| 1. |
- Arunraj, Kondetharayil Soman, et al.
(författare)
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Linking Coherent Anticyclonic Eddies in the Iceland Basin to Decadal Oceanic Variability in the Subpolar North Atlantic
- 2022
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Ingår i: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 127:5
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Tidskriftsartikel (refereegranskat)abstract
- The Iceland Basin in the eastern Subpolar North Atlantic is an eddy-rich region characterized by intense anticyclonic eddy activity. Our study present the variability of coherent Anticyclonic Eddies (AEs) generated in this region, using satellite altimetry and two ocean eddy tracking algorithms. The yearly count of AEs in the Iceland Basin reveals a decadal variability similar to that of ocean heat content change in the eastern subpolar gyre. Periods with higher number of AEs coincide with periods of increased ocean heat content, and vice versa. However, both algorithms agree that more than 50% of the detected AEs are confined to the central Iceland Basin. The annual number of AEs also tracks zonal shifts of the subpolar front, a variable that can explain about 53 (77)% of the interannual (decadal) variability of AEs in the Iceland Basin. Finally, a Lagrangian approach is used to demonstrate that the amount of subtropical versus subpolar water masses reaching the Iceland Basin appears to influence, via baroclinic instability, the generation of AEs.
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| 2. |
- Berglund, Sara, et al.
(författare)
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The Water Mass Transformation in the Upper Limb of the Overturning Circulation in the Southern Hemisphere
- 2021
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Ingår i: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 126:8
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Tidskriftsartikel (refereegranskat)abstract
- The warming and salinification of the northwards flowing water masses from the Southern Ocean to the tropics are studied with Lagrangian trajectories simulated using fields from an Earth System Model. The trajectories are used to trace the geographical distribution of the water mass transformation and connect it with the pathways of the upper limb of the overturning circulation in the Southern Hemisphere. In the Antarctic Circumpolar Current water gains heat just below the mixed layer, mainly when the layer is thin during Austral spring and summer. This gain is therefore suggested to be a consequence of heat flux from the atmosphere and mixing processes at the base of the mixed layer. In the Southern Hemispheric subtropical gyres on the other hand, a large warming and salinification of the northwards flowing water results from internal mixing with other warmer and more saline water masses. Close to the Antarctic shelf waters are getting fresher as a result of ice melting, whereas further north, in the Antarctic Circumpolar current, waters are getting more saline as a result of evaporation. Our results show that it is not only the heat and freshwater fluxes through the sea surface that control the heat and salt changes of the upper limb of the overturning circulation in the Southern Hemisphere. In fact, internal mixing accounts for 25% of the heat change, and 22% of the salinity change.
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| 3. |
- Broomé, Sara, et al.
(författare)
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A Satellite-Based Lagrangian Perspective on Atlantic Water Fractionation Between Arctic Gateways
- 2021
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Ingår i: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 126:11
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Tidskriftsartikel (refereegranskat)abstract
- Warm Atlantic Water reaches the Arctic Ocean via two gateways: the Barents Sea Opening (BSO) and Fram Strait. Here, we study the near-surface flow of the Atlantic Water in the Nordic Seas and its fractionation between these Arctic gateways, using simulated Lagrangian trajectories based on satellite altimetry for 1994–2018. Lagrangian particles are released in the eastern Nordic Seas, where Atlantic Water flows poleward in two current cores: an inner branch along the Norwegian Continental Slope and an outer sea ward branch. The trajectories toward Fram Strait and the BSO are, in an averaged sense, largely steered by the bottom topography, and on inter-annual timescales we find an anticorrelation in the number of particles that reach the two gateways. Most of the particles released in the inner branch enter the Barents Sea and most of the particles seeded in the outer branch reach Fram Strait. However, there is a significant cross-over of particles from the outer to the inner branch in the Lofoten Basin, and nearly half of the total number of particles entering the BSO originate in the outer branch. This cross-over is accomplished solely by the time-fluctuating part of the velocity field, and it becomes stronger when the eddy kinetic energy in the Lofoten Basin is anomalously high. Thus, the outer branch may, via processes in the Lofoten Basin, be important for Barents Sea climate variability.
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| 4. |
- Chafik, Léon, 1985-, et al.
