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  • Copernicus Publications (EGU)  (20)
  • Kiel : GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel  (3)
  • 1
    Online Resource
    Online Resource
    CASISAC - Änderungen im Agulhasstromsystem und Auswirkungen auf die Küsten im südlichen Afrika, Vorhaben: Ozean- und Klimamodellierung, Schlussbericht GEOMAR, Verbundprojekt WTZ SPACES II : Laufzeit des Vorhabens und Berichtszeitraum: 01.07.2018 bis 31.12.2021 (2022)
    Kiel : GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel
    Details Availability
    Keywords: Forschungsbericht
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (15 Seiten, 11,35 MB) , Illustrationen, Diagramme
    URL: https://edocs.tib.eu/files/e01fb24/1879707098.pdf
    URL: https://doi.org/10.2314/KXP:1879707098
    DOI: 10.2314/KXP:1879707098
    Language: German
    Note: Förderkennzeichen BMBF 03F0796A , Verbundnummer 01183026
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  • 2
    Online Resource
    Online Resource
    BMBF-Verbundvorhaben "RACE - Beobachtungen, Simulation und Synthese (multi-)dekadischer Schwankungen der Atlantikzirkulation" : Abschlussbericht der Teilprojekte am GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel : TP 1.1 Die Rolle des tropischen Atlantiks für Klimaschwankungen im atlantischen Raum, TP 1.4 Mechanismen für Zirkulationsschwankungen im Nordatlantik in gekoppelten Ozean-Atmosphäre Modellen, TP 2.2 Die veränderliche Ozeanzirkulation am Westrand des subpolaren Nordatlantiks, TP 3.1 Veränderungen der atlantischen Zirkulationssysteme in hochauflösenden Ozeanmodellen: Ursachen und regionale Auswirkungen (2016)
    Kiel : GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel
    Details Availability
    Keywords: Forschungsbericht
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (31 Seiten, 8,99 MB) , Illustrationen, Diagramme
    URL: https://edocs.tib.eu/files/e01fb16/875667473.pdf
    URL: https://doi.org/10.2314/GBV:875667473
    DOI: 10.2314/GBV:875667473
    Language: German
    Note: Förderkennzeichen BMBF 03F065B. - Verbund-Nummer 01071018 , "Autor(en): Biastoch, Arne; Böning, Claus; Brandt, Peter; Dengler, Marcus; Fischer, Jürgen; Karstensen, Johannes; Latif, Mojib, Visbeck, Martin" - Berichtsblatt , Unterschiede zwischen dem gedruckten Dokument und der elektronischen Ressource können nicht ausgeschlossen werden , Mit deutscher und englischer Zusammenfassung
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  • 3
    Online Resource
    Online Resource
    BMBF-Verbundprojekt WTZ südliches Afrika „SPACES - AGULHAS II - Regionale und globale Bedeutung“ : gemeinsamer Schlussbericht : Teilprojekt GEOMAR: Hochauflösende Klimamodellierung des Agulhasstromsystems als Schlüsselregion für die globale Ozeanzirkulation, Teilprojekt AWI: Agulhassystem in Klimamodellen der nächsten Generation : Teilprojekt Universität Hamburg: Analyse der Zwischenwassermassen im Südantlantik und ihrer Beiträge zum Benguela-Auftrieb : Laufzeit des Vorhabens: 01.07.2016 bis 30.06.2018, Berichtszeitraum: 01.07.2016 bis 30.06.2018 (2018)
    Kiel : GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel
    Details Availability
    Keywords: Forschungsbericht ; Agulhasstrom ; Klima ; Modell
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (24 Seiten, 2,40 MB) , Diagramme
    URL: https://doi.org/10.2314/GBV:1662719485
    DOI: 10.2314/GBV:1662719485
    Language: German
    Note: Förderkennzeichen BMBF 03F0750A-C. - Verbund-Nummer 01170298 , Unterschiede zwischen dem gedruckten Dokument und der elektronischen Ressource können nicht ausgeschlossen werden
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  • 4
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    An observed 20-year time series of Agulhas leakage (2014)
    Copernicus Publications (EGU)
    In:  Ocean Science, 10 (4). pp. 601-609.
