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  • 11
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    The annual review of CLIVAR by the SSG (2003)
    CLIVAR
    In:  CLIVAR Exchanges, 26 (8 (2-3)). pp. 3-5.
    Details
    Publication Date: 2019-02-01
    Type: Article , NonPeerReviewed
    Format: text
    Location Call Number Limitation Availability
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    OceanRep: Article in a Scientific Journal - without review
  • 12
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    Physical Climate Processes and Feedbacks (2001)
    Cambridge University Press
    In:  In: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernemental Panel on Climate Change. , ed. by Houghton, J. T., Ding, Y. and Griggs, D. J. Cambridge University Press, Cambridge, United Kingdom, pp. 417-470. ISBN 0521-01495-6
    Details
    Publication Date: 2020-03-30
    Type: Book chapter , NonPeerReviewed
    Format: text
    Location Call Number Limitation Availability
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    OceanRep: Book chapter
  • 13
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    Parameter estimates of a zero-dimensional ecosystem model applying the adjoint method (2001)
    Pergamon Press
    In:  Deep Sea Research Part II: Topical Studies in Oceanography, 48 . pp. 1769-1800.
    Details
    Publication Date: 2020-08-05
    Description: Assimilation experiments with data from the Bermuda Atlantic Time-series Study (BATS, 1989¯1993) were performed with a simple mixed-layer ecosystem model of dissolvedinorganic nitrogen (N), phytoplankton (P) and herbivorous zooplankton (H). Our aim is to optimize the biological model parameters, such that the misfits between model results andobservations are minimized. The utilized assimilation method is the variational adjoint technique, starting from a wide range of first-parameter guesses. A twin experiment displayedtwo kinds of solutions, when Gaussian noise was added to the model-generated data. The expected solution refers to the global minimum of the misfit model-data function, whereasthe other solution is biologically implausible and is associated with a local minimum. Experiments with real data showed either bottom-up or top-down controlled ecosystemdynamics, depending on the deep nutrient availability. To confine the solutions, an additional constraint on zooplankton biomass was added to the optimization procedure. Thisinclusion did not produce optimal model results that were consistent with observations. The modelled zooplankton biomass still exceeded the observations. From the model-datadiscrepancies systematic model errors could be determined, in particular when the chlorophyll concentration started to decline before primary production reached its maximum. Adirect comparision of measured 14C-production data with modelled phytoplankton production rates is inadequate at BATS, at least when a constant carbon to nitrogen C : N ratio isassumed for data assimilation.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/S0967-0645(00)00161-2
    Location Call Number Limitation Availability
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 14
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    Numerical Ocean Circulation Modelling: Present Status and Future Directions (2001)
    Academic Press
    In:  In: Ocean Circulation and Climate - Observing and Modelling the Global Ocean. , ed. by Siedler, G., Church, J. and Gould, J. Academic Press, London, UK; San Diego, USA, pp. 547-556. ISBN 0-12-641351-7
    Details
    Publication Date: 2020-05-05
    Type: Book chapter , NonPeerReviewed
    Format: text
    Location Call Number Limitation Availability
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    OceanRep: Book chapter
  • 15
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    Variational assimilation of SSH variability from TOPEX/POSEIDON and ERSI into an eddy-permitting model of the North Atlantic (2003)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 108 (C3). p. 3092.
    Details
    Publication Date: 2019-04-04
    Description: A first step for improving the climatological state of high‐resolution general circulation models by means of data assimilation is presented. A method developed for the assimilation of statistical characteristics into chaotic ocean models is applied to assimilate SSH variability from TOPEX/POSEIDON and ERS1 in association with temperature and salinity from the World Ocean Atlas 1997 in order to estimate the underlying mean circulation. The method requires a parameterization of SSH variability which derives from the approach of Green and Stone. By estimating initial conditions for temperature and salinity, a mean state is achieved which, although not fully consistent with the altimetric and climatological data, is markedly improved on time scales of one year in comparison to the control run. The assimilation of SSH variability data introduces complementary information about the main frontal structures consistent with climatological observations. The state is however not an equilibrium state and returns back to the first guess quasi‐equilibrium state for longer integration periods.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2001JC000982
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 16
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    An adjoint method for the assimilation of statistical characteristics into eddy-resolving ocean models (2002)
    Taylor & Francis
    In:  Tellus A: Dynamic meteorology and oceanography, 54 . pp. 406-425.
