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  • 1
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    Neutral Density Revisited (1999)
    Elsevier
    In:  Deep Sea Research Part II: Topical Studies in Oceanography, 46 . pp. 33-54.
    Details
    Publication Date: 2019-09-23
    Description: The possibilities of defining and computing an approximately neutral density variable are reexamined in this paper. There are three desirable properties that a neutral density variable should possess. Firstly, the isosurfaces of this variable should coincide with (approximately) neutral surfaces. This would facilitate the analysis of hydrographic data on the most appropriate mixing and spreading surfaces. Secondly, the horizontal gradients of the neutral density should agree with the gradients of the in situ density, and thirdly the vertical gradient of the neutral density variable should be proportional to the static stability of the water column. A density variable that approximates the latter two properties can be used in ocean circulation models based on layer coordinates, and would reduce substantial errors in present isopycnal models due to the use of a potential density variable. No variable can possess all the three properties simultaneously. The variable γn introduced by Jackett and McDougall (1997, J. Phys. Oceanogr. 27, 237–263) satisfies the first of the properties exactly but is not designed for the use in models. Based on climatological data in the North Atlantic, an alternative neutral density variable ν̃(S, Θ) is defined, which is shown to approximate the two gradient criteria much better than any potential density. We suggest that this neutral density variable may be useful in isopycnal ocean models as an alternative to potential density, since it could significantly reduce errors in thermal wind relation and vertical stability
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/S0967-0645(98)00113-1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Friedrich A. Schott (1939-2008) (2008)
    AGU (American Geophysical Union)
    In:  Eos, Transactions American Geophysical Union, 89 (41). p. 391.
    Details
    Publication Date: 2017-02-24
    Description: The physical oceanography community recently lost one of its most influential and productive scientists. Friedrich A. (“Fritz”) Schott, who had been fighting leukemia for about a year, died on 30 April 2008 at the age of 69.
    Type: Article , NonPeerReviewed
    Format: text
    DOI: 10.1029/2008EO410005
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    OceanRep: Article in a Scientific Journal - without review
  • 3
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    Decadal to Multidecadal Variability of the Atlantic MOC: Mechanisms and Predictability (2007)
    AGU (American Geophysical Union)
    In:  In: Ocean Circulation: Mechanisms and Impacts - Past and Future Changes of the Ocean's Meridional Overturning. , ed. by Schmittner, A., Chiang, J. and Hemming, S. AGU Monograph, 173 . AGU (American Geophysical Union), Washington D.C., pp. 149-166. ISBN 978-0-87590-438-2
    Details
    Publication Date: 2020-07-28
    Description: The dynamics and predictability of the decadal to multidecadal Atlantic merid­ional overturning circulation (MOC) variability are described from observations and models. The investigation focuses on two modes that involve the MOC: One mode exhibits a quasi-decadal period, while the other is multidecadal. The two modes have completely different underlying dynamics, which is reflected in their rather different spatial characteristics. While the quasi-decadal mode represents a damped mode of the coupled ocean-atmosphere system, the multidecadal mode can be basically understood as the MOC response to the multidecadal forcing by the North Atlantic Oscillation (NAO). "Perfect model" predictability studies indicate a rather high predictability potential of the MOC variability on decadal timescales. Variations of the MOC are associated with variations in the meridional heat trans­port that drive sea surface temperature (SST) anomalies. SST anomalies in the North Atlantic thus exhibit a similar decadal predictability potential as the MOC. The decadal predictability carries over to the atmosphere. The probability density function of European surface air temperature anomalies, for instance, changes sig­nificantly with the state of the MOC. A reconstruction of the MOC for the 20th cen­tury from observed SSTs shows considerable variability on decadal timescales, but no strong sustained long-term trend. Furthermore, an assessment of the observed hydrographical changes in the Nordic Seas, with the aid of ocean general circula­tion model experiments and the analysis of recent scenario integrations with global climate models, indicates that the expected anthropogenic weakening of the MOC may not exceed the level of the internal variability within the next decades.
    Type: Book chapter , NonPeerReviewed
    Format: text
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    OceanRep: Book chapter
  • 4
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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
  • 5
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    Assimilation of Geosat altimeter data into an eddy-resolving primitive equation model of the North Atlantic Ocean (1996)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 101 (C6). pp. 14175-14190.
    Details
    Publication Date: 2019-09-23
    Description: We present a new method for assimilating observations of sea surface height (SSH) into a high‐resolution primitive equation model. The method is based on the concept of reinitialization. First, the surface velocity increments necessary to adjust the model forecast to the observed geostrophic surface currents are projected onto deep velocity increments by a linear regression method. Second, changes in the density field required to balance the changes in the velocity field geostrophically are obtained from an inversion of the thermal wind equation. A unique partition of the density increments into corresponding temperature and salinity changes is realized by conserving the local θ‐S relation of the model forecast. In contrast to pure statistical methods that infer temperature and salinity changes from correlations with SSH anomalies, our approach explicitly conserves water mass properties on isopycnals. For the assimilation experiment we use optimally interpolated maps of Geosat SSH anomalies (the mean topography is taken from the model), which are assimilated into the World Ocean Circulation Experiment (WOCE) Community Modeling Effort (CME) model of the North Atlantic Ocean at 5‐day intervals covering the year 1987. It is shown that the assimilation significantly improves the model's representation of eddy activity, with the hydrographic structure of individual eddies agreeing well with independent hydrographic observations. The importance of a careful treatment of water mass properties in the assimilation process is discussed and further illustrated by comparing different assimilation schemes.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/95JC03801
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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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
  • 7
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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
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