Published manuscripts
C. Mohn, J. Hansen, M. Carreiro-Silva, S. Cunningham, E. de Froe, C. Dominguez-Carrió, S. Gary, R. Glud, C. G ̈oke, C. Johnson, T. Morato, E. Møller, L. Rovelli, K. Schulz, K. Soetaert, A. van der Kaaden, D. van Oevelen (2023). Tidal to decadal scale hydrodynamics at two contrasting cold-water coral sites in the Northeast Atlantic. Progress in Oceanography, 214, 103031. https://doi.org/10.1016/j.pocean.2023.103031
Abstract
Cold-water corals (CWCs) thrive in areas with complex and rough topography favoring the development of highly diverse benthic communities. Several biotic and abiotic factors including organic matter supply, temperature, bottom roughness and currents are important drivers of ecosystem structure and functioning in deep-sea environments at different spatial and temporal scales. Little is known, however, how basin-scale changes in the ocean climate affect these drivers at local scales. Here, we use high-resolution implementations of the hydrodynamic model ROMS-AGRIF for estimating characteristic spatial and temporal scales of local hydrodynamics in response to variations of basin-scale currents imposed by distinct changes of the Atlantic Meridional Overturning Circulation (AMOC) in the past century. We focus on two CWC communities on the SE Rockall Bank slope and at Condor Seamount. We considered two contrasting AMOC states that were identified from the 1958–2009 hindcast of the 1/20° resolution VIKING20 North Atlantic basin-scale ocean circulation model and used as boundary conditions for the high-resolution local area models. At SE Rockall Bank, variability of near-bottom currents in both regions was largely dominated by tidal dynamics, but strongly modified by AMOC induced basin-scale variations of water mass properties and bottom currents. During strong AMOC years, waters in the main CWC depth corridor (600–1200 m) were cooler and less saline but were dominated by stronger bottom currents when compared with conditions during weak AMOC years. At Condor Seamount, bottom currents were largely unaffected by AMOC related changes close to the summit at water depths < 400 m. Kinetic energy dissipation rates derived from the 3D near-bottom velocity field appeared to positively relate with the in-situ CWC distribution. Kinetic energy dissipation is therefore proposed as a mechanistic descriptor of CWC presence as it provides a more mechanistic view of hydrodynamics driving organic matter supply to filter and suspension-feeding communities.
Cold-water corals (CWCs) thrive in areas with complex and rough topography favoring the development of highly diverse benthic communities. Several biotic and abiotic factors including organic matter supply, temperature, bottom roughness and currents are important drivers of ecosystem structure and functioning in deep-sea environments at different spatial and temporal scales. Little is known, however, how basin-scale changes in the ocean climate affect these drivers at local scales. Here, we use high-resolution implementations of the hydrodynamic model ROMS-AGRIF for estimating characteristic spatial and temporal scales of local hydrodynamics in response to variations of basin-scale currents imposed by distinct changes of the Atlantic Meridional Overturning Circulation (AMOC) in the past century. We focus on two CWC communities on the SE Rockall Bank slope and at Condor Seamount. We considered two contrasting AMOC states that were identified from the 1958–2009 hindcast of the 1/20° resolution VIKING20 North Atlantic basin-scale ocean circulation model and used as boundary conditions for the high-resolution local area models. At SE Rockall Bank, variability of near-bottom currents in both regions was largely dominated by tidal dynamics, but strongly modified by AMOC induced basin-scale variations of water mass properties and bottom currents. During strong AMOC years, waters in the main CWC depth corridor (600–1200 m) were cooler and less saline but were dominated by stronger bottom currents when compared with conditions during weak AMOC years. At Condor Seamount, bottom currents were largely unaffected by AMOC related changes close to the summit at water depths < 400 m. Kinetic energy dissipation rates derived from the 3D near-bottom velocity field appeared to positively relate with the in-situ CWC distribution. Kinetic energy dissipation is therefore proposed as a mechanistic descriptor of CWC presence as it provides a more mechanistic view of hydrodynamics driving organic matter supply to filter and suspension-feeding communities.
K. Schulz, D. Kadko, V. Mohrholz, M. Stephens, I. Fer (2023). Winter vertical diffusion rates in the Arctic Ocean, estimated from 7Be measurements and dissipation rate profiles. Journal of Geophysical Research: Oceans, e2022JC019197.
https://doi.org/10.1029/2022JC019197
https://doi.org/10.1029/2022JC019197
Abstract
Ocean turbulent mixing is a key process affecting the uptake and redistribution of heat, carbon, nutrients, oxygen and other dissolved gasses. Vertical turbulent diffusivity sets the rates of water mass transformations and ocean mixing, and is intrinsically an average quantity over process time scales. Estimates based on microstructure profiling, however, are typically obtained as averages over individual profiles. How representative such averaged diffusivities are, remains unexplored in the quiescent Arctic Ocean. Here, we compare upper ocean vertical diffusivities in winter, derived from the 7Be tracer-based approach to those estimated from direct turbulence measurements during the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, 2019–2020. We found that diffusivity estimates from both methods agree within their respective measurement uncertainties. Diffusivity estimates obtained from dissipation rate profiles are sensitive to the averaging method applied, and the processing and analysis of similar data sets must take this sensitivity into account. Our findings indicate low characteristic diffusivities around 10−6 m2 s−1 and correspondingly low vertical heat fluxes.
Ocean turbulent mixing is a key process affecting the uptake and redistribution of heat, carbon, nutrients, oxygen and other dissolved gasses. Vertical turbulent diffusivity sets the rates of water mass transformations and ocean mixing, and is intrinsically an average quantity over process time scales. Estimates based on microstructure profiling, however, are typically obtained as averages over individual profiles. How representative such averaged diffusivities are, remains unexplored in the quiescent Arctic Ocean. Here, we compare upper ocean vertical diffusivities in winter, derived from the 7Be tracer-based approach to those estimated from direct turbulence measurements during the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, 2019–2020. We found that diffusivity estimates from both methods agree within their respective measurement uncertainties. Diffusivity estimates obtained from dissipation rate profiles are sensitive to the averaging method applied, and the processing and analysis of similar data sets must take this sensitivity into account. Our findings indicate low characteristic diffusivities around 10−6 m2 s−1 and correspondingly low vertical heat fluxes.
E. Ruiz-Castillo, M. Janout, J. Hölemann, T. Kanzow, K. Schulz, V. Ivanov (2023). Structure and seasonal variability of the Arctic Boundary Current north of Severnaya Zemlya. Journal of Geophysical Research: Oceans, 128(1), e2022JC019197.
https://doi.org/10.5194/egusphere-egu22-11472
https://doi.org/10.5194/egusphere-egu22-11472
Abstract
We assessed the spatial and temporal variability of the Arctic Boundary Current (ABC) using seven oceanographic moorings, deployed across the continental slope north of Severnaya Zemlya in 2015–2018. Transports and individual water masses were quantified based on temperature and salinity recorders and current profilers. Our results were compared with observations from the northeast Svalbard and the central Laptev Sea continental slopes to evaluate the hydrographic transformation along the ABC pathway. The highest velocities (>0.30 m s−1) of the ABC occurred at the upper continental slope and decreased offshore to below 0.03 m s−1 in the deep basin. The ABC showed seasonal variability with velocities two times higher in winter than in summer. Compared to upstream conditions in Svalbard, water mass distribution changed significantly within 20 km of the shelf edge due to mixing with- and intrusion of shelf waters. The ABC transported 4.15 ± 0.3 Sv in the depth range 50–1,000 m, where 0.88 ± 0.1, 1.5 ± 0.2, 0.61 ± 0.1 and 1.0 ± 0.15 Sv corresponded to Atlantic Water (AW), Dense Atlantic Water (DAW), Barents Sea Branch Water (BSBW) and Transformed Atlantic Water (TAW). 62–70% of transport was constrained to within 30–40 km of the shelf edge, and beyond 84 km, transport increases were estimated to be 0.54 Sv. Seasonality of TAW derived from local shelf-processes and advection of seasonal-variable Fram Strait waters, while BSBW transport variability was dominated by temperature changes with maximum transport coinciding with minimum temperatures. Further Barents Sea warming will likely reduce TAW and BSBW transport leading to warmer conditions along the ABC pathway.
