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  • 1
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    The Deep‐Water Plume in the Northwestern Weddell Sea, Antarctica: Mean State, Seasonal Cycle and Interannual Variability Influenced by Climate Modes (2023)
    Details
    Publication Date: 2023-06-21
    Description: We provide an updated estimate of the annual‐mean, seasonal cycle and interannual variability of the transports and properties of the Weddell Sea Bottom Water (WSBW) plume in the northwestern Weddell Sea. For this we used a densely instrumented mooring array deployed across the continental slope between January 2017 and January 2019. We found that the annual‐mean WSBW transport is 3.4 ± 1.5 Sv, corresponding to a cross‐section area of 35 km2 and a maximum thickness of 203 m. The annual mean transport‐weighted properties of WSBW are −0.99°C (Θ), 34.803 g/kg (SA) and 28.44 kg/m3 (γn). The WSBW is characterized by 3 bottom‐intensified velocity cores, which display seasonal variations in flow speed and transport different varieties of WSBW. The seasonal peak of WSBW transport and density is reached in May (4.7 Sv, 28.443 kg m−3) while the minimum values are observed in February (2.8 Sv, 28.435 kg m−3). The coldest WSBW is found between March and May, and the warmest between August and October. The density decrease of WSBW observed in the austral autumn of 2018 can be explained by warmer ambient waters being entrained during the formation of WSBW. This was enabled by the weakening of the along‐shore winds associated with a positive Southern Annular Mode index, reinforced by a La Niña event in early 2018. The synchronous decrease of total WSBW transport and volume between September 2018 and February 2019 indicates a reduction in the export of the dense precursors of WSBW from the Weddell Sea continental shelf.
    Description: Plain Language Summary: The Meridional Overturning Circulation (MOC) redistributes heat and carbon dioxide in the world ocean. Thus, it plays an important role in the regulation of our planet's climate. The Weddell Sea is the main contributor to the deep branch of the MOC in the Southern Hemisphere. Despite the importance of this contribution, uncertainties still remain associated to the plume of dense waters transported along the continental slope of the Weddell Sea. To reduce these uncertainties, we analyzed the most densely instrumented mooring array deployed across the continental slope in the northwestern Weddell Sea. We found that this plume flows faster close to the seafloor and that it presents important seasonal and interannual variability. The Weddell Sea Bottom Water interannual variability is influenced by changes in the along‐shore winds driven by the phase of two important climate modes, the Southern Annular Mode and the El Niño‐Southern Oscillation, but also by changes in the export of the dense precursors of WSBW in its formation areas. Increasing our knowledge on the along‐slope plume variability and properties is important to better understand the causes behind the variability of the MOC observed further downstream.
    Description: Key Points: The Weddell Sea Bottom Water (WSBW) plume presents 3 velocity cores and a clear seasonal cycle, with maximum transports and densities in May and minimum in February. A +SAM, reinforced by a ‐ENSO, favors the warming of WSBW via a wind‐driven warming of the ambient waters entrained during its formation. We observed a marked decrease in WSBW density and transports between September 2018 and February 2019 compared to the previous year.
    Description: EU Horizon 2020 Research and Innovation Program
    Description: German Research Foundation
    Description: Alfred Wegener Institute Helmholtz‐Center
    Description: https://doi.org/10.5281/zenodo.7500163
    Keywords: ddc:551.46 ; Weddell Sea ; WSBW ; Meridional Overturning Circulation ; SAM ; ENSO ; deep‐water plume
    Language: English
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  • 2
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    Impact of Cyclonic Wind Anomalies Caused by Massive Winter Sea Ice Retreat in the Barents Sea on Atlantic Water Transport Toward the Arctic: A Model Study (2023)
    Details
    Publication Date: 2023-12-05
    Description: The Arctic is warming much faster than the global average. This is known as Arctic Amplification and is caused by feedbacks in the local climate system. In this study, we explore a previously proposed hypothesis that an associated wind feedback in the Barents Sea could play an important role by increasing the warm water inflow into the Barents Sea. We find that the strong recent decrease in Barents Sea winter sea ice cover causes enhanced ocean‐atmosphere heat flux and a local air temperature increase, thus a reduction in sea level pressure and a local cyclonic wind anomaly with eastward winds in the Barents Sea Opening. By investigating various reanalysis products and performing high‐resolution perturbation experiments with the ocean and sea ice model FESOM2.1, we studied the impact of cyclonic atmospheric circulation changes on the warm Atlantic Water import into the Arctic via the Barents Sea and Fram Strait. We found that the observed wind changes do not significantly affect the warm water transport into the Barents Sea, which rejects the wind‐feedback hypothesis. At the same time, the cyclonic wind anomalies in the Barents Sea increase the amount of Atlantic Water recirculating westwards in Fram Strait by a downslope shift of the West Spitsbergen Current, and thus reduce Atlantic Water reaching the Arctic basin via Fram Strait. The resulting warm‐water anomaly in the Greenland Sea Gyre drives a local anticyclonic circulation anomaly.
