| 1. |
- Jakobsson, Martin, et al.
(författare)
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Bathymetric properties of the Baltic Sea
- 2019
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Ingår i: Ocean Science. - : Copernicus GmbH. - 1812-0784 .- 1812-0792. ; 15:4, s. 905-924
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Tidskriftsartikel (refereegranskat)abstract
- Baltic Sea bathymetric properties are analysed here using the newly released digital bathymetric model (DBM) by the European Marine Observation and Data Network (EMODnet). The analyses include hypsometry, volume, descriptive depth statistics, and kilometre-scale seafloor ruggedness, i.e. terrain heterogeneity, for the Baltic Sea as a whole as well as for 17 sub-basins defined by the Baltic Marine Environment Protection Commission (HELCOM). We compare the new EMODnet DBM with IOWTOPO the previously most widely used DBM of the Baltic Se aproduced by the Leibniz-Institut fur Ostseeforschung Warnemiinde (IOW), which has served as the primary gridded bathymetric resource in physical and environmental studies for nearly two decades. The area of deep water exchange between the Bothnian Sea and the Northern Baltic Proper across the Aland Sea is specifically analysed in terms of depths and locations of critical bathymetric sills. The EMODnet DBM provides a bathymetric sill depth of 88 m at the northern side of the Aland Sea and 60 m at the southern side, differing from previously identified sill depths of 100 and 70 m, respectively. High-resolution multibeam bathymetry acquired from this deep water exchange path, where vigorous bottom currents interacted with the seafloor, allows us to assess what presently available DBMs are missing in terms of physical characterization of the seafloor. Our study highlights the need for continued work towards complete high-resolution mapping of the Baltic Sea seafloor.
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| 2. |
- Jakobsson, Martin, et al.
(författare)
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Potential links between Baltic Sea submarine terraces and groundwater seeping
- 2020
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Ingår i: Earth Surface Dynamics. - : Copernicus GmbH. - 2196-6311 .- 2196-632X. ; 8:1, s. 1-15
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Tidskriftsartikel (refereegranskat)abstract
- Submarine groundwater discharge (SGD) influences ocean chemistry, circulation, and the spreading of nutrients and pollutants; it also shapes sea floor morphology. In the Baltic Sea, SGD was linked to the development of terraces and semicircular depressions mapped in an area of the southern Stockholm archipelago, Sweden, in the 1990s. We mapped additional parts of the Stockholm archipelago, areas in Blekinge, southern Sweden, and southern Finland using high-resolution multibeam sonars and sub-bottom profilers to investigate if the sea floor morphological features discovered in the 1990s are widespread and to further address the hypothesis linking their formation to SGD. Sediment coring and sea floor photography conducted with a remotely operated vehicle (ROV) and divers add additional information to the geophysical mapping results. We find that terraces, with general bathymetric expressions of about 1 m and lateral extents of sometimes > 100 m, are widespread in the surveyed areas of the Baltic Sea and are consistently formed in glacial clay. Semicircular depressions, however, are only found in a limited part of a surveyed area east of the island of Asko, southern Stockholm archipelago. While submarine terraces can be produced by several processes, we interpret our results to be in support of the basic hypothesis of terrace formation initially proposed in the 1990s; i.e. groundwater flows through siltier, more permeable layers in glacial clay to discharge at the sea floor, leading to the formation of a sharp terrace when the clay layers above seepage zones are undermined enough to collapse. By linking the terraces to a specific geologic setting, our study further refines the formation hypothesis and thereby forms the foundation for a future assessment of SGD in the Baltic Sea that may use marine geological mapping as a starting point. We propose that SGD through the submarine sea floor terraces is plausible and could be intermittent and linked to periods of higher groundwater levels, implying that to quantify the contribution of freshwater to the Baltic Sea through this potential mechanism, more complex hydrogeological studies are required.
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| 3. |
- Jakobsson, Martin, et al.
(författare)
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Ryder Glacier in northwest Greenland is shielded from warm Atlantic water by a bathymetric sill
- 2020
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Ingår i: Communications Earth & Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 1
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Tidskriftsartikel (refereegranskat)abstract
- The processes controlling advance and retreat of outlet glaciers in fjords draining the Greenland Ice Sheet remain poorly known, undermining assessments of their dynamics and associated sea-level rise in a warming climate. Mass loss of the Greenland Ice Sheet has increased six-fold over the last four decades, with discharge and melt from outlet glaciers comprising key components of this loss. Here we acquired oceanographic data and multibeam bathymetry in the previously uncharted Sherard Osborn Fjord in northwest Greenland where Ryder Glacier drains into the Arctic Ocean. Our data show that warmer subsurface water of Atlantic origin enters the fjord, but Ryder Glacier’s floating tongue at its present location is partly protected from the inflow by a bathymetric sill located in the innermost fjord. This reduces under-ice melting of the glacier, providing insight into Ryder Glacier’s dynamics and its vulnerability to inflow of Atlantic warmer water.
