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  • 1
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    An approximate Kalman filter for ocean data assimilation: An example with an idealized Gulf Stream model (1995)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A practical method of data assimilation for use with large, nonlinear, ocean general circulation models is explored. A Kalman filter based on approximation of the state error covariance matrix is presented, employing a reduction of the effective model dimension, the error's asymptotic steady state limit, and a time-invariant linearization of the dynamic model for the error integration. The approximations lead to dramatic computational savings in applying estimation theory to large complex systems. We examine the utility of the approximate filter in assimilating different measurement types using a twin experiment of an idealized Gulf Stream. A nonlinear primitive equation model of an unstable east-west jet is studied with a state dimension exceeding 170,000 elements. Assimilation of various pseudomeasurements are examined, including velocity, density, and volume transport at localized arrays and realistic distributions of satellite altimetry and acoustic tomography observations. Results are compared in terms of their effects on the accuracies of the estimation. The approximate filter is shown to outperform an empirical nudging scheme used in a previous study. The examples demonstrate that useful approximate estimation errors can be computed in a practical manner for general circulation models.
    Keywords: OCEANOGRAPHY
    Type: Journal of Geophysical Research (ISSN 0148-0227); 100; C4; p. 6777-6793
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  • 2
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    Assimilation of Altimeter Data into a Quasigeostrophic Model of the Gulf Stream System (1995)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: The dynamical consequences of constraining a numerical model with sea surface height data have been investigated. The model used for this study is a quasigeostrophic model of the Gulf Stream region. The data that have been assimilated are maps of sea surface height obtained as the superposition of sea surface height variability deduced from the Geosat altimeter measurements and a mean field constructed from historical hydrographic data. The method used for assimilating the data is the nudging technique. Nudging has been implemented in such a way as to achieve a high degree of convergence of the surface model fields toward the observations. The assimilation of the surface data is thus equivalent to the prescription of a surface pressure boundary condition. The authors analyzed the mechanisms of the model adjustment and the characteristics of the resultant equilibrium state when the surface data are assimilated. Since the surface data are the superposition of a mean component and an eddy component, in order to understand the relative role of these two components in determining the characteristics of the final equilibrium state, two different experiments have been considered: in the first experiment only the climatological mean field is assimilated, while in the second experiment the total surface streamfunction field (mean plus eddies) has been used. It is shown that the model behavior in the presence of the surface data constraint can be conveniently described in terms of baroclinic Fofonoff modes. The prescribed mean component of the surface data acts as a 'surface topography' in this problem. Its presence determines a distortion of the geostrophic contours in the subsurface layers, thus constraining the mean circulation in those layers. The intensity of the mean flow is determined by the inflow/outflow conditions at the open boundaries, as well as by eddy forcing and dissipation.
    Keywords: Oceanography
    Type: NASA-CR-205501 , NAS 1.26:205501 , Journal of Physical Oceanography; 25; 6; 1130-1152
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  • 3
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    Coherent structures in a baroclinic atmosphere. IV - A comparison between theory and data (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: This paper presents the results of a data analysis, on an extensive series of Atlantic blocks, for the composites of both the positive and the negative anomaly cases and for the winter climatology, using the composite approach of Dole (1982, 1986). The results are compared with predictions of the theory formulated by Malguzzi and Malanotte-Rizzoli (1984, 1985) and Malanotte-Rizzoli and Malguzzi (1987) to model the blocking events. The results of the analyses indicate that the composite of the positive anomaly cases shows a strong northern barrier centered in the latitude band 62-72 deg N, in agreement with the northern confinement of the block. The northern, positive wall is not present in the climatology; its presence and significance are doubtful and debatable for the negative anomaly composite. For the individual positive cases for which a smooth and zonal upstream wind could be defined, the V-function shows the appearance of both a northern and southern wall below the tropopause.