    Description: Plain Language Summary: The Barents Sea has been experiencing a rapid decrease in its winter sea ice extent during the last 30 years. The loss of sea ice creates new areas where, in winter, the relatively warm ocean loses heat to the cold atmosphere. As warm air rises, the warming reduces the sea level air pressure, changing the atmospheric circulation to develop a local anticlockwise wind system centered over the northern Barents Sea. The associated eastward winds in the Barents Sea Opening and southeastward winds in Fram Strait affect how warm water from the North Atlantic moves toward the Arctic. There has been a long debate on whether this wind anomaly can increase the warm Atlantic Water transport into the Barents Sea and thus cause a positive feedback mechanism for further reducing the sea ice through melting. We find that the observed atmospheric circulation changes have no significant impact on the Barents Sea warm water inflow and thus reject the wind feedback as a strong player in contributing to Arctic Amplification. However, strong anomalous southeastward winds in Fram Strait and the northern Nordic Seas cause a southward shift of the warm Atlantic Water recirculation and reduce its flow toward the Arctic.
    Description: Key Points: A hypothesis that a wind feedback contributes to Arctic Amplification is rejected by performing dedicated wind perturbation simulations. Winter sea ice retreat in the northern Barents Sea causes anomalous cyclonic winds by locally enhancing ocean heat loss. Anomalous cyclonic winds result in less Atlantic Water transport through Fram Strait.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: North‐German Supercomputing Alliance
    Description: https://github.com/FESOM/fesom2
    Description: https://doi.org/10.7265/N5K072F8
    Description: https://doi.org/10.5065/D6HH6H41
    Description: https://doi.org/10.5065/D6WH2N0S
    Description: https://github.com/FESOM/pyfesom2
    Description: https://doi.org/10.5281/zenodo.7458143
    Keywords: ddc:551 ; Barents Sea ; Arctic Amplification ; feedback ; Atlantic water ; modeling ; Fram Strait
    Language: English
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  • 3
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    Structure and Seasonal Variability of the Arctic Boundary Current North of Severnaya Zemlya (2023)
    Details
    Publication Date: 2023-11-13
    Description: 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〈sup〉−1〈/sup〉) of the ABC occurred at the upper continental slope and decreased offshore to below 0.03 m s〈sup〉−1〈/sup〉 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.
    Description: Plain Language Summary: We assessed the structure and seasonal variability of the flow and water masses of the Arctic Boundary Current (ABC) in the region north of Severnaya Zemlya. This current is important in the Arctic Ocean as it transports relatively warm and saline waters along the Eurasian Arctic continental slope. We quantified the flow, transport and hydrographic variability of the ABC. Compared to observations from upstream, our results indicate that the water masses away from the shelf break maintained the hydrographic characteristics from upstream. In contrast, the water masses near the shelf break were significantly cooled and freshened due to intrusion of‐ and mixing with shelf waters. The water masses near the shelf break showed a seasonal signal in volume transport and temperature which derives from local shelf processes, advection of seasonal‐variable waters along the ABC pathway and the seasonal cooling of the Barents Sea. If the warming trend in the Barents Sea continues, warmer waters are expected to be advected eastward along the Eurasian continental slope by the ABC.
    Description: Key Points: We quantify the Arctic Boundary Current (ABC) transport north of Severnaya Zemlya with a 2015–2018 mooring array. Hydrographic changes along the ABC pathway are most prominent at the continental slope due to the interaction with shelf water. Seasonality of water masses from the shelf sea was observed in transport, temperature and off‐shelf excursions within the ABC.
    Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347
    Description: EC Horizon 2020 Framework Programme http://dx.doi.org/10.13039/100010661
    Description: Russian Science Foundation http://dx.doi.org/10.13039/501100006769
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: https://doi.pangaea.de/10.1594/PANGAEA.951363
    Description: https://doi.pangaea.de/10.1594/PANGAEA.951394
    Description: https://doi.pangaea.de/10.1594/PANGAEA.951394
    Description: https://doi.pangaea.de/10.1594/PANGAEA.954244
    Description: https://doi.pangaea.de/10.1594/PANGAEA.954249
    Description: https://doi.pangaea.de/10.1594/PANGAEA.954299
    Description: https://doi.pangaea.de/10.1594/PANGAEA.954352
    Keywords: ddc:551.48 ; Arctic Boundary Current ; seasonal transport variability ; water mass transport ; along‐slope current
    Language: English
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  • 4
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    Enhanced turbulence driven by mesoscale motions and flow-topography interaction in the Denmark Strait Overflow plume (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 121 (10). pp. 7650-7672.
    Details
    Publication Date: 2019-09-23
    Description: The Denmark Strait Overflow (DSO) contributes roughly half to the total volume transport of the Nordic overflows. The overflow increases its volume by entraining ambient water as it descends into the subpolar North Atlantic, feeding into the deep branch of the Atlantic Meridional Overturning Circulation. In June 2012, a multiplatform experiment was carried out in the DSO plume on the continental slope off Greenland (180 km downstream of the sill in Denmark Strait), to observe the variability associated with the entrainment of ambient waters into the DSO plume. In this study, we report on two high-dissipation events captured by an autonomous underwater vehicle (AUV) by horizontal profiling in the interfacial layer between the DSO plume and the ambient water. Strong dissipation of turbulent kinetic energy of O( math formula) W kg−1 was associated with enhanced small-scale temperature variance at wavelengths between 0.05 and 500 m as deduced from a fast-response thermistor. Isotherm displacement slope spectra reveal a wave number-dependence characteristic of turbulence in the inertial-convective subrange ( math formula) at wavelengths between 0.14 and 100 m. The first event captured by the AUV was transient, and occurred near the edge of a bottom-intensified energetic eddy. Our observations imply that both horizontal advection of warm water and vertical mixing of it into the plume are eddy-driven and go hand in hand in entraining ambient water into the DSO plume. The second event was found to be a stationary feature on the upstream side of a topographic elevation located in the plume pathway. Flow-topography interaction is suggested to drive the intense mixing at this site.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 5
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    Overflow induced turbulence in a deep ocean channel (2016)
    In:  [Talk] In: Ocean Sciences Meeting 2016, 21.-26.02.2016, New Orleans, USA .
    Details
    Publication Date: 2017-02-24
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
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    One stop shop web site for TMAs (2017)
    AtlantOS
    In:  AtlantOS Deliverable, D3.1 . AtlantOS, 5 pp.
    Details
    Publication Date: 2019-03-11
    Description: Establishment of community shared “one stop shop” web site for TMAs where 1) the TMA network as a whole is introduced (partners, common themes, data products); 2) High-level TMA products (volume transport, heat transport) can be obtained; 3) documentation (calculation, error bars) of the products is provided; 4) links to data behind the high-level products (OCEAN SITES); 5) joint cross-TMA network analyses are provided. To achieve this deliverable one workshop will be held (in project month 8).
    Type: Report , NonPeerReviewed , info:eu-repo/semantics/book
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  • 7
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    Submesoscale CO2 variability across an upwelling front off Peru (2017)
    Copernicus Publications (EGU)
    In:  Ocean Science, 13 (6). pp. 1017-1033.