We assessed the spatial and temporal variability of the Arctic Boundary Current (ABC) using seven oceanographic moorings, deployed across the continental slope north of Severnaya Zemlya in 2015–2018. Transports and individual water masses were quantified based on temperature and salinity recorders and current profilers. Our results were compared with observations from the northeast Svalbard and the central Laptev Sea continental slopes to evaluate the hydrographic transformation along the ABC pathway. The highest velocities (>0.30 m s−1) of the ABC occurred at the upper continental slope and decreased offshore to below 0.03 m s−1 in the deep basin. The ABC showed seasonal variability with velocities two times higher in winter than in summer. Compared to upstream conditions in Svalbard, water mass distribution changed significantly within 20 km of the shelf edge due to mixing with- and intrusion of shelf waters. The ABC transported 4.15 ± 0.3 Sv in the depth range 50–1,000 m, where 0.88 ± 0.1, 1.5 ± 0.2, 0.61 ± 0.1 and 1.0 ± 0.15 Sv corresponded to Atlantic Water (AW), Dense Atlantic Water (DAW), Barents Sea Branch Water (BSBW) and Transformed Atlantic Water (TAW). 62–70% of transport was constrained to within 30–40 km of the shelf edge, and beyond 84 km, transport increases were estimated to be 0.54 Sv. Seasonality of TAW derived from local shelf-processes and advection of seasonal-variable Fram Strait waters, while BSBW transport variability was dominated by temperature changes with maximum transport coinciding with minimum temperatures. Further Barents Sea warming will likely reduce TAW and BSBW transport leading to warmer conditions along the ABC pathway.
D. S. van Maren, C. Maushake, J.-W. Mol, D. van Keulen, J. Ju ̈rges, J. Vroom, H. Schuttelaars, T. Gerkema, K. Schulz, T. H. Badewien, M. Gerriets, A. Engels, A. Wurpts, D. Oberrecht, A. J. Manning, T. Bailey, L. Ross, V. Mohrholz, D. M. L. Horemans, M. Becker, D. Post, C. Schmidt, P. J. T. Dankers (2023). Synoptic observations of sediment transport and exchange mechanisms in the turbid Ems estuary: The EDoM campaign. Earth System Science Data, 15(1), 53-73.
https://doi.org/10.4121/c.6056564.v3
https://doi.org/10.4121/c.6056564.v3
Abstract
An extensive field campaign, the Ems-Dollard Measurements (EDoM), was executed in the Ems Estuary, bordering the Netherlands and Germany, aimed at better understanding the mechanisms that drive the exchange of water and sediments between a relatively exposed outer estuary and a hyper-turbid tidal river. More specifically, the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using eight concurrently operating ships and 10 moorings measuring for at least one spring–neap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and sediment settling properties. These observations have provided important new insights into horizontal sediment fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these observations suggests that large-scale residual transport is surprisingly similar during periods of high and low discharge, with higher river discharge resulting in both higher seaward-directed fluxes near the surface and landward-directed fluxes near the bed. Sediment exchange seems to be strongly influenced by a previously undocumented lateral circulation cell driving residual transport. Vertical density-driven flows in the outer estuary are influenced by variations in river discharge, with a near-bed landward flow being most pronounced in the days following a period with elevated river discharge. The study site is more turbid during winter conditions, when the estuarine turbidity maximum (ETM) is pushed seaward by river flow, resulting in a more pronounced impact of suspended sediments on hydrodynamics. All data collected during the EDoM campaign, but also standard monitoring data (waves, water levels, discharge, turbidity, and salinity) collected by Dutch and German authorities are made publicly available at 4TU Centre for Research Data.
An extensive field campaign, the Ems-Dollard Measurements (EDoM), was executed in the Ems Estuary, bordering the Netherlands and Germany, aimed at better understanding the mechanisms that drive the exchange of water and sediments between a relatively exposed outer estuary and a hyper-turbid tidal river. More specifically, the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using eight concurrently operating ships and 10 moorings measuring for at least one spring–neap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and sediment settling properties. These observations have provided important new insights into horizontal sediment fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these observations suggests that large-scale residual transport is surprisingly similar during periods of high and low discharge, with higher river discharge resulting in both higher seaward-directed fluxes near the surface and landward-directed fluxes near the bed. Sediment exchange seems to be strongly influenced by a previously undocumented lateral circulation cell driving residual transport. Vertical density-driven flows in the outer estuary are influenced by variations in river discharge, with a near-bed landward flow being most pronounced in the days following a period with elevated river discharge. The study site is more turbid during winter conditions, when the estuarine turbidity maximum (ETM) is pushed seaward by river flow, resulting in a more pronounced impact of suspended sediments on hydrodynamics. All data collected during the EDoM campaign, but also standard monitoring data (waves, water levels, discharge, turbidity, and salinity) collected by Dutch and German authorities are made publicly available at 4TU Centre for Research Data.
A. Rogge, M. Janout, N. Zakharova, E. Trudnowska, C. Hoerstmann, C. Wekerle, L. Oziel, V. Schourup-Kristensen, E. Ruiz-Castillo, K. Schulz, V. Povazhnyy, M. Iversen, A. Waite (2023). Cross-shelf transport of Barents Sea dense water as a sink for atmospheric CO2 in the Arctic Ocean. Nature Geoscience, 16(1), 82-88. https://doi.org/10.1038/S41561-022-01069-z
Abstract
Large amounts of atmospheric carbon can be exported and retained in the deep sea on millennial time scales, buffering global warming. However, while the Barents Sea is one of the most biologically productive areas of the Arctic Ocean, carbon retention times were thought to be short. Here we present observations, complemented by numerical model simulations, that revealed a deep and widespread lateral injection of approximately 2.33 kt C d−1 from the Barents Sea shelf to some 1,200 m of the Nansen Basin, driven by Barents Sea Bottom Water transport. With increasing distance from the out ow region, the plume expanded and penetrated into even deeper waters and the sediment. The seasonally uctuating but continuous injection increases the carbon sequestration of the Barents Sea by 1/3 and feeds the deep sea community of the Nansen Basin. Our ndings combined with those from other out ow regions of carbon-rich polar dense waters highlight the importance of lateral injection as a global carbon sink. Resolving uncertainties around negative feedbacks of global warming due to sea ice decline will necessitate observation of changes in bottom water formation and biological productivity at a resolution high enough to quantify future deep carbon injection.
Large amounts of atmospheric carbon can be exported and retained in the deep sea on millennial time scales, buffering global warming. However, while the Barents Sea is one of the most biologically productive areas of the Arctic Ocean, carbon retention times were thought to be short. Here we present observations, complemented by numerical model simulations, that revealed a deep and widespread lateral injection of approximately 2.33 kt C d−1 from the Barents Sea shelf to some 1,200 m of the Nansen Basin, driven by Barents Sea Bottom Water transport. With increasing distance from the out ow region, the plume expanded and penetrated into even deeper waters and the sediment. The seasonally uctuating but continuous injection increases the carbon sequestration of the Barents Sea by 1/3 and feeds the deep sea community of the Nansen Basin. Our ndings combined with those from other out ow regions of carbon-rich polar dense waters highlight the importance of lateral injection as a global carbon sink. Resolving uncertainties around negative feedbacks of global warming due to sea ice decline will necessitate observation of changes in bottom water formation and biological productivity at a resolution high enough to quantify future deep carbon injection.
K. Schulz, A. Nguyen, H. Pillar (2022). Towards an improved and observationally- constrained melt rate parameterization for vertical ice fronts of marine terminating glaciers. Geophysical Research Letters, 49(18), e2022GL100654. https://doi.org/10.1029/2022GL100654
Abstract
Submarine melting at Greenland's marine terminating glaciers is a crucial, yet poorly constrained process in the coupled ice-ocean system. Application of Antarctic melt rate representations, derived for floating glacier tongues, to non-floating marine terminating glaciers commonly found in Greenland, results in a dramatic underestimation of submarine melting. Here, we revisit the physical theory underlying melt rate parameterizations and leverage recently published observational data to derive a novel melt rate parameterization. This is the first parameterization that (a) consistently comprises both convective- and shear-dominated melt regimes, (b) includes coefficients quantitatively constrained using observational data, and (c) is applicable to any vertical glacier front. We show that, compared to the current state-of-the-art approach, the scheme provides an improved fit to observed melt rates on the scale of the terminating front, offering an opportunity to incorporate this critical missing forcing into ocean circulation models.