    Details
    Publication Date: 2019-09-23
    Description: We provide a time series of Agulhas leakage anomalies over the last 20-years from satellite altimetry. Until now, measuring the interannual variability of Indo-Atlantic exchange has been the major barrier in the investigation of the dynamics and large scale impact of Agulhas leakage. We compute the difference of transport between the Agulhas Current and Agulhas Return Current, which allows us to deduce Agulhas leakage. The main difficulty is to separate the Agulhas Return Current from the southern limb of the subtropical "supergyre" south of Africa. For this purpose, an algorithm that uses absolute dynamic topography data is developed. The algorithm is applied to a state-of-the-art ocean model. The comparison with a Lagrangian method to measure the leakage allows us to validate the new method. An important result is that it is possible to measure Agulhas leakage in this model using the velocity field along a section that crosses both the Agulhas Current and the Agulhas Return Current. In the model a good correlation is found between measuring leakage using the full depth velocities and using only the surface geostrophic velocities. This allows us to extend the method to along-track absolute dynamic topography from satellites. It is shown that the accuracy of the mean dynamic topography does not allow us to determine the mean leakage but that leakage anomalies can be accurately computed.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.5194/os-10-601-2014
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Variability in the subtropical-tropical cells and its effect on near-surface temperature of the equatorial Pacific: a model study (2008)
    Copernicus Publications (EGU)
    In:  Ocean Science, 4 (1). pp. 73-88.
    Details
    Publication Date: 2015-02-26
    Description: A set of experiments utilizing different implementations of the global ORCA-LIM model with horizontal resolutions of 2°, 0.5° and 0.25° is used to investigate tropical and extra-tropical influences on equatorial Pacific SST variability at interannual to decadal time scales. The model experiments use a bulk forcing methodology building on the global forcing data set for 1958 to 2000 developed by Large and Yeager (2004) that is based on a blend of atmospheric reanalysis data and satellite products. Whereas representation of the mean structure and transports of the (sub-) tropical Pacific current fields is much improved with the enhanced horizontal resolution, there is only little difference in the simulation of the interannual variability in the equatorial regime between the 0.5° and 0.25° model versions, with both solutions capturing the observed SST variability in the Niño3-region. The question of remotely forced oceanic contributions to the equatorial variability, in particular, the role of low-frequency changes in the transports of the Subtropical Cells (STCs), is addressed by a sequence of perturbation experiments using different combinations of fluxes. The solutions show the near-surface temperature variability to be governed by wind-driven changes in the Equatorial Undercurrent. The relative contributions of equatorial and off-equatorial atmospheric forcing differ between interannual and longer, (multi-) decadal timescales: for the latter there is a significant impact of changes in the equatorward transport of subtropical thermocline water associated with the lower branches of the STCs, related to variations in the off-equatorial trade winds. A conspicuous feature of the STC variability is that the equatorward transports in the interior and along the western boundary partially compensate each other at both decadal and interannual time scales, with the strongest transport extrema occurring during El Niño episodes. The behaviour is rationalized in terms of a wobbling in the poleward extents of the tropical gyres, which is manifested also in a meridional shifting of the bifurcation latitudes of the North and South Equatorial Current systems.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.5194/os-4-73-2008
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Relating Agulhas leakage to the Agulhas Current retroflection location (2009)
    Copernicus Publications (EGU)
    In:  Ocean Science, 5 . pp. 511-521.