    Keywords: METEOROLOGY AND CLIMATOLOGY
    Type: Journal of the Atmospheric Sciences (ISSN 0022-4928); 44; 2506-252
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  • 4
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    Space and time variability of the surface color field in the northern Adriatic Sea (1986)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A time series of coastal zone color scanner images for the years 1979 and 1980 was used to observe the spatial and temporal variability of bio-optical processes and circulation patterns of the northern Adriatic Sea on monthly, seasonal, and interannual scales. The chlorophyll-like pigment concentrations derived from satellite data exhibited a high correlation with sea truth measurements performed during seven surveys in the summer of both years. Comparison of the mean pigment fields indicates a general increase in concentration values and larger scales of coastal features from 1979 to 1980. This variability may be linked to the different patterns of nutrient influx due to coastal runoff in the 2 years. The distribution of surface features is consistent with the general cyclonic circulation pattern. The pigment heterogeneity appears to be governed by fluctuations of freshwater discharge, while the dominant wind fields do not appear to have important direct effects. The Po River presents a plume spreading predominantly in a southeastern direction, with scales positively correlated with its outflow. The spatial scales of the western coastal layer, in contrast, are negatively correlated with this outflow and the plume scales. Both results are consistent with, and may be rationalized by, recent theoretical and experimental results involving a dynamical balance between nonlinear advection and bottom friction, with alternate predominance of one of the two effects.
    Keywords: OCEANOGRAPHY
    Type: Journal of Geophysical Research (ISSN 0148-0227); 91; 12957-12
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  • 5
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    Assimilation of Altimeter Data into a Quasigeostrophic Model of the Gulf Stream System (1995)
    In:  CASI
    Details
    Publication Date: 2019-08-16
    Description: The improvement in the climatological behavior of a numerical model as a consequence of the assimilation of surface data is investigated. The model used for this study is a quasigeostrophic (QG) model of the Gulf Stream region. The data that have been assimilated are maps of sea surface height that have been obtained as the superposition of sea surface height variability deduced from the Geosat altimeter measurements and a mean field constructed from historical hydrographic data. The method used for assimilating the data is the nudging technique. Nudging has been implemented in such a way as to achieve a high degree of convergence of the surface model fields toward the observations. Comparisons of the assimilation results with available in situ observations show a significant improvement in the degree of realism of the climatological model behavior, with respect to the model in which no data are assimilated. The remaining discrepancies in the model mean circulation seem to be mainly associated with deficiencies in the mean component of the surface data that are assimilated. On the other hand, the possibility of building into the model more realistic eddy characteristics through the assimilation of the surface eddy field proves very successful in driving components of the mean model circulation that are in relatively good agreement with the available observations. Comparisons with current meter time series during a time period partially overlapping the Geosat mission show that the model is able to 'correctly' extrapolate the instantaneous surface eddy signals to depths of approximately 1500 m. The correlation coefficient between current meter and model time series varies from values close to 0.7 in the top 1500 m to values as low as 0.1-0.2 in the deep ocean.
    Keywords: Oceanography
    Type: NASA-CR-204983 , NAS 1.26:204983 , Journal of Physical Oceanography: Part 1; 25; 6; 1153-1173
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  • 6
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    Physical forcing and physical/biochemical variability of the Mediterranean Sea: A review of unresolved issues and directions of future research - Report of the Workshop “Variability of the Eastern and Western Mediterranean circulation and thermohaline properties : similarities and differences” Rome, 7-9 November 2011 (2012)
    Ciesm, Unesco and IOC
    In:  Ciesm, Unesco and IOC, Monaco, 48 pp.
    Details
    Publication Date: 2014-02-18
    Type: Report , NonPeerReviewed
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  • 7
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    Tropical instability waves in the Atlantic Ocean (2004)
    Elsevier
    In:  Ocean Modelling, 7 (1-2). pp. 145-163.
    Details
    Publication Date: 2020-10-16
    Description: An idealized numerical model of the tropical Atlantic Ocean is used to study the structure, energetics and heat flux of the Atlantic tropical instability waves (TIWs). The model results compare well with the observations, and they both show that, unlike commonly assumed, the TIWs in the Atlantic exist on both sides of the equator and are generated not only in the summer but from May to January. Furthermore it is demonstrated that the Atlantic TIWs are generated by barotropic instability of the shear between the equatorial undercurrent and the northern south equatorial current and make a surprisingly small contribution to the heat budget of the equatorial mixed layer. The model results reveal that the often published strong meridional heat flux divergence of the TIWs is largely compensated for by their vertical heat flux divergence.
    Type: Article , PeerReviewed
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