Submarine melting at Greenland's marine terminating glaciers is a crucial, yet poorly constrained process in the coupled ice-ocean system. Application of Antarctic melt rate representations, derived for floating glacier tongues, to non-floating marine terminating glaciers commonly found in Greenland, results in a dramatic underestimation of submarine melting. Here, we revisit the physical theory underlying melt rate parameterizations and leverage recently published observational data to derive a novel melt rate parameterization. This is the first parameterization that (a) consistently comprises both convective- and shear-dominated melt regimes, (b) includes coefficients quantitatively constrained using observational data, and (c) is applicable to any vertical glacier front. We show that, compared to the current state-of-the-art approach, the scheme provides an improved fit to observed melt rates on the scale of the terminating front, offering an opportunity to incorporate this critical missing forcing into ocean circulation models.
K. Schulz, V. Mohrholz, I. Fer, M. Janout, M. Hoppmann, J. Schaffer, Z. Koenig (2022). A full year of turbulence measurements in the Arctic Ocean, MOSAiC drift campaign, 2019–2020. Scientific Data 9(1), 1-11. https://doi.org/10.1038/s41597-022-01574-1
Abstract
Ocean turbulent mixing is a key process in the global climate system, regulating ocean circulation and the uptake and redistribution of heat, carbon, nutrients, oxygen and other tracers. In polar oceans, turbulent heat transport additionally affects the sea ice mass balance. Due to the inaccessibility of polar regions, direct observations of turbulent mixing are sparse in the Arctic Ocean. During the year-long drift expedition “Multidisciplinary drifting Observatory for the Study of Arctic Climate” (MOSAiC) from September 2019 to September 2020, we obtained an unprecedented data set of vertical profiles of turbulent dissipation rate and water column properties, including oxygen concentration and fluorescence. Nearly 1,700 profiles, covering the upper ocean down to approximately 400 m, were collected in sets of 3 or more consecutive profiles every day, and complemented with several intensive sampling periods. This data set allows for the systematic assessment of upper ocean mixing in the Arctic, and the quantification of turbulent heat and nutrient fluxes, and can help to better constrain turbulence parameterizations in ocean circulation models.
Ocean turbulent mixing is a key process in the global climate system, regulating ocean circulation and the uptake and redistribution of heat, carbon, nutrients, oxygen and other tracers. In polar oceans, turbulent heat transport additionally affects the sea ice mass balance. Due to the inaccessibility of polar regions, direct observations of turbulent mixing are sparse in the Arctic Ocean. During the year-long drift expedition “Multidisciplinary drifting Observatory for the Study of Arctic Climate” (MOSAiC) from September 2019 to September 2020, we obtained an unprecedented data set of vertical profiles of turbulent dissipation rate and water column properties, including oxygen concentration and fluorescence. Nearly 1,700 profiles, covering the upper ocean down to approximately 400 m, were collected in sets of 3 or more consecutive profiles every day, and complemented with several intensive sampling periods. This data set allows for the systematic assessment of upper ocean mixing in the Arctic, and the quantification of turbulent heat and nutrient fluxes, and can help to better constrain turbulence parameterizations in ocean circulation models.
Y. Kawaguchi, Z. Koenig, D. Nomura, M. Hoppmann, J. Inoue, Y.-C. Fang, K. Schulz, M. Gallagher, C. Katlein, M. Nicolaus, B. Rabe (2022). Turbulent mixing in the ice-ocean boundary layer in the Central Arctic Ocean: Results from the MOSAiC campaign. Journal of Geophysical Research: Oceans 127(8), e2021JC017975. https://doi.org/10.1029/2021JC017975
Abstract
We examined mixing processes within the ice–ocean boundary layer (IOBL) close to the geographic North Pole, with an emphasis on wind-driven sea ice drift. Observations were conducted from late August to late September 2020, during the final leg of the international Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. Measurements of ice motion, and profiles of currents, hydrography, and microstructure turbulence were conducted. The multifarious direct observations of sea ice and the upper ocean were used to quantify the transport of momentum, heat, and salt in the IOBL. The ice drift was mostly characterized by the inertial oscillation at a semi-diurnal frequency, which forced an inertial current in the mixed layer. Observation-derived heat and salinity fluxes at the ice–ocean interface suggest early termination of basal melting and transitioning to refreezing, resulting from a rise in the freezing point temperature by the presence of freshened near-surface water. Based on the friction velocity, the measured dissipation rate (ε) of turbulent energy can be approximated as 1.4–1.7 times of the “Law of the Wall” criterion. We also observed a spiraling Ekman flow and find its vertical extent in line with the estimate from ε-based diffusivity. Following passage of a storm, the enhanced oscillatory motions of the ice drift caused trapping of the near-inertial waves (NIWs) that exclusively propagated through the base of the weakly stratified mixed layer. We accounted Holmboe instabilities and NIWs for the observed distinct peak of the dissipation rate near the bottom of the mixed layer.
We examined mixing processes within the ice–ocean boundary layer (IOBL) close to the geographic North Pole, with an emphasis on wind-driven sea ice drift. Observations were conducted from late August to late September 2020, during the final leg of the international Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. Measurements of ice motion, and profiles of currents, hydrography, and microstructure turbulence were conducted. The multifarious direct observations of sea ice and the upper ocean were used to quantify the transport of momentum, heat, and salt in the IOBL. The ice drift was mostly characterized by the inertial oscillation at a semi-diurnal frequency, which forced an inertial current in the mixed layer. Observation-derived heat and salinity fluxes at the ice–ocean interface suggest early termination of basal melting and transitioning to refreezing, resulting from a rise in the freezing point temperature by the presence of freshened near-surface water. Based on the friction velocity, the measured dissipation rate (ε) of turbulent energy can be approximated as 1.4–1.7 times of the “Law of the Wall” criterion. We also observed a spiraling Ekman flow and find its vertical extent in line with the estimate from ε-based diffusivity. Following passage of a storm, the enhanced oscillatory motions of the ice drift caused trapping of the near-inertial waves (NIWs) that exclusively propagated through the base of the weakly stratified mixed layer. We accounted Holmboe instabilities and NIWs for the observed distinct peak of the dissipation rate near the bottom of the mixed layer.
K. Schulz, B. Lincoln, V. Povazhnyy, T. Rippeth, Y.-D. Lenn, M. Janout, M. Alkire, B. Scannell, S. Torres-Valdés (2022). Increasing nutrient fluxes and mixing regime changes in the eastern Arctic Ocean. Geophysical Research Letters, e2021GL096152. https://doi.org/10.1029/2021GL096152
Abstract
Primary productivity in the Arctic Ocean is experiencing dramatic changes linked to the receding sea ice cover. The vertical transport of nutrients from deeper water layers is the limiting factor for primary production. Here, we compare coincident profiles of turbulence and nutrients from the Siberian Seas in 2007, 2008, and 2018. In all years, the water column structure in the upstream region of the Arctic Boundary Current promotes upward nutrient transport, in contrast to the regions further downstream, and there are first indications for an eastward progression of these conditions. In summer 2018, strongly enhanced vertical nitrate flux and primary production above the continental slope were observed, likely related to a remote storm. The estimated contribution of these elevated fluxes above the slope to the Pan-Arctic vertical nitrate supply is comparable with the basin-wide transport, and is predicted to increase with declining sea ice cover in the future.
Primary productivity in the Arctic Ocean is experiencing dramatic changes linked to the receding sea ice cover. The vertical transport of nutrients from deeper water layers is the limiting factor for primary production. Here, we compare coincident profiles of turbulence and nutrients from the Siberian Seas in 2007, 2008, and 2018. In all years, the water column structure in the upstream region of the Arctic Boundary Current promotes upward nutrient transport, in contrast to the regions further downstream, and there are first indications for an eastward progression of these conditions. In summer 2018, strongly enhanced vertical nitrate flux and primary production above the continental slope were observed, likely related to a remote storm. The estimated contribution of these elevated fluxes above the slope to the Pan-Arctic vertical nitrate supply is comparable with the basin-wide transport, and is predicted to increase with declining sea ice cover in the future.