    Details
    Publication Date: 2016-10-11
    Description: The relation between the Agulhas Current retroflection location and the magnitude of Agulhas leakage, the transport of water from the Indian to the Atlantic Ocean, is investigated in a high-resolution numerical ocean model. Sudden eastward retreats of the Agulhas Current retroflection loop are linearly related to the shedding of Agulhas rings, where larger retreats generate larger rings. Using numerical Lagrangian floats a 37 year time series of the magnitude of Agulhas leakage in the model is constructed. The time series exhibits large amounts of variability, both on weekly and annual time scales. A linear relation is found between the magnitude of Agulhas leakage and the location of the Agulhas Current retroflection, both binned to three month averages. In the relation, a more westward location of the Agulhas Current retroflection corresponds to an increased transport from the Indian Ocean to the Atlantic Ocean. When this relation is used in a linear regression and applied to almost 20 years of altimetry data, it yields a best estimate of the mean magnitude of Agulhas leakage of 13.2 Sv. The early retroflection of 2000, when Agulhas leakage was probably halved, can be identified using the regression.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.5194/os-5-511-2009
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  • 7
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    Twenty-first century Southern Hemisphere impacts of ozone recovery and climate change from the stratosphere to the ocean (2022)
    Copernicus Publications (EGU)
    Details
    Publication Date: 2024-04-18
    Description: Changes in stratospheric ozone concentrations and increasing concentrations of greenhouse gases (GHGs) alter the temperature structure of the atmosphere and drive changes in the atmospheric and oceanic circulation. We systematically investigate the impacts of ozone recovery and increasing GHGs on the atmospheric and oceanic circulation in the Southern Hemisphere during the twenty-first century using a unique coupled ocean–atmosphere climate model with interactive ozone chemistry and enhanced oceanic resolution. We use the high-emission scenario SSP5-8.5 for GHGs under which the springtime Antarctic total column ozone returns to 1980s levels by 2048 in our model, warming the lower stratosphere and strengthening the stratospheric westerly winds. We perform a spatial analysis and show for the first time that the austral spring stratospheric response to GHGs exhibits a marked planetary wavenumber 1 (PW1) pattern, which reinforces the response to ozone recovery over the Western Hemisphere and weakens it over the Eastern Hemisphere. These changes, which imply an eastward phase shift in the PW1, largely cancel out in the zonal mean. The Southern Hemisphere residual circulation strengthens during most of the year due to the increase in GHGs and weakens in spring due to ozone recovery. However, we find that in November the GHGs also drive a weakening of the residual circulation, reinforcing the effect of ozone recovery, which represents another novel result. At the surface, the westerly winds weaken and shift equatorward due to ozone recovery, driving a weak decrease in the transport of the Antarctic Circumpolar Current and in the Agulhas leakage and a cooling of the upper ocean, which is most pronounced in the latitudinal band 35–45∘ S. The increasing GHGs drive changes in the opposite direction that overwhelm the ozone effect. The total changes at the surface and in the oceanic circulation are nevertheless weaker in the presence of ozone recovery than those induced by GHGs alone, highlighting the importance of the Montreal Protocol in mitigating some of the impacts of climate change. We additionally compare the combined effect of interactively calculated ozone recovery and increasing GHGs with their combined effect in an ensemble in which we prescribe the CMIP6 ozone field. This second ensemble simulates a weaker ozone effect in all the examined fields, consistent with its weaker increase in ozone. The magnitude of the difference between the simulated changes at the surface and in the oceanic circulation in the two ensembles is as large as the ozone effect itself. This shows the large uncertainty that is associated with the choice of the ozone field and how the ozone is treated.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Cold vs. warm water route – sources for the upper limb of the Atlantic Meridional Overturning Circulation revisited in a high-resolution ocean model (2019)
    Copernicus Publications (EGU)
    Details
    Publication Date: 2022-04-06
    Description: The northward flow of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC) is fed by waters entering the South Atlantic from the Indian Ocean mainly via the Agulhas Current (AC) system and by waters entering from the Pacific through Drake Passage (DP), commonly referred to as the “warm” and “cold” water routes, respectively. However, there is no final consensus on the relative importance of these two routes for the upper limb's volume transport and thermohaline properties. In this study we revisited the AC and DP contributions by performing Lagrangian analyses between the two source regions and the North Brazil Current (NBC) at 6∘ S in a realistically forced high-resolution (1∕20∘) ocean model. Our results agree with the prevailing conception that the AC contribution is the major source for the upper limb transport of the AMOC in the tropical South Atlantic. However, they also suggest a non-negligible DP contribution of around 40 %, which is substantially higher than estimates from previous Lagrangian studies with coarser-resolution models but now better matches estimates from Lagrangian observations. Moreover, idealized analyses of decadal changes in the DP and AC contributions indicate that the ongoing increase in Agulhas leakage indeed may have induced an increase