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| 4. |
- Muchowski, Julia, et al.
(författare)
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Potential and Limitations of a Commercial Broadband Echo Sounder for Remote Observations of Turbulent Mixing
- 2022
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Ingår i: Journal of Atmospheric and Oceanic Technology. - 0739-0572 .- 1520-0426. ; 39:12, s. 1985-2003
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Tidskriftsartikel (refereegranskat)abstract
- Stratified oceanic turbulence is strongly intermittent in time and space, and therefore generally underresolved by currently available in situ observational approaches. A promising tool to at least partly overcome this constraint are broadband acoustic observations of turbulent microstructure that have the potential to provide mixing parameters at orders of magnitude higher resolution compared to conventional approaches. Here, we discuss the applicability, limitations, and measurement uncertainties of this approach for some prototypical turbulent flows (stratified shear layers, turbulent flow across a sill), based on a comparison of broadband acoustic observations and data from a free-falling turbulence microstructure profiler. We find that broadband acoustics are able to provide a quantitative description of turbulence energy dissipation in stratified shear layers (correlation coefficient r = 0.84) if the stratification parameters required by the method are carefully preprocessed. Essential components of our suggested preprocessing algorithm are 1) a vertical low-pass filtering of temperature and salinity profiles at a scale slightly larger than the Ozmidov length scale of turbulence and 2) an automated elimination of weakly stratified layers according to a gradient threshold criterion. We also show that in weakly stratified conditions, the acoustic approach may yield acceptable results if representative averaged vertical temperature and salinity gradients rather than local gradients are used. Our findings provide a step toward routine turbulence measurements in the upper ocean from moving vessels by combining broadband acoustics with in situ CTD profiles.
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| 5. |
- Reed-Weidner, Elizabeth, 1990-
(författare)
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The layer that did not swim away : Broadband acoustic discrimination and characterization of ocean stratification
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ocean stratification plays a critical role in many oceanographic processes. The magnitude of mixing between stable water masses is regulated, in part, by the intensity of stratification. As such, stratification modulates the vertical transport of heat and many important dissolved constituents in the water column, influencing such processes as ocean ventilation, and ocean heat and carbon uptake. As human induced climate change alters oceanic stratification a thorough understanding of its distribution and variability is critical in the study of the world’s oceans. However, traditional methods are limited in terms of spatial context and rapid, near-synoptic observational methods, such as those provided by active acoustic systems, are needed to fill in the gaps. Broadband acoustic water column data have already been used to observe ocean structure and commercial systems are becoming increasingly available. However, broadband acoustic methods for characterizing oceanic stratification are not well developed, limiting of applicability of broadband data to the study, quantification, and monitor ocean stratification as the world warms.This thesis aims to develop quantitative acoustic methods for characterization of ocean stratification using active broadband acoustic systems. This work leveraged the high range resolution and signal to noise ratios, as well as the frequency-modulated scattering response of broadband acoustic systems. Broadband acoustic methods were developed and established through 1) the analysis of field data from different ocean basins and 2) the development and application of acoustic scattering models. This work will provide the means to better understand the physical mechanisms responsible for acoustic backscattering from stratification, working towards rapid, remote, high-resolution measurements of ocean stratification through acoustic inversion, in order to monitor and quantify changes in ocean structure.
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| 6. |
- Stranne, Christian, et al.
(författare)
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Acoustic Mapping of Thermohaline Staircases in the Arctic Ocean
- 2017
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Although there is enough heat contained in inflowing warm Atlantic Ocean water to melt all Arctic sea ice within a few years, a cold halocline limits upward heat transport from the Atlantic water. The amount of heat that penetrates the halocline to reach the sea ice is not well known, but vertical heat transport through the halocline layer can significantly increase in the presence of double diffusive convection. Such convection can occur when salinity and temperature gradients share the same sign, often resulting in the formation of thermohaline staircases. Staircase structures in the Arctic Ocean have been previously identified and the associated double diffusive convection has been suggested to influence the Arctic Ocean in general and the fate of the Arctic sea ice cover in particular. A central challenge to understanding the role of double diffusive convection in vertical heat transport is one of observation. Here, we use broadband echo sounders to characterize Arctic thermohaline staircases at their full vertical and horizontal resolution over large spatial areas (100 s of kms). In doing so, we offer new insight into the mechanism of thermohaline staircase evolution and scale, and hence fluxes, with implications for understanding ocean mixing processes and ocean-sea ice interactions.