(författare)
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The Faroe‐Shetland Channel Jet : Structure, Variability, and Driving Mechanisms
- 2023
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Ingår i: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 128:4
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Tidskriftsartikel (refereegranskat)abstract
- The Faroe-Bank Channel (FBC) is a key gateway through which dense overflow water of the Nordic Seas supplies the lower limb of the Atlantic Meridional Overturning Circulation. Most recently, it was discovered that a deep jet through the Faroe-Shetland Channel carries the bulk of this overflow water, but numerous questions regarding its structure, seasonality, and interannual variability as well as its linkage to atmospheric forcing remain poorly understood. A realistic high-resolution ocean reanalysis (GLORYS12; 1993–2018) is, therefore, employed to address these questions. We first confirm that the Faroe-Shetland Channel Jet is a permanent feature in GLORYS12 as well as in an ensemble of low-resolution reanalyses. On seasonal time scales, we find a strong transport covariability between this deep jet and the observed FBC overflow. On interannual time scales, the strength of this deep jet is governed by the wind-forced circulation in the Nordic Seas. Due to the largely barotropic structure of these flows, they have a signature detectable in satellite sea-surface heights. Further, we suggest that the structure of the deep jet is qualitatively consistent with a geostrophic dynamical model that accounts for along-isobath density variations. This study indicates that GLORYS12 is a promising product to study the dense water pathways and dynamics in the Nordic Seas.
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| 5. |
- Chamizo, E., et al.
(författare)
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The Potential of U-233/U-236 as a Water Mass Tracer in the Arctic Ocean
- 2022
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Ingår i: Journal of Geophysical Research - Oceans. - : Wiley-Blackwell Publishing, Inc.. - 2169-9275 .- 2169-9291. ; 127:3
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Tidskriftsartikel (refereegranskat)abstract
- This study explores for the first time the possibilities that the U-233/U-236 atom ratio offers to distinguish waters of Atlantic or Pacific origin in the Arctic Ocean. Atlantic waters entering the Arctic Ocean often carry an isotopic signature dominantly originating from European reprocessing facilities with some smaller contribution from global fallout nuclides, whereas northern Pacific waters are labeled with nuclides released during the atmospheric nuclear testing period only. In the Arctic Ocean, U-233 originates from global fallout while U-236 carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the U-233/U-236 ratio provides a tool to identify water masses with distinct U sources. In this work, U-233 and U-236 were analyzed in samples from the GN01 GEOTRACES expedition to the western Arctic Ocean in 2015. The study of depth profiles and surface seawater samples shows that: (a) Pacific and Atlantic waters show enhanced signals of both radionuclides, which can be unraveled based on their U-233/U-236 signature; and (b) Deep and Bottom Waters show extremely low U-233 and U-236 concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high U-233/U-236 ratios are interpreted as a relative increase of naturally occurring U-233 and U-236 and thus for gradually reaching natural U-233/U-236 levels in the deep Arctic Ocean. Our results set the basis for future studies using the U-233/U-236 ratio to distinguish anthropogenic and pre-anthropogenic U in the Arctic Ocean and beyond.
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| 6. |
- Charette, M. A., et al.
(författare)
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The Transpolar Drift as a Source of Riverine and Shelf-Derived Trace Elements to the Central Arctic Ocean
- 2020
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Ingår i: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 125:5
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Tidskriftsartikel (refereegranskat)abstract
- A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river-influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high-resolution pan-Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and similar to 25-50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle-reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the open ocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 +/- 0.4 Sv (10(6) m(3)s(-1)). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologic cycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean. Plain Language Summary A major feature of the Arctic Ocean circulation is the Transpolar Drift (TPD), a surface current that carries ice and continental shelf-derived materials from Siberia across the North Pole to the North Atlantic Ocean. In 2015, an international team of oceanographers conducted a survey of trace elements in the Arctic Ocean, traversing the TPD. Near the North Pole, they observed much higher concentrations of trace elements in surface waters than in regions on either side of the current. These trace elements originated from land, and their journey across the Arctic Ocean is made possible by chemical reactions with dissolved organic matter that originates mainly in Arctic rivers. This study reveals the importance of rivers and shelf processes combined with strong ocean currents in supplying trace elements to the central Arctic Ocean and onward to the Atlantic. These trace element inputs are expected to increase as a result of permafrost thawing and increased river runoff in the Arctic, which is warming at a rate much faster than anywhere else on Earth. Since many of the trace elements are essential building blocks for ocean life, these processes could lead to significant changes in the marine ecosystems and fisheries of the Arctic Ocean.
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| 7. |
- Charette, M, et al.
(författare)
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The Transpolar Drift as a Source of Riverine and Shelf‐Derived Trace Elements to the Central Arctic Ocean
- 2020
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Ingår i: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 125, s. 1-34
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Tidskriftsartikel (refereegranskat)abstract
- A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river‐influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high‐resolution pan‐Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and ~25–50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle‐reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the openocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 ± 0.4 Sv(106m3 s−1). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologicc ycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean.