D. Thomas, D. Arévalo-Martínez, K. Crocket, F. Große, J. Grosse, K. Schulz, R. Sühring, A. Tessin. A changing Arctic Ocean. Ambio 51, 293–297 (2022). https://doi.org/10.1007/s13280-021-01677-w
B. Rabe, C. Heuzé, J. Regnery, Y. Aksenov, J. Allerholt, M. Athanase, Y. Bai, C. Basque, D. Bauch, T. Baumann, D. Chen, S. Cole, L. Craw, A. Davies, E. Damm, K. Dethloff, D. Divine, F. Doglioni, F. Ebert, Y.-C. Fang, I. Fer, A. Fong, R. Gradinger, M. Granskog, R. Graupner, C. Haas, H. He, Y. He, M. Hoppmann, M. Janout, D. Kadko, T. Kanzow, S. Karam, Y. Kawaguchi, Z. Koenig, B. Kong, R. Krishfield, T. Krumpen, D. Kuhlmey, I. Kuznetsov, M. Lan, R. Lei, T. Li, S. Torres-Valdés, L. Lin, L. Lin, H. Liu, N. Liu, B. Loose, X. Ma, R. MacKay, M. Mallet, R. Mallett, W. Maslowski, C. Mertens, V. Mohrholz, M. Muilwijk, M. Nicolaus, J. O’Brien, D. Perovich, J. Ren, M. Rex, N. Ribeiro, A. Rinke, J. Schaffer, I. Schuffenhauer, K. Schulz, M. Shupe, W. Shaw, V. Sokolov, A. Sommerfeld, G. Spreen, T. Stanton, M. Stephens, J. Su, N. Sukhikh, A. Sundfjord, K. Thomisch, S. Tippenhauer, J. Toole, M. Vredenborg, M. Walter, H. Wang, L. Wang, Y. Wang, M. Wendisch, J. Zhao, M. Zhou, J. Zhu (2022). Overview of the MOSAiC expedition: Physical Oceanography. Elementa: Science of the Anthropocene 10(1). https://doi.org/10.1525/elementa.2021.00062
Abstract
Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present along-drift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean.
Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present along-drift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean.
K. Schulz, S. Büttner, A. Rogge, M. Janout, J. Hölemann, T. Rippeth (2021). Turbulent mixing and the formation of an intermediate nepheloid layer above the Siberian continental slope. Geophysical Research Letters, 48(9), e2021GL092988. https://doi.org/10.1029/2021GL092988
Abstract
Intermediate nepheloid layers (INLs) form important pathways for the cross-slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid-water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross-slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment.
Intermediate nepheloid layers (INLs) form important pathways for the cross-slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid-water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross-slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment.
K. Schulz, M. Janout, Y.-D. Lenn, E. Ruiz-Castillo, I. Polyakov, V. Mohrholz, S. Tippenhauer, K. Reeve, J. Hölemann, B. Rabe, M. Vredenborg (2021). On the along-slope heat loss of the boundary current in the eastern Arctic Ocean. JGR: Oceans, 126(2), e2020JC016375. https://doi.org/10.1029/2020JC016375
Abstract
This study presents recent observations to quantify oceanic heat fluxes along the continental slope of the Eurasian part of the Arctic Ocean, in order to understand the dominant processes leading to the observed along-track heat loss of the Arctic Boundary Current (ABC). We investigate the fate of warm Atlantic Water (AW) along the Arctic Ocean continental margin of the Siberian Seas based on 11 cross-slope conductivity, temperature, depth transects and direct heat flux estimates from microstructure profiles obtained in summer 2018. The ABC loses on average
(108) J m−2 per 100 km during its propagation along the Siberian shelves, corresponding to an average heat flux of 47 W m−2 out of the AW layer. The measured vertical heat flux on the upper AW interface of on average 10 W m−2 in the deep basin, and 3.7 W m−2 above the continental slope is larger than previously reported values. Still, these heat fluxes explain less than 20% of the observed heat loss within the boundary current. Heat fluxes are significantly increased in the turbulent near-bottom layer, where AW intersects the continental slope, and at the lee side of a topographic irregularity. This indicates that mixing with ambient colder water along the continental margins is an important contribution to AW heat loss. Furthermore, the cold halocline layer receives approximately the same amount of heat due to upward mixing from the AW, compared to heat input from the summer-warmed surface layer above. This underlines the importance of both surface warming and increased vertical mixing in a future ice-free Arctic Ocean in summer.
This study presents recent observations to quantify oceanic heat fluxes along the continental slope of the Eurasian part of the Arctic Ocean, in order to understand the dominant processes leading to the observed along-track heat loss of the Arctic Boundary Current (ABC). We investigate the fate of warm Atlantic Water (AW) along the Arctic Ocean continental margin of the Siberian Seas based on 11 cross-slope conductivity, temperature, depth transects and direct heat flux estimates from microstructure profiles obtained in summer 2018. The ABC loses on average
(108) J m−2 per 100 km during its propagation along the Siberian shelves, corresponding to an average heat flux of 47 W m−2 out of the AW layer. The measured vertical heat flux on the upper AW interface of on average 10 W m−2 in the deep basin, and 3.7 W m−2 above the continental slope is larger than previously reported values. Still, these heat fluxes explain less than 20% of the observed heat loss within the boundary current. Heat fluxes are significantly increased in the turbulent near-bottom layer, where AW intersects the continental slope, and at the lee side of a topographic irregularity. This indicates that mixing with ambient colder water along the continental margins is an important contribution to AW heat loss. Furthermore, the cold halocline layer receives approximately the same amount of heat due to upward mixing from the AW, compared to heat input from the summer-warmed surface layer above. This underlines the importance of both surface warming and increased vertical mixing in a future ice-free Arctic Ocean in summer. K. Schulz, K. Klingbeil, C. Morys, T. Gerkema (2021). The fate of mud nourishment in response to short-term wind forcing. Estuaries and Coasts, 44.1: 88-102. https://doi.org/10.1007/s12237-021-00964-9
Abstract
In this study, results from a realistic 3D hydrodynamic and sediment transport model, applied to a channel in the Dutch Wadden Sea, are analyzed in order to assess the effect of short-term wind forcing, the impact of fresh water effects, and the variability induced by the spring-neap cycle on the transport of suspended sediment. In the investigated region, a pilot study for sediment nourishment, the so-called Mud Motor, is executed. This project aims for the beneficial re-use of dredged harbor sediments through the disposal of these sediments at a location where natural currents are expected to transport them toward a nearby salt marsh area. The model results presented in this study advance the understanding of the driving forces that determine sediment transport in shallow, near-coastal zones, and can help to improve the design of the Mud Motor. In the investigated channel, which is oriented parallel to the coastline, tidal asymmetries generally drive a transport of sediment in flood direction. It was found that already moderate winds along the channel axis reverse (wind in ebb direction), or greatly enhance this transport, up to an export of sediment over the adjacent water shed (wind in flood direction). The most beneficial wind conditions (moderate westerly winds) can cause an accumulation of more than 90% of the initial 200 tons sediment pool on the intertidal area; during less favorable conditions (northeasterly winds), less than a third of the dumped sediment is transported onto the mudflat. On-shore winds induce a transport toward the coast. Surprisingly, sediment pathways are only sensitive to the exact disposal location in the channel during wind conditions that counteract the tidally driven transport, and freshwater effects play no significant role for the dispersal of sediment.
In this study, results from a realistic 3D hydrodynamic and sediment transport model, applied to a channel in the Dutch Wadden Sea, are analyzed in order to assess the effect of short-term wind forcing, the impact of fresh water effects, and the variability induced by the spring-neap cycle on the transport of suspended sediment. In the investigated region, a pilot study for sediment nourishment, the so-called Mud Motor, is executed. This project aims for the beneficial re-use of dredged harbor sediments through the disposal of these sediments at a location where natural currents are expected to transport them toward a nearby salt marsh area. The model results presented in this study advance the understanding of the driving forces that determine sediment transport in shallow, near-coastal zones, and can help to improve the design of the Mud Motor. In the investigated channel, which is oriented parallel to the coastline, tidal asymmetries generally drive a transport of sediment in flood direction. It was found that already moderate winds along the channel axis reverse (wind in ebb direction), or greatly enhance this transport, up to an export of sediment over the adjacent water shed (wind in flood direction). The most beneficial wind conditions (moderate westerly winds) can cause an accumulation of more than 90% of the initial 200 tons sediment pool on the intertidal area; during less favorable conditions (northeasterly winds), less than a third of the dumped sediment is transported onto the mudflat. On-shore winds induce a transport toward the coast. Surprisingly, sediment pathways are only sensitive to the exact disposal location in the channel during wind conditions that counteract the tidally driven transport, and freshwater effects play no significant role for the dispersal of sediment.