in the AC contribution to the upper limb of the AMOC in the tropics, while the DP contribution decreased. In terms of thermohaline properties, our study highlights the fact that the AC and DP contributions cannot be unambiguously distinguished by their temperature, as the commonly adopted terminology may imply, but rather by their salinity when entering the South Atlantic. During their transit towards the NBC the bulk of DP waters experiences a net density loss through a net warming, whereas the bulk of AC waters experiences a slight net density gain through a net increase in salinity. Notably, these density changes are nearly completely captured by Lagrangian particle trajectories that reach the surface mixed layer at least once during their transit, which amount to 66 % and 49 % for DP and AC waters, respectively. This implies that more than half of the water masses supplying the upper limb of the AMOC are actually formed within the South Atlantic and do not get their characteristic properties in the Pacific and Indian Oceans.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Simulating the spread of disinfection by-products and anthropogenic bromoform emissions from ballast water discharge in Southeast Asia (2019)
    Copernicus Publications (EGU)
    Details
    Publication Date: 2022-04-06
    Description: Ballast water treatment is required for vessels to prevent the introduction of potentially invasive neobiota. Some treatment methods use chemical disinfectants which produce a variety of halogenated compounds as disinfection by-products (DBPs). One of the most abundant DBP from oxidative ballast water treatment is bromoform (CHBr3) where we find an average concentration of 894±560nmolL-1 (226±142μgL-1) in the undiluted ballast water from measurements and literature. Bromoform is a relevant gas for atmospheric chemistry and ozone depletion, especially in the tropics where entrainment into the stratosphere is possible. The spread of DBPs in the tropics over months to years is assessed here for the first time. With Lagrangian trajectories based on the NEMO-ORCA12 model velocity field, we simulate DBP spread in the sea surface and try to quantify the oceanic bromoform concentration and emission to the atmosphere from ballast water discharge at major harbours in the tropical region of Southeast Asia. The exemplary simulations of two important regions, Singapore and the Pearl River Delta, reveal major transport pathways of the DBPs and the anthropogenic bromoform concentrations in the sea surface. Based on our simulations, we expect DBPs to spread into the open ocean, along the coast and also an advection with monsoon-driven currents into the North Pacific and Indian Ocean. Furthermore, anthropogenic bromoform concentrations and emissions are predicted to increase locally around large harbours. In the sea surface around Singapore we estimate an increase in bromoform concentration by 9% compared to recent measurement. In a moderate scenario where 70% of the ballast water is chemically treated bromoform emissions to the atmosphere can locally exceed 1000pmolm-2h-1 and double climatological emissions. In the Pearl River Delta all bromoform is directly outgassed which leads to an additional bromine (Br) input into the atmosphere of 495kmolBr (∼42tCHBr3) a-1. From Singapore ports the additional atmospheric Br input is calculated as 312kmolBr (∼26tCHBr3) a-1. We estimate the global anthropogenic Br input from ballast water into the atmosphere of up to 13Mmola-1. This is 0.1% global Br input from background bromoform emissions and thus probably not relevant for stratospheric ozone depletion.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    The INALT family – a set of high-resolution nests for the Agulhas Current system within global NEMO ocean/sea-ice configurations (2019)
    Copernicus Publications (EGU)
    Details
    Publication Date: 2022-04-06
    Description: The Agulhas Current, the western boundary current of the South Indian Ocean, has been shown to play an important role in the connectivity between the Indian and Atlantic oceans. The greater Agulhas Current system is highly dominated by mesoscale dynamics. To investigate their influence on the regional and global circulations, a family of high-resolution ocean general circulation model configurations based on the NEMO code has been developed. Horizontal resolution refinement is achieved by embedding “nests” covering the South Atlantic and the western Indian oceans at 1/10∘ (INALT10) and 1/20∘ (INALT20) within global hosts with coarser resolutions. Nests and hosts are connected through two-way interaction, allowing the nests not only to receive boundary conditions from their respective host but also to feed back the impact of regional dynamics onto the global ocean. A double-nested configuration at 1/60∘ resolution (INALT60) has been developed to gain insights into submesoscale processes within the Agulhas Current system. Large-scale measures such as the Drake Passage transport and the strength of the Atlantic meridional overturning circulation are rather robust among the different configurations, indicating the important role of the hosts in providing a consistent embedment of the regionally refined grids into the global circulation. The dynamics of the Agulhas Current system strongly depend on the representation of mesoscale processes. Both the southward-flowing Agulhas Current and the northward-flowing Agulhas Undercurrent increase in strength with increasing resolution towards more realistic values, which suggests the importance of improving mesoscale dynamics as well as bathymetric slopes along this narrow western boundary current regime. The exploration of numerical choices such as lateral boundary conditions and details of the implementation of surface wind stress forcing demonstrates the range of solutions within any given configuration.
    Type: Article , PeerReviewed
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