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  • 1
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    Towards the impact of eddies on the response of the Southern Ocean to climate change (2010)
    Elsevier
    In:  Ocean Modelling, 34 (3-4). pp. 150-165.
    Details
    Publication Date: 2019-01-23
    Description: The sensitivity of the meridional overturning circulation (MOC) of the Southern Ocean (SO) to wind stress changes is discussed. Using an idealised SO model in both non- and eddy-permitting configurations, we assess the effects of both, coarsening the horizontal resolution and implementing different parameterisations for the lateral eddy diffusivity appropriate to the Gent and McWilliams (1990) parameterisation, K. We find that the MOC is characterised by an eddy-driven part ψ* which generally opposes the wind-driven part and that the increase of the MOC diminishes with amplifying winds, with the possibility that the MOC in the SO may become completely insensitive to wind stress changes. However, for moderate wind stress, the MOC is still significantly increasing in our configuration. The diagnosed lateral eddy diffusivity K in the eddy-permitting version shows strong spatial variability and is increasing with increasing wind stress. Similar to the MOC (but in contrast to ψ*) the increase of K diminishes with amplifying winds. It turns out that a small increase in the isopycnal slopes is also relevant in order to capture the correct sensitivity of ψ* on wind stress. This relation also holds in model configurations with coarser but still eddy-permitting horizontal resolution: decreasing the horizontal resolution decreases K, but increases the isopycnal slopes such that the strength of the MOC including its sensitivity to wind stress is almost unchanged. The parameterisations are able to reproduce the MOC for certain wind stresses, but all parameterisations underestimate the sensitivity of K and thus overestimate the sensitivity of the MOC on wind stress. Our results show that it is indispensable to incorporate the correct sensitivity of K into climate models in order to reproduce the correct sensitivity of the MOC to wind stress and that up-to-date parameterisations for K are only partially successful.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1016/j.ocemod.2010.05.005
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Lagrangian and Eulerian lateral diffusivities in zonal jets (2011)
    Elsevier
    In:  Ocean Modelling, 39 (1-2). pp. 114-124.
    Details
    Publication Date: 2017-02-06
    Description: Meridional diffusivities from Lagrangian particle dispersion and Eulerian diffusivities from a flux-gradient relationship are estimated in an idealized primitive equation channel model featuring eddy-driven zonal jets. The Eulerian estimate shows an increase with depth and clear minima of meridional diffusivities within the zonal jets, indicating mixing barriers. The Lagrangian estimates agree with the Eulerian method on the vertical variation and also show indications of meridional mimima, although meridional variations are poorly resolved. We found early maxima in the particle spreading rates which should not be related to diffusivities since they are caused by the meandering zonal jets. The meanders also produce rotational eddy fluxes, which can obscure the Eulerian diffusivity estimates. Zonal particle dispersion rates do not converge within the chosen lag interval, because of shear dispersion by the mean flow, i.e. it is not possible to estimate Lagrangian zonal diffusivities representative for regions of similar size of the zonal jet spacing. Removing the zonal mean flow, zonal and meridional dispersion rates converge and show much higher zonal than meridional diffusivities. Further, the pronounced vertical increase and indications of meridional minima in the Lagrangian meridional diffusivities disappear, pointing towards the importance of shear dispersion by the mean flow for the suppression of meridional mixing by zonal jets. (C) 2011 Elsevier Ltd. All rights reserved.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.ocemod.2011.02.002
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Why western boundary currents are diffusive: A link between bottom pressure torque and bolus velocity (2010)
    Elsevier
    In:  Ocean Modelling, 32 . pp. 14-24.
    Details
    Publication Date: 2019-09-23
    Description: The bottom pressure torque is known to vanish in the interior ocean but to play a dominant role in the western boundary layer in balancing the planetary vorticity on spatial scales larger than the Rossby radius of deformation. In this study, the appearance of the bottom pressure torque and thus any deviation of wind-driven flow from classical Sverdrup balance is locally related in steady state to non-zero bolus velocity and/or friction, under the assumption that horizontal density advection is small compared to the lifting of isopycnals. To first order approximation, the vortex stretching by the vertical bolus velocity is related to the bottom pressure torque. The bolus vortex stretching becomes a significant term in the barotropic vorticity budget of the western boundary layer and is formally equivalent to bottom friction as in the classical models of the wind-driven gyre circulation.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.ocemod.2009.07.003
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Nordatlantik I - 1.1 Die Bedeutung des Süßwasserhaushalts für die zukünftige Entwicklung der thermohalinen Zirkulation; 1.2 Rolle des äquatorialen Atlantiks als Schlüsselregion für Klimaschwankungen im atlantischen Raum; 2.2 Schwankungen der tiefen Randstromzirkulation am Ausgang des Labradorsee; 3.2 Schwankungen der thermohalinen Zirkulation in hoch auflösenden Modellen : Schlussbericht (2010)
    BMBF / IFM-GEOMAR
    In:  In: Der Nordatlantik als Teil des Erdsystems: vom System-Verständnis zur Analyse regionaler Auswirkungen : BMBF Verbundvorhaben als Beitrag zur nationalen und internationalen Erdsystemforschung ; Abschlußbericht Nordatlantik-Projekt. BMBF / IFM-GEOMAR, Kiel, Germany, pp. 1-15.
    Details
    Publication Date: 2019-04-18
    Description: Visible results were obtained by the working groups in Kiel on the first four aspects of the project, resulting in improvements of the quantitative knowledge of key processes and key regions in the Atlantic Ocean. New ocean and coupled ocean-atmosphere models were analyzed with regard to seasonal and decadal climate changes, as well as optimization techniques. In addition, the influence of circulation variability on oceanic CO2 uptake was investigated. Intense field studies were carried out successfully in two regions: The measurements obtained in the equatorial Atlantic serve as the basis for a better understanding of the role of the tropical Atlantic for climate fluctuations in the Atlantic in general, and also provide predictability indicators for seasonal forecasts. The second focal area of field work was the southern region of the Labrador Sea near 53°N where different components of the North Atlantic Deep Water merge to form the deep western boundary current (DWBC). Here a mooring array has been deployed for the past 13 years to monitor this branch of the thermohaline circulation exiting the Labrador Sea. In collaboration with other national and international large-scale observations (ship-based measurements, Argo floats, etc.) and modeling efforts, the field work carried out by the Kiel working groups provides a significant contribution toward a sustainable regional ocean-climate analysis system.
    Type: Book chapter , NonPeerReviewed
    Format: text
    DOI: 10.2314/GBV:639459536
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    OceanRep: Book chapter
  • 5
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    Parameterising meso-scale eddy momentum fluxes based on potential vorticity mixing and a gauge term (2010)
    Elsevier
    Details
    Publication Date: 2023-03-09
    Description: Meso-scale fluctuations are known to drive large-scale zonal flows in the ocean, a mechanism which is currently missing in non-eddy-resolving ocean models. A closure for meso-scale eddy momentum fluxes is evaluated in a suite of idealised eddying channel models, featuring eddy-driven zonal jets. It is shown how the appearance of zonal jets, which act as mixing barriers for turbulent exchange, and reduced lateral diffusivities are linked in a natural way by implementing mixing of potential vorticity and using a gauge term to insure that no spurious forces are introduced. It appears, therefore, possible to parameterise the appearance of zonal jets and its effect on the ventilation of interior ocean basins in non-eddy-resolving, realistic ocean models.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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