K. Schulz, K. Soetaert, C. Mohn, L. Korte, F. Mienis, G. Duineveld, D. van Oevelen (2020). Linking large-scale circulation patterns to the distribution of cold water corals along the eastern Rockall Bank (northeast Atlantic). Journal of Marine Systems, 212, 103456. https://doi.org/10.1016/j.jmarsys.2020.103456
Abstract
Cold-water corals (CWC) are known to tolerate a relatively wide range of environmental conditions. However, along the basin margins of the Rockall Trough (NE Atlantic), the habitat of CWC is confined to a narrow range of 525–1200 m water depth, and the reason for that is not fully understood. To investigate the distribution of CWCs in this area, current velocities and water mass distribution in the Logachev Mound Province were measured with two long-term (1 year) moorings equipped with an acoustic doppler current profiler and fluorescence+turbidity sensors. Additional ship-based water column profiles and discrete water samples, covering a full diurnal tidal cycle, were taken for chemical parameters. The results indicate the presence of a boundary current along the eastern flank of the Rockall Bank, that transports a nutrient-rich water mass southwards and governs the direction of particle transport at the depth band of the CWC mounds. Based on literature results, this northern water mass is identified as Wyville Thomson Ridge Overflow Water (WTOW). The density envelope and depth distribution of WTOW match the reported occurrence of CWC mounds in the Rockall Trough. Hence, the presence of WTOW may be a necessary condition for coral growth and therefore mound formation in the Rockall Trough, e.g. by forming a conduit for particles and coral larvae.
Cold-water corals (CWC) are known to tolerate a relatively wide range of environmental conditions. However, along the basin margins of the Rockall Trough (NE Atlantic), the habitat of CWC is confined to a narrow range of 525–1200 m water depth, and the reason for that is not fully understood. To investigate the distribution of CWCs in this area, current velocities and water mass distribution in the Logachev Mound Province were measured with two long-term (1 year) moorings equipped with an acoustic doppler current profiler and fluorescence+turbidity sensors. Additional ship-based water column profiles and discrete water samples, covering a full diurnal tidal cycle, were taken for chemical parameters. The results indicate the presence of a boundary current along the eastern flank of the Rockall Bank, that transports a nutrient-rich water mass southwards and governs the direction of particle transport at the depth band of the CWC mounds. Based on literature results, this northern water mass is identified as Wyville Thomson Ridge Overflow Water (WTOW). The density envelope and depth distribution of WTOW match the reported occurrence of CWC mounds in the Rockall Trough. Hence, the presence of WTOW may be a necessary condition for coral growth and therefore mound formation in the Rockall Trough, e.g. by forming a conduit for particles and coral larvae.
K. Schulz, H. Burchard, V. Mohrholz, P. Holtermann, H. Schuttelaars, M. Becker, C. Maushake, T. Gerkema (2020). Intratidal and spatial variability over a slope in the Ems estuary: robust along-channel SPM transport versus episodic events. Estuarine, Coastal and Shelf Science, 243, 106902. https://doi.org/10.1016/j.ecss.2020.106902
Abstract
Results from measurements are presented that were collected during a full tidal cycle in the Ems estuary, involving two landers and an anchored research vessel. The conditions were characterized by very weak winds, no wave effects, and low river run-off, so that the state was close to tide-only. We find that the lateral (i.e., cross-slope) transport of water and suspended particulate matter (SPM) much of the time shows a vertically layered structure, which is however subject to sudden transitions. Moreover, even on the small spatial scale of these measurements (i.e., within a distance of 200 m), a strong lateral variability is observed in the circulation patterns. We analyze its dynamics by means of dimensionless parameters. In addition, near-bottom peaks in SPM concentration are observed, notably during early flood. However, these episodic events have little effect on the overall transport of SPM, which involves the whole water column: the measurements show a high vertically integrated SPM signal during late ebb, resulting in an ebb-dominance in the transport at this position in the tidal channel.
Results from measurements are presented that were collected during a full tidal cycle in the Ems estuary, involving two landers and an anchored research vessel. The conditions were characterized by very weak winds, no wave effects, and low river run-off, so that the state was close to tide-only. We find that the lateral (i.e., cross-slope) transport of water and suspended particulate matter (SPM) much of the time shows a vertically layered structure, which is however subject to sudden transitions. Moreover, even on the small spatial scale of these measurements (i.e., within a distance of 200 m), a strong lateral variability is observed in the circulation patterns. We analyze its dynamics by means of dimensionless parameters. In addition, near-bottom peaks in SPM concentration are observed, notably during early flood. However, these episodic events have little effect on the overall transport of SPM, which involves the whole water column: the measurements show a high vertically integrated SPM signal during late ebb, resulting in an ebb-dominance in the transport at this position in the tidal channel.
I. Bartl, D. Hellemann, C. Rabouille, K. Schulz, P. Tallberg, S. Hietanen, M. Voss (2019). Particulate organic matter controls benthic microbial N retention and N removal in contrasting estuaries of the Baltic Sea. Biogeosciences 16.18, 3543-3564. https://doi.org/10.5194/bg-16-3543-2019
Abstract
Estuaries worldwide act as “filters” of land-derived nitrogen (N) loads, yet differences in coastal environmental settings can affect the N filter function. We investigated microbial N retention (nitrification, ammonium assimilation) and N removal (denitrification, anammox) processes in the aphotic benthic system (bottom boundary layer (BBL) and sediment) of two Baltic Sea estuaries differing in riverine N loads, trophic state, geomorphology, and sediment type. In the BBL, rates of nitrification (5–227 nmol N L−1 d−1) and ammonium assimilation (9–704 nmol N L−1 d−1) were not enhanced in the eutrophied Vistula Estuary compared to the oligotrophic Öre Estuary. No anammox was detected in the sediment of either estuary, while denitrification rates were twice as high in the eutrophied (352±123 µmol N m−2 d−1) as in the oligotrophic estuary. Particulate organic matter (POM) was mainly of phytoplankton origin in the benthic systems of both estuaries. It seemed to control heterotrophic denitrification and ammonium assimilation as well as autotrophic nitrification by functioning as a substrate source of N and organic carbon. Our data suggest that in stratified estuaries, POM is an essential link between riverine N loads and benthic N turnover and may furthermore function as a temporary N reservoir. During long particle residence times or alongshore transport pathways, increased time is available for the recycling of N until its eventual removal, allowing effective coastal filtering even at low process rates. Understanding the key controls and microbial N processes in the coastal N filter therefore requires to also consider the effects of geomorphological and hydrological features.
Estuaries worldwide act as “filters” of land-derived nitrogen (N) loads, yet differences in coastal environmental settings can affect the N filter function. We investigated microbial N retention (nitrification, ammonium assimilation) and N removal (denitrification, anammox) processes in the aphotic benthic system (bottom boundary layer (BBL) and sediment) of two Baltic Sea estuaries differing in riverine N loads, trophic state, geomorphology, and sediment type. In the BBL, rates of nitrification (5–227 nmol N L−1 d−1) and ammonium assimilation (9–704 nmol N L−1 d−1) were not enhanced in the eutrophied Vistula Estuary compared to the oligotrophic Öre Estuary. No anammox was detected in the sediment of either estuary, while denitrification rates were twice as high in the eutrophied (352±123 µmol N m−2 d−1) as in the oligotrophic estuary. Particulate organic matter (POM) was mainly of phytoplankton origin in the benthic systems of both estuaries. It seemed to control heterotrophic denitrification and ammonium assimilation as well as autotrophic nitrification by functioning as a substrate source of N and organic carbon. Our data suggest that in stratified estuaries, POM is an essential link between riverine N loads and benthic N turnover and may furthermore function as a temporary N reservoir. During long particle residence times or alongshore transport pathways, increased time is available for the recycling of N until its eventual removal, allowing effective coastal filtering even at low process rates. Understanding the key controls and microbial N processes in the coastal N filter therefore requires to also consider the effects of geomorphological and hydrological features.