    Details
    Publication Date: 2020-02-06
    Description: As a major source for atmospheric CO2, the Peruvian upwelling region exhibits strong variability in surface fCO2 on short spatial and temporal scales. Understanding the physical processes driving the strong variability is of fundamental importance for constraining the effect of marine emissions from upwelling regions on the global CO2 budget. In this study, a frontal decay on length scales of 𝒪(10 km) was observed off the Peruvian coast following a pronounced decrease in down-frontal (equatorward) wind speed with a time lag of 9 h. Simultaneously, the sea-to-air flux of CO2 on the inshore (cold) side of the front dropped from up to 80 to 10 mmol m−2 day−1, while the offshore (warm) side of the front was constantly outgassing at a rate of 10–20 mmol m−2 day−1. Based on repeated ship transects the decay of the front was observed to occur in two phases. The first phase was characterized by a development of coherent surface temperature anomalies which gained in amplitude over 6–9 h. The second phase was characterized by a disappearance of the surface temperature front within 6 h. Submesoscale mixed-layer instabilities were present but seem too slow to completely remove the temperature gradient in this short time period. Dynamics such as a pressure-driven gravity current appear to be a likely mechanism behind the evolution of the front.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 8
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    North Atlantic Ventilation - Cruise No. MSM 21/1b, June 9 - June 22, 2012, Reykjavik (Iceland) - Reykjavik (Iceland) (2015)
    DFG-Senatskommission für Ozeanographie
    In:  Maria S. Merian-Berichte, MSM21/1b . DFG-Senatskommission für Ozeanographie, Bremen, 73 pp.
    Details
    Publication Date: 2017-09-25
    Description: The R/V MARIA S. MERIAN cruise MSM21/1b was carried out jointly by GEOMAR Kiel and the Institut für Meereskunde at the Centre for Marine and Atmospheric Sciences of Hamburg University. Scientists and technicians from the Atlantic Branch of the P.P. Shirshov Institute for Oceanology (Kaliningrad, Russia) and the Finnish Meteorological Institute (Helsinki; Finland) also participated in the cruise. The measurements mainly contributed to three projects: 1) "Nordatlantik", funded by the German Ministry of Education and Research (BMBF), 2) "THOR", funded by the European Commission within the framework of FP7, 3) "Valley Mixing", funded by the Deutsche Forschungsgemeinschaft (DFG). The main objectives of the cruise were to characterize the spatio-temporal variability of the Denmark Strait overflow and to identify processes responsible for the exchange of the overflow plume with ambient water downstream of Denmark Strait. A multi-platform approach was taken to achieve the goals, based on moorings, an autonomous underwater vehicle (AUV), as well as lowered and vessel-mounted observations. From these platforms, measurements of temperature, salinity, dissolved oxygen, current velocity, dissipation of turbulent kinetic energy, and bottom pressure were obtained. Altogether, twelve moorings were deployed, one mooring was recovered, data from two PIES were uploaded via acoustic telemetry, turbulence measurements on six extended AUV dives were carried out, and 92 CTD / LADCP casts were taken, 25 of which had additionally been equipped with a profiling turbulence (MSS) probe. Along with this, underway data of surface hydrography and current velocity were collected throughout the cruise. Our research was conducted in the spirit of both the "OSPAR Code of Conduct for Responsible Marine Research" and the "Commitment of responsible marine research" by the DFG Senatskommission.
    Type: Report , NonPeerReviewed
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  • 9
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    From pole to pole: 33 years of physical oceanography onboard R/V Polarstern (2017)
    Copernicus Publications (EGU)
    In:  Earth System Science Data, 9 (1). pp. 211-220.
    Details
    Publication Date: 2020-02-06
    Type: Article , PeerReviewed
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  • 10
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    Greater role for Atlantic inflows on sea-ice loss in the Eurasian Basin of the Arctic Ocean (2017)
    American Association for the Advancement of Science (AAAS)
    In:  Science, 356 (6335). pp. 285-291.
    Details
    Publication Date: 2020-02-06
    Description: Arctic sea-ice loss is a leading indicator of climate change and can be attributed, in large part, to atmospheric forcing. Here, we show that recent ice reductions, weakening of the halocline, and shoaling of intermediate-depth Atlantic Water layer in the eastern Eurasian Basin have increased winter ventilation in the ocean interior, making this region structurally similar to that of the western Eurasian Basin. The associated enhanced release of oceanic heat has reduced winter sea-ice formation at a rate now comparable to losses from atmospheric thermodynamic forcing, thus explaining the recent reduction in sea-ice cover in the eastern Eurasian Basin. This encroaching “atlantification” of the Eurasian Basin represents an essential step toward a new Arctic climate state, with a substantially greater role for Atlantic inflows.
    Type: Article , PeerReviewed
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