade fish finder (single-beam acoustic) system, and presence–absence modelling using generalized linear models were performed. For the collected clams, catch per unit effort (CPUE; number of clams collected/h) was calculated and used as a proxy for clam individual density. The results showed that the presence–absence variation in the spatial distribution of (sub-)adult clams was associated with the bottom sediment type. (Sub-)adult clams occurred frequently in the sediments with higher gravel (particles with diameters ≥2 mm, mainly shell fragments) and lower mud (diameters

Arctic Ocean primary productivity is limited by light and inorganic nutrients. With sea ice cover declining in recent decades, nitrate limitation has been speculated to become more prominent. Although much has been learned about nitrate supply from general patterns of ocean circulation and water column stability, a quantitative analysis requires dedicated turbulence measurements that have only started to accumulate in the last dozen years. Here we present new observations of the turbulent vertical nitrate flux in the Laptev Sea, Baffin Bay, and Young Sound (North-East Greenland), supplementing a compilation of 13 published estimates throughout the Arctic Ocean. Combining all flux estimates with a Pan-Arctic database of in situ measurements of nitrate concentration and density, we found the annual nitrate inventory to be largely determined by the strength of stratification and by bathymetry. Nitrate fluxes explained the observed regional patterns and magnitudes of both new primary production and particle export on annual scales. We argue that with few regional exceptions, vertical turbulent nitrate fluxes can be a reliable proxy of Arctic primary production accessible through autonomous and large-scale measurements. They may also provide a framework to assess nutrient limitation scenarios based on clear energetic and mass budget constraints resulting from turbulent mixing and freshwater flows.

In this study, a framework for simulating the flow field and heat transfer processes in small shallow inland water bodies has been developed. As the dynamics and thermal structure of these water bodies are crucial in studying the quality of stored water , and in assessing the heat fluxes from their surfaces as well, the heat transfer and temperature simulations were modeled. The proposed model is able to simulate the full 3-D water flow and heat transfer in the water body by applying complex and time varying boundary conditions. In this model, the continuity, momentum and temperature equations together with the turbulence equations, which comprise the buoyancy effect, have been solved. This model is built on the Reynolds Averaged Navier Stokes (RANS) equations with the widely used Boussinesq approach to solve the turbulence issues of the flow field. Micrometeorological data were obtained from an Automatic Weather Station (AWS) installed on the site and combined with field bathymetric measurements for the model. In the framework developed, a simple, applicable and generalizable approach is proposed for preparing the geometry of small shallow water bodies using coarsely measured bathymetry. All parts of the framework are based on open-source tools, which is essential for developing countries.