C. van der Boog, M. F. de Jong, M. Scheidat, M. Leopold, S. Geelhoed, K. Schulz, H. Dijkstra, J. Pietrzak, C. Katsmann (2019). Hydrographic and Biological Survey of a Surface-Intensified Anticyclonic Eddy in the Caribbean Sea. Journal of Geophysical Research: Oceans, 124(8), 6235– 6251. https://doi.org/10.1029/2018JC014877
Abstract
In the Caribbean Sea, mesoscale anticyclonic ocean eddies impact the local ecosystem by mixing of low salinity river outflow with the nutrient-rich waters upwelling along the Venezuelan and Colombian coast. To gain insight into the physics and the ecological impact of these anticyclones, we performed a combined hydrographic and biological survey of one Caribbean anticyclone in February 2018. We found that the anticyclone had a radius of 90 km and was surface intensified with the strongest velocities (0.72 m/s) in the upper 150 m of the water column. Below, isopycnal displacements were found down to 700 dbar. The core of the anticyclone entrained waters from the Orinoco River plume and contained slightly elevated chlorophyll concentrations compared to the surroundings. At the edge of the anticyclone we observed higher densities of flying fish but not higher densities of predators like seabirds and cetaceans. Below the surface, a strong temperature inversion (0.98 °C) was present within a barrier layer. In addition, we found thermohaline staircases that originated from double diffusion processes within Tropical Atlantic Central Water.
In the Caribbean Sea, mesoscale anticyclonic ocean eddies impact the local ecosystem by mixing of low salinity river outflow with the nutrient-rich waters upwelling along the Venezuelan and Colombian coast. To gain insight into the physics and the ecological impact of these anticyclones, we performed a combined hydrographic and biological survey of one Caribbean anticyclone in February 2018. We found that the anticyclone had a radius of 90 km and was surface intensified with the strongest velocities (0.72 m/s) in the upper 150 m of the water column. Below, isopycnal displacements were found down to 700 dbar. The core of the anticyclone entrained waters from the Orinoco River plume and contained slightly elevated chlorophyll concentrations compared to the surroundings. At the edge of the anticyclone we observed higher densities of flying fish but not higher densities of predators like seabirds and cetaceans. Below the surface, a strong temperature inversion (0.98 °C) was present within a barrier layer. In addition, we found thermohaline staircases that originated from double diffusion processes within Tropical Atlantic Central Water.
M. Baptist, T. Gerkema, B.C. van Prooijen, D.S. van Maren, M. van Regteren, K. Schulz, I. Colosimo, J. Vroom, T. van Kessel, B. Grasmeijer, P. Willemsen, K. Elschot, A.V. de Groot, J. Cleveringa, E.E. van Eekelen, F. Schuurman, H.J. de Lange, M.E.B. van Puijenbroek (2019). Beneficial use of dredged sediment to enhance salt marsh development applying a Mud Motor. Ecological Engineering, 127, 312–323. https://doi.org/10.1016/j.ecoleng.2018.11.019
I. Bartl, I. Liskow, K. Schulz, L. Umlauf and M. Voss (2018). River plume and bottom boundary layer – hotspots for nitrification in a coastal bay? Estuarine, Coastal & Shelf Sciences, 208, 70–82. https://doi.org/10.1016/j.ecss.2018.04.023
Abstract
Coastal zones, impacted by major rivers, comprise distinct environments, such as river plumes and bottom boundary layers (BBL). These environments are characterized by high nutrient concentrations and high microbial activities and thus offer favourable conditions for nitrification, a key process in the coastal nitrogen cycle. Because nitrification provides substrates for both primary production and denitrification, elucidation of its magnitude and regulation is crucial for understanding the nitrogen cycle in coastal zones. During three research cruises covering three seasons, the enhancement of nitrification rates and their regulation by environmental variables, including salinity, temperature, oxygen, and inorganic and organic nitrogen were investigated in river plume and BBL of the Vistula Estuary (Bay of Gdansk, Southern Baltic Sea). Nitrification rates were not enhanced in the river plume (39 ± 38 nmol L−1 d−1) compared to coastal surface water (45 ± 18 nmol−1 d−1) but the relationship to salinity and particulate organic nitrogen changed, suggesting different regulatory mechanisms along the salinity gradient. Nitrification rates in the BBL covered a range from 1 to 227 nmol L−1 d−1 and did not differ seasonally. NH4+ turnover was dominated by assimilation into biomass in summer and by nitrification in winter and spring. In summer, rates were only slightly enhanced in the BBL and clearly related to particulate organic nitrogen and carbon concentrations, indicating particle attachment of nitrifiers and close coupling to organic matter degradation. The lack of correlations between nitrification and environmental variables in winter and spring suggested other regulatory mechanisms than in summer. Short-term changes, including the oxygenation of anoxic deep offshore water and particle resuspension clearly enhanced nitrification and further highlighted the variable mechanisms regulating nitrification in the Bay of Gdansk. Although nitrification rates did not greatly differ between seasons or water layers, the variability in regulatory mechanisms and the seasonal switch in NH4+ recycling are likely to have implications on coastal N-turnover and hence on the filter function of coastal waters.
Coastal zones, impacted by major rivers, comprise distinct environments, such as river plumes and bottom boundary layers (BBL). These environments are characterized by high nutrient concentrations and high microbial activities and thus offer favourable conditions for nitrification, a key process in the coastal nitrogen cycle. Because nitrification provides substrates for both primary production and denitrification, elucidation of its magnitude and regulation is crucial for understanding the nitrogen cycle in coastal zones. During three research cruises covering three seasons, the enhancement of nitrification rates and their regulation by environmental variables, including salinity, temperature, oxygen, and inorganic and organic nitrogen were investigated in river plume and BBL of the Vistula Estuary (Bay of Gdansk, Southern Baltic Sea). Nitrification rates were not enhanced in the river plume (39 ± 38 nmol L−1 d−1) compared to coastal surface water (45 ± 18 nmol−1 d−1) but the relationship to salinity and particulate organic nitrogen changed, suggesting different regulatory mechanisms along the salinity gradient. Nitrification rates in the BBL covered a range from 1 to 227 nmol L−1 d−1 and did not differ seasonally. NH4+ turnover was dominated by assimilation into biomass in summer and by nitrification in winter and spring. In summer, rates were only slightly enhanced in the BBL and clearly related to particulate organic nitrogen and carbon concentrations, indicating particle attachment of nitrifiers and close coupling to organic matter degradation. The lack of correlations between nitrification and environmental variables in winter and spring suggested other regulatory mechanisms than in summer. Short-term changes, including the oxygenation of anoxic deep offshore water and particle resuspension clearly enhanced nitrification and further highlighted the variable mechanisms regulating nitrification in the Bay of Gdansk. Although nitrification rates did not greatly differ between seasons or water layers, the variability in regulatory mechanisms and the seasonal switch in NH4+ recycling are likely to have implications on coastal N-turnover and hence on the filter function of coastal waters.
K. Schulz, T. Gerkema (2018). An inversion of the estuarine circulation by sluice water discharge and its impact on sediment transport. Estuarine, Coastal & Shelf Sciences, 200, 31–40. https://doi.org/10.1016/j.ecss.2017.09.031
Abstract
The Wadden Sea is characterized by a complex topography of branching channels and intertidal flats, in which the interplay between fresh water discharges, wind forcing and the tidal current causes sediment transport rates and direction to be highly variable in space and time. During three field campaigns, indications of a negative estuarine circulation have been found in a channel adjacent to the coast in the Western Dutch Wadden Sea. Contrary to the classical picture of estuarine circulation, a periodic density stratification was observed that builds up during flood and breaks down during ebb. This can be related to a large freshwater source at the mouth of the channel, the sluice in Kornwerderzand. In this study, observations of this phenomenon are presented, and with the help of a numerical model the different drivers for residual suspended matter transport in this area, namely tidal asymmetries in the current velocity and the above mentioned periodic stratification, are investigated. It is found that the residual current in the area of interest points in ebb direction, caused by both the elongated ebb flow phase and the periodic stratification. On the contrary, the stronger flood currents cause a transport of suspended matter in flood direction. This transport is counteracted and therefore diminished by the effects of the sluice discharge.