    Details
    Publication Date: 2016-06-14
    Description: The study investigates perspectives of the parameter estimation problem with the adjoint method in eddy-resolving models. Sensitivity to initial conditions resulting from the chaotic nature of this type of model limits the direct application of the adjoint method by predictability. Prolonging the period of assimilation is accompanied by the appearance of an increasing number of secondary minima of the cost function that prevents the convergence of this method. In the framework of the Lorenz model it is shown that averaged quantities are suitable for describing invariant properties, and that secondary minima are for this type of data transformed into stochastic deviations. An adjoint method suitable for the assimilation of statistical characteristics of data and applicable on time scales beyond the predictability limit is presented. The approach assumes a greater predictability for averaged quantities. The adjoint to a prognostic model for statistical moments is employed for calculating cost function gradients that ignore the fine structure resulting from secondary minima. Coarse resolution versions of eddy-resolving models are used for this purpose. Identical twin experiments are performed with a quasigeostrophic model to evaluate the performance and limitations of this approach in improving models by estimating parameters. The wind stress curl is estimated from a simulated mean stream function. A very simple parameterization scheme for the assimilation of second-order moments is shown to permit the estimation of gradients that perform efficiently in minimizing cost functions.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1034/j.1600-0870.2002.01294.x
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 17
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    Circulation Characteristics in Three Eddy-Permitting Models of the North Atlantic (2001)
    Elsevier
    In:  Progress in Oceanography, 48/2-3 . pp. 123-162.
    Details
    Publication Date: 2016-10-07
    Description: A systematic intercomparison of three realistic eddy-permitting models of the North Atlantic circulation has been performed. The models use different concepts for the discretization of the vertical coordinate, namely geopotential levels, isopycnal layers, terrain-following (sigma) coordinates, respectively. Although these models were integrated under nearly identical conditions, the resulting large-scale model circulations show substantial differences. The results demonstrate that the large-scale thermohaline circulation is very sensitive to the model representation of certain localised processes, in particular to the amount and water mass properties of the overflow across the Greenland–Scotland region, to the amount of mixing within a few hundred kilometers south of the sills, and to several other processes at small or sub-grid scales. The different behaviour of the three models can to a large extent be explained as a consequence of the different model representation of these processes.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/S0079-6611(01)00003-9
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 18
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    Assimilation of Altimetric Data into an Eddy-Permitting Model of the North Atlantic (2001)
    Elsevier
    In:  Progress in Oceanography, 48 (2-3). pp. 313-315.
    Details
    Publication Date: 2019-09-23
    Description: This paper shows that the mean flow of an eddy-permitting model can be altered by assimilation of surface height variability, providing that information about the mean sea surface is included, using an adaption of a statistical–dynamical method devised by Oschlies and Willebrand. We show that for a restricted depth range (about 1000 m), dynamical knowledge can make up for the null space present in surface data whose temporal extent may be too short to distinguish between vertical modes. The lack of an accurate geoid has meant that most assimilation methods, while representing variability well, have been unable to modify the mean flow to any extent. However, we show that by including several approximate forms for the mean sea surface, the mean interior flow in the upper kilometer can be rapidly adjusted towards reality by the assimilation, with the location of major current systems moved by hundreds of kilometers.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/S0079-6611(01)00009-X
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 19
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    Mechanism of interannual to decadal variability of the North Atlantic circulation (2001)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2022-03-10
    Description: A model of the Atlantic Ocean was forced with decadal-scale time series of surface fluxes taken from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis. The bulk of the variability of the oceanic circulation is found to be related to the North Atlantic oscillation (NAO). Both realistic experiments and idealized sensitivity studies with the model show a fast (intraseasonal timescale) barotropic response and a delayed (timescale about 6–8 yr) baroclinic oceanic response to the NAO. The fast response to a high NAO constitutes a barotropic anticyclonic circulation anomaly near the subpolar front with a substantial decrease of the northward heat transport and an increase of northward heat transport in the subtropics due to changes in Ekman transport. The delayed response is an increase in subpolar heat transport due to enhanced meridional overturning and due to a spinup of the subpolar gyre. The corresponding subpolar and subtropical heat content changes could in principle act as an immediate positive feedback and a delayed negative feedback to the NAO.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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