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| 7. |
- Stranne, Christian, et al.
(författare)
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The climate sensitivity of northern Greenland fjords is amplified through sea-ice damming
- 2021
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Ingår i: Communications Earth & Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- Record-high air temperatures were observed over Greenland in the summer of 2019 and melting of the northern Greenland Ice Sheet was particularly extensive. Here we show, through direct measurements, that near surface ocean temperatures in Sherard Osborn Fjord, northern Greenland, reached 4 °C in August 2019, while in the neighboring Petermann Fjord, they never exceeded 0 °C. We show that this disparity in temperature between the two fjords occurred because thick multi-year sea ice at the entrance of Sherard Osborn Fjord trapped the surface waters inside the fjord, which led to the formation of a warm and fresh surface layer. These results suggest that the presence of multi-year sea ice increases the sensitivity of Greenland fjords abutting the Arctic Ocean to climate warming, with potential consequences for the long-term stability of the northern sector of the Greenland Ice Sheet.
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| 8. |
- Weidner, Elizabeth, 1990-, et al.
(författare)
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A wideband acoustic method for direct assessment of bubble-mediated methane flux
- 2019
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Ingår i: Continental Shelf Research. - : Elsevier BV. - 0278-4343 .- 1873-6955. ; 173, s. 104-115
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Tidskriftsartikel (refereegranskat)abstract
- The bubble-mediated transport and eventual fate of methane escaping from the seafloor is of great interest to researchers in many fields. Acoustic systems are frequently used to study gas seep sites, as they provide broad synoptic observations of processes in the water column. However, the visualization and characterization of individual gas bubbles needed for quantitative studies has routinely required the use of optical sensors which offer a limited field of view and require extended amounts of time for deployment and data collection. In this paper, we present an innovative method for studying individual bubbles and estimating gas flux using a calibrated wideband from the Bolin Centre for Climate Research database: http://bolin.su.se/data/.and split-beam echosounder. The extended bandwidth (16 – 26 kHz) affords vertical range resolution of approximately 7.5 cm, allowing for the differentiation of individual bubbles in acoustic data. Split-aperture processing provides phase-angle data used to compensate for transducer beam-pattern effects and to precisely locate bubbles in the transducer field of view. The target strength of individual bubbles is measured and compared to an analytical scattering model to estimate bubble radius, and bubbles are tracked through the water column to estimate rise velocity. The resulting range of bubble radii (0.68–8.40 mm in radius) agrees with those found in other investigations with optical measurements, and the rise velocities trends are consistent with published models. Together, the observations of bubble radius and rise velocity offer a measure of gas flux, requiring nothing more than vessel transit over a seep site, bypassing the need to deploy time-consuming and expensive optical systems.
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| 9. |
- Weidner, Elizabeth, 1990-, et al.
(författare)
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An acoustic scattering model for stratification interfaces
- 2021
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Ingår i: Journal of the Acoustical Society of America. - : Acoustical Society of America (ASA). - 0001-4966 .- 1520-8524. ; 150:6, s. 4353-4361
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Tidskriftsartikel (refereegranskat)abstract
- Stable fluid bodies, such as the ocean and atmosphere, are composed of a series of increasingly dense layers, defined by density stratification interfaces in which the medium properties (e.g., temperature, salinity) change. The intensity of the stratification between the layers influences the internal mixing dynamics and entrainment, facilitating the transport of dissolved constituents within the fluid medium. Acoustic systems offer the means for high resolution observations of these interfaces, which allow for continuous data collection over broad spatial scales. Here, a one-dimensional acoustic scattering model is presented for predicting acoustic backscatter from stratification interfaces, which is widely applicable to the acoustic water column data collected with ship-mounted sonars. Model predictions based on hydrographic profiles suggest that in many oceanic cases, the density gradient perturbations can be disregarded, and sound speed perturbations alone drive the majority of the acoustic scattering. A frequency-dependent scattering intensity based on the sharpness of the stratification interface is predicted by the model, suggesting a path to remote estimations of the physical medium properties through broadband acoustic inversion.
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| 10. |
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