The Wadden Sea is characterized by a complex topography of branching channels and intertidal flats, in which the interplay between fresh water discharges, wind forcing and the tidal current causes sediment transport rates and direction to be highly variable in space and time. During three field campaigns, indications of a negative estuarine circulation have been found in a channel adjacent to the coast in the Western Dutch Wadden Sea. Contrary to the classical picture of estuarine circulation, a periodic density stratification was observed that builds up during flood and breaks down during ebb. This can be related to a large freshwater source at the mouth of the channel, the sluice in Kornwerderzand. In this study, observations of this phenomenon are presented, and with the help of a numerical model the different drivers for residual suspended matter transport in this area, namely tidal asymmetries in the current velocity and the above mentioned periodic stratification, are investigated. It is found that the residual current in the area of interest points in ebb direction, caused by both the elongated ebb flow phase and the periodic stratification. On the contrary, the stronger flood currents cause a transport of suspended matter in flood direction. This transport is counteracted and therefore diminished by the effects of the sluice discharge.
K. Schulz, T. Endoh and L. Umlauf (2017). Slope-induced tidal straining: Analysis of rotational effects. Journal of Geophysical Research: Oceans, 122(3), 2069–2089. https://doi.org/10.1002/2016JC012448
Abstract
Tidal straining is known to be an important factor for the generation of residual currents and transports of suspended matter in the coastal ocean. Recent modeling studies and field experiments have revealed a new type of “slope-induced” tidal straining, in which the horizontal density gradient required for this process is induced by the presence of a slope rather than by river runoff (as in classical tidal straining). Slope-induced tidal straining is investigated here with the help of an idealized numerical model, and results are compared to a recent data set from the East China Sea providing first direct observational evidence. The focus of this study is on the effect of rotation that was ignored in previous investigations. The model is shown to reproduce the key features of the observations, in particular the strain-induced generation of unstable stratification in the bottom boundary layer during periods of upslope flow. Rotation effects are found to significantly reduce the upslope tidal pumping of suspended material and also give rise to a newly identified pumping mechanism that results in a vigorous transport of suspended material along the slope. It is shown that slope-induced tidal straining is likely to be relevant for a wide range of oceanic slopes exposed to tidal motions.
Tidal straining is known to be an important factor for the generation of residual currents and transports of suspended matter in the coastal ocean. Recent modeling studies and field experiments have revealed a new type of “slope-induced” tidal straining, in which the horizontal density gradient required for this process is induced by the presence of a slope rather than by river runoff (as in classical tidal straining). Slope-induced tidal straining is investigated here with the help of an idealized numerical model, and results are compared to a recent data set from the East China Sea providing first direct observational evidence. The focus of this study is on the effect of rotation that was ignored in previous investigations. The model is shown to reproduce the key features of the observations, in particular the strain-induced generation of unstable stratification in the bottom boundary layer during periods of upslope flow. Rotation effects are found to significantly reduce the upslope tidal pumping of suspended material and also give rise to a newly identified pumping mechanism that results in a vigorous transport of suspended material along the slope. It is shown that slope-induced tidal straining is likely to be relevant for a wide range of oceanic slopes exposed to tidal motions.
K. Schulz, and L. Umlauf (2016). Residual transport of suspended material by tidal straining near sloping topography. Journal of Physical Oceanography, 46(7), 2083 – 2102. https://doi.org/10.1175/JPO-D-15-0218.1
Abstract
Tidal straining is known to have an important impact on the generation of residual currents and the transport of suspended material in estuaries and the coastal ocean. Essential for this process is an externally imposed horizontal density gradient, typically resulting from either freshwater runoff or differential heating. Here, it is shown that near sloping topography, tidal straining may effectively transport suspended material across isobaths even if freshwater runoff and differential heating do not play a significant role. A combined theoretical and idealized modeling approach is used to illustrate the basic mechanisms and implications of this new process. The main finding of this study is that, for a wide range of conditions, suspended material is transported upslope by a pumping mechanism that is in many respects similar to classical tidal pumping. Downslope transport may also occur, however, only for the special cases of slowly sinking material in the vicinity of slopes with a slope angle larger than a critical threshold. The effective residual velocity at which suspended material is transported across isobaths is a significant fraction of the tidal velocity amplitude (up to 40% in some cases), suggesting that suspended material may be transported over large distances during a single tidal cycle.
Tidal straining is known to have an important impact on the generation of residual currents and the transport of suspended material in estuaries and the coastal ocean. Essential for this process is an externally imposed horizontal density gradient, typically resulting from either freshwater runoff or differential heating. Here, it is shown that near sloping topography, tidal straining may effectively transport suspended material across isobaths even if freshwater runoff and differential heating do not play a significant role. A combined theoretical and idealized modeling approach is used to illustrate the basic mechanisms and implications of this new process. The main finding of this study is that, for a wide range of conditions, suspended material is transported upslope by a pumping mechanism that is in many respects similar to classical tidal pumping. Downslope transport may also occur, however, only for the special cases of slowly sinking material in the vicinity of slopes with a slope angle larger than a critical threshold. The effective residual velocity at which suspended material is transported across isobaths is a significant fraction of the tidal velocity amplitude (up to 40% in some cases), suggesting that suspended material may be transported over large distances during a single tidal cycle.
Data Sets
Schulz, Kirstin; Mohrholz, Volker; Fer, Ilker; Janout, Markus A; Hoppmann, Mario; Schaffer, Janin; Koenig, Zoé; Rabe, Benjamin; Heuzé, Céline; Regnery, Julia; Allerholt, Jacob; Fang, Ying-Chih; He, Hailun; Kanzow, Torsten; Karam, Salar; Kuznetsov, Ivan; Kong, Bin; Liu, Hailong; Muilwijk, Morven; Schuffenhauer, Ingo; Sukhikh, Natalia; Sundfjord, Arild; Tippenhauer, Sandra (2022): Turbulent microstructure profile (MSS) measurements from the MOSAiC drift, Arctic Ocean. PANGAEA
Abstract
During the year-long drift expedition MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) from September 2019 to September 2020, we obtained an unprecedented data set of vertical turbulent dissipation rate profiles and high resolved hydrodynamic properties, including oxygen concentration and fluorescence, also covering the winter season. Nearly 1,700 individual profiles, covering the upper ocean down to approximately 400~m, were collected on a near-daily base and complemented with several periods of intensified continuous sampling.
During the year-long drift expedition MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) from September 2019 to September 2020, we obtained an unprecedented data set of vertical turbulent dissipation rate profiles and high resolved hydrodynamic properties, including oxygen concentration and fluorescence, also covering the winter season. Nearly 1,700 individual profiles, covering the upper ocean down to approximately 400~m, were collected on a near-daily base and complemented with several periods of intensified continuous sampling.
Baumann, Till; Fer, Ilker; Bryhni, Helge; Peterson, Algot K; Allerholt, Jacob; Fang, Ying-Chih; Hoppmann, Mario; Karam, Salar; Koenig, Zoé; Kong, Bin; Mohrholz, Volker; Muilwijk, Morven; Schaffer, Janin; Schulz, Kirstin; Sukhikh, Natalia; Tippenhauer, Sandra (2021): Under-ice current measurements during MOSAiC from a 75 kHz acoustic Doppler profiler. PANGAEA
Abstract
Horizontal ocean currents were measured from drifting sea ice, using an RD-Instruments 75 kHz ADCP (acoustic Doppler current profiler), Longranger, deployed pointing downward through an hydrohole on ice for all three drifts of the MOSAiC expedition between 30 November 2019 and 19 September 2020. Due to the unreliability of magnetic compasses at high latitudes, a GPS compass was used and the current profiles were recorded in beam coordinates. Geo-referenced, eastward and northward velocity components in the upper 500 m were obtained during post processing.