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| 8. |
- Chen, Sheng, et al.
(författare)
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On the First Observed Wave-Induced Stress Over the Global Ocean
- 2020
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Ingår i: Journal of Geophysical Research - Oceans. - : AMER GEOPHYSICAL UNION. - 2169-9275 .- 2169-9291. ; 125:12
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Tidskriftsartikel (refereegranskat)abstract
- Despite many investigations/studies on the surface wave-induced stress, the global feature of the wave-induced stress has not been obtained previously as that requires a simultaneous observation of wave spectra and wind on a global scale. The China France Oceanography Satellite (CFOSAT) provided an opportunity for the first time to evaluate the global wave-induced stress and its contribution to the total wind stress. In this study, the global spatial distributions of wave-induced stress and its correlated index for August to November in 2019 are presented using the simultaneous ocean surface winds and wave spectra from the CFOSAT. The main results show that the wave-induced stress is fundamentally dependent on the wind and wave fields on a global scale and shows significant temporal and spatial variations. Further analyses indicate that there is an upward momentum flux under strong swells and low wind speeds (below similar to 5 m/s), and an anticorrelation between the dimensionless wave-induced stress and the proportion of swell energy to the total. Finally, the variations of the surface wave induced wind stress are clear asymmetric between northern and southern hemispheres in late summer but symmetric in late fall, which are closely associated with the seasonal changes in large-scale atmospheric circulation.
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| 9. |
- Dotto, Tiago S., et al.
(författare)
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Control of the Oceanic Heat Content of the Getz-Dotson Trough, Antarctica, by the Amundsen Sea Low
- 2020
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Ingår i: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291. ; 125:8
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Tidskriftsartikel (refereegranskat)abstract
- The changing supply of warm Circumpolar Deep Water (CDW) to the West Antarctic continental shelf is responsible for the basal melting and thinning of the West Antarctic ice shelves that has occurred in recent decades. Here we assess the variability in CDW supply, and its drivers, from a multiyear mooring deployed in, and a regional ocean model spanning, the Getz-Dotson Trough, Amundsen Sea. Between 2010 to 2015, the CDW within the trough underwent a pronounced cooling and freshening, associated with changes in thermohaline properties on isopycnals. Variability in the rate of CDW inflow is controlled by local wind forcing of a shelf break undercurrent, which determines the hydrographic properties of inflowing CDW via tilting of density surfaces above the continental slope. Local wind is coupled to the Amundsen Sea Low (ASL) low-pressure system, which is modulated by large-scale climatic modes via atmospheric teleconnections. For the period analyzed, a deeper ASL was associated with westward wind anomaly at the shelf break. Changes in the sea surface slope decelerated the shelf break undercurrent, resulting in less heat accessing the continental shelf and, consequently, a cooling of the Getz-Dotson Trough. Therefore, the present work suggests that the fate of the West Antarctic ice shelves is closely tied to the future evolution of the ASL.
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| 10. |
- du Plessis, Marcel, 1990, et al.
(författare)
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The Daily-Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations
- 2022
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Ingår i: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 127:4
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Tidskriftsartikel (refereegranskat)abstract
- Water mass transformation in the Southern Ocean is vital for driving the large-scale overturning circulation, which transports heat from the surface to the ocean interior. Using profiling gliders, this study investigates the role of summertime buoyancy forcing and wind-driven processes on the intraseasonal (1-10 days) mixed layer thermohaline variability in three Southern Ocean regions southwest of Africa important for water mass transformation-the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), and Marginal Ice Zone (MIZ). At intraseasonal time scales, heat flux was shown as the main driver of buoyancy gain in all regions. In the SAZ and MIZ, shallow mixed layers and strong stratification enhanced mixed layer buoyancy gain by trapping incoming heat, while buoyancy loss resulted primarily from the entrainment of cold, salty water from below. In the PFZ, rapid mixing linked to Southern Ocean storms set persistently deep mixed layers and suppressed mixed layer intraseasonal thermohaline variability. In the polar regions, lateral stirring of meltwater from seasonal sea-ice melt dominated daily mixed layer salinity variability. We propose that these meltwater fronts are advected to the PFZ during late summer, indicating the potential for seasonal sea-ice freshwater to impact a region where the upwelling limb of overturning circulation reaches the surface. This study reveals a regional dependence of how the mixed layer thermohaline properties respond to small spatiotemporal processes, emphasizing the importance of surface forcing occurring between 1 and 10 days on the mixed layer water mass transformation in the Southern Ocean.
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