Horizontal ocean currents were measured from drifting sea ice, using an RD-Instruments 75 kHz ADCP (acoustic Doppler current profiler), Longranger, deployed pointing downward through an hydrohole on ice for all three drifts of the MOSAiC expedition between 30 November 2019 and 19 September 2020. Due to the unreliability of magnetic compasses at high latitudes, a GPS compass was used and the current profiles were recorded in beam coordinates. Geo-referenced, eastward and northward velocity components in the upper 500 m were obtained during post processing.
Janout, Markus; Tippenhauer, Sandra; Schulz, Kirstin; Ivanov, Vladimir; Polyakov, Igor (2020): Microstructure measurements during Akademik Tryoshnikov cruise AT2018 to the Arctic Ocean. PANGAEA
Abstract
Shipboard loosely-tethered free-falling microstructure (MSS) measurements were carried out during expedition Transdrift-XXIV to the eastern Arctic Ocean onboard the Akademik Tryoshnikov (AT2018). The expedition was jointly organized between the US-Russian NABOS (Nansen and Amundsen Basin Observational System), the German-Russian CATS (Changing Arctic Transpolar System, funded by BMBF), and the TICE-project funded by the Alfred-Wegener-Institute. 236 stations were carried out between 25 August and 23 September 2018. The profiler MSS90L manufactured by Sea and Sun Technology samples at 1024 Hz and was equipped with temperature, salinity, shear, and fluorescence sensors.
Shipboard loosely-tethered free-falling microstructure (MSS) measurements were carried out during expedition Transdrift-XXIV to the eastern Arctic Ocean onboard the Akademik Tryoshnikov (AT2018). The expedition was jointly organized between the US-Russian NABOS (Nansen and Amundsen Basin Observational System), the German-Russian CATS (Changing Arctic Transpolar System, funded by BMBF), and the TICE-project funded by the Alfred-Wegener-Institute. 236 stations were carried out between 25 August and 23 September 2018. The profiler MSS90L manufactured by Sea and Sun Technology samples at 1024 Hz and was equipped with temperature, salinity, shear, and fluorescence sensors.
Korte, Laura F; Schulz, Kirstin; Mienis, Furu; van Oevelen, Dick; Mohn, Christian; Reichart, Gert-Jan; Duineveld, Gerard C A (2020): Long-term moorings observations in the southwestern Rockall Trough (Logachev cold-water coral mound province). PANGAEA / dataset in review
Abstract
Two long-term moorings were deployed in the Logachev mound province from May 2017-May 2018 recording physical parameters and collecting particle fluxes. Moorings were deployed on the relatively shallow Rockall Bank and the deep Oreo Mound. In addition, CTD water-profiling was conducted during Pelagia cruise 64PE420 in May 2017 and Pelagia cruise 64PE436 in May 2018.
Two long-term moorings were deployed in the Logachev mound province from May 2017-May 2018 recording physical parameters and collecting particle fluxes. Moorings were deployed on the relatively shallow Rockall Bank and the deep Oreo Mound. In addition, CTD water-profiling was conducted during Pelagia cruise 64PE420 in May 2017 and Pelagia cruise 64PE436 in May 2018.
Abstract
These data sets were obtained in the context of the STW project “Sediment for salt marshes: physical and ecological aspects of a Mud Motor” with Project Number 13888, which is partly financed by The Netherlands Organisation for Scientific Research (NWO). The data set consists of 8 netcdf-files named cruiseXdeployment1/2.nc and 4 netcdf-files named filtration_cruiseX.nc. Each of the first 8 netcdf-files contains a 12.5h record of vertical profiles of current velocities, temperature, salinity, optical backscatter and the suspended particulate matter (SPM) concentrations derived from the optical backscatter using filtration samples. All data is interpolated to the same temporal and vertical grid. Measurements were carried out either near the port of Harlingen (deployment 1 on cruises 1-3, deployment 2 on cruise 4) or at the end of the Kimstergat channel (deployment 2 on cruise 1-3, deployment 1 on cruise 4). The files filtration_cruiseX.nc contain the raw data of the filtration: dry and wet weight of the filters, filtrated volume, corresponding optical backscatter values and multiplication factors for the OBS in the box on the ship. The intercalibration between the OBS in the box and the in-situ OBS (mounted on a frame) is: OBS_box = 2.01*OBS_frame - 0.01. Details on the data acquisition and postprocessing (for the first three cruises) can be found in Schulz, K., Gerkema, T., 2018. An inversion of the estuarine circulation by sluice water discharge and its impact on suspended sediment transport. Estuar. Coast. Shelf Sci. 200, 31–40. Please note that this publication contains an error: SPM concentrations were underestimated by a factor of 4 for cruises 1 and 2 (SPM_correct = 4* SPM_publication) and by a factor of 5 for cruise 3 (SPM_correct = 5* SPM_publication). The SPM concentrations in the netcdf-files are corrected. A corrigendum will be published soon. Raw data is available upon request.
These data sets were obtained in the context of the STW project “Sediment for salt marshes: physical and ecological aspects of a Mud Motor” with Project Number 13888, which is partly financed by The Netherlands Organisation for Scientific Research (NWO). The data set consists of 8 netcdf-files named cruiseXdeployment1/2.nc and 4 netcdf-files named filtration_cruiseX.nc. Each of the first 8 netcdf-files contains a 12.5h record of vertical profiles of current velocities, temperature, salinity, optical backscatter and the suspended particulate matter (SPM) concentrations derived from the optical backscatter using filtration samples. All data is interpolated to the same temporal and vertical grid. Measurements were carried out either near the port of Harlingen (deployment 1 on cruises 1-3, deployment 2 on cruise 4) or at the end of the Kimstergat channel (deployment 2 on cruise 1-3, deployment 1 on cruise 4). The files filtration_cruiseX.nc contain the raw data of the filtration: dry and wet weight of the filters, filtrated volume, corresponding optical backscatter values and multiplication factors for the OBS in the box on the ship. The intercalibration between the OBS in the box and the in-situ OBS (mounted on a frame) is: OBS_box = 2.01*OBS_frame - 0.01. Details on the data acquisition and postprocessing (for the first three cruises) can be found in Schulz, K., Gerkema, T., 2018. An inversion of the estuarine circulation by sluice water discharge and its impact on suspended sediment transport. Estuar. Coast. Shelf Sci. 200, 31–40. Please note that this publication contains an error: SPM concentrations were underestimated by a factor of 4 for cruises 1 and 2 (SPM_correct = 4* SPM_publication) and by a factor of 5 for cruise 3 (SPM_correct = 5* SPM_publication). The SPM concentrations in the netcdf-files are corrected. A corrigendum will be published soon. Raw data is available upon request.
Umlauf, Lars; Heene, Toralf; Weinreben, Stefan; Schulz, Kirstin: Physical oceanographical data from cruise MSM50 in the Western Baltic Sea. Leibniz-Institute for Baltic Sea Research
Abstract
This data set includes all physical oceanographical measurements made during cruise MSM50 in January 2016 in the Western Baltic Sea. The data file (zip format) unpacks in a self-explanatory directory structure, which contains (a) ship-based measurements, (b) measurements from moored instrumentation, and (c) supplementary data. The ship-based measurements include: CTD measurements, measurements with a turbulence microstructure profiler, meteorological data, thermosalinograph data. Moorings were deployed at six stations in the Western Baltic Sea. Mooring data include data from CTD loggers, turbidity sensors, and high-resolution acoustic current meters (ADCPs, ADVs). The cruise report, which is also part of this data set, contains exact instrument specifications, drawings of all moorings, and tables with times, positions, and stations names. Keywords: Baltic Sea, CTD, currents, turbulence, microstructure, turbidity
This data set includes all physical oceanographical measurements made during cruise MSM50 in January 2016 in the Western Baltic Sea. The data file (zip format) unpacks in a self-explanatory directory structure, which contains (a) ship-based measurements, (b) measurements from moored instrumentation, and (c) supplementary data. The ship-based measurements include: CTD measurements, measurements with a turbulence microstructure profiler, meteorological data, thermosalinograph data. Moorings were deployed at six stations in the Western Baltic Sea. Mooring data include data from CTD loggers, turbidity sensors, and high-resolution acoustic current meters (ADCPs, ADVs). The cruise report, which is also part of this data set, contains exact instrument specifications, drawings of all moorings, and tables with times, positions, and stations names. Keywords: Baltic Sea, CTD, currents, turbulence, microstructure, turbidity