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  • 1
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    Ekman drift in homogeneous water (1993)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 98 (C11). p. 20187.
    Details
    Publication Date: 2018-03-05
    Description: Measurements made with satellite-tracked buoys drogued in different layers between the sea surface and 30-m depth under homogeneous winter conditions in the North Sea allow analysis of the Ekman currents under a large variety of wind conditions. The experiment lasted from November 20, 1991, until February 29, 1992. The first 4 weeks of this period, during which the buoys stayed close together, are used to determine the Ekman stresses. The total current field is a superposition of barotropic currents due to sea level variations and Ekman currents. The classical Ekman theory is not able to describe properly the observed deflection of the currents to the right of the wind direction and their decay with depth. This deflection is 10° near the sea surface and increases to approximately 50° in 25-m depth. The relation between wind stress and the stress field in the interior of the water is given by a tensor, which describes the rotation and the variation of the stress with increasing depth. The concept of eddy viscosity is applicable, if a viscosity tensor is used to relate stress and vertical shear. The viscosity tensor is a function of the vertical coordinate only and is independent from the wind stress. It shows maximum values in 15- to 20-m depth and may be due to Langmuir circulation cells. Further studies are needed to determine the physics of this tensor. Its magnitude in the interior of the mixed layer exceeds 1000 cgs units. Consequently, Ekman currents are weak and may not be the dominant currents within the mixed layer.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/93JC01898
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    The role of mesoscale eddies in the source regions of the Agulhas Current (1999)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 29 . pp. 2303-2317.
    Details
    Publication Date: 2020-08-04
    Description: A primitive equation model to study the dynamics of the Agulhas system has been developed. The model domain covers the South Atlantic and the south Indian Ocean with a resolution of ⅓° in the Agulhas region while coarser outside. It is driven by a climatology of the European Centre for Medium-Range Weather Forecasts. It is shown that the model simulates the Agulhas Current, its retroflection, and the ring shedding successfully. The model results show baroclinic anticyclonic eddies in the Mozambique Channel and east of Madagascar, which travel toward the northern Agulhas Current. After the eddies reach the current they are advected southward with the mean flow. Due to the limited numerical resolution only a few eddies reach the retroflection region without much modification. These eddies are responsible for drastic enhancement of the heat transfer from the Indian Ocean to the South Atlantic and lead to periodicities in the interoceanic heat transport of about 50 days superimposed on the seasonal variability. Combined satellite data from TOPEX/Poseidon and ERS-1 show that the observed vortices in the Mozambique Channel are comparable to those seen in the model. In contrast to this the simulated eddies east of Madagascar seem not to be well reproduced. Analyses of the energy conversion terms between the mean flow and the eddies suggest that barotropic instability plays an important role in the generation of Mozambique Channel eddies. For the generation of Agulhas rings and other eddy structures in the model the barotropic instability mechanism seems to be minor, and baroclinic instability mechanisms are more likely.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(1999)029〈2303:TROMEI〉2.0.CO;2
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Preliminary results from a low resolution model of the South Atlantic Ocean (1996)
    In:  International WOCE Newsletter, 24 . pp. 18-20.
    Details
    Publication Date: 2017-07-03
    Type: Article , NonPeerReviewed
    Format: text
    Location Call Number Limitation Availability
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    OceanRep: Article in a Scientific Journal - without review
  • 4
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    Sonderforschungsbereich 133 Warmwassersphäre des Atlantiks - Eine Dokumentation - (1994)
    Institut für Meereskunde
    In:  Berichte aus dem Institut für Meereskunde an der Christian-Albrechts-Universität Kiel, 258 . Institut für Meereskunde, Kiel, Germany, 129 pp.
    Details
    Publication Date: 2014-10-13
    Type: Report , NonPeerReviewed
    Format: text
    DOI: 10.3289/ifm_ber_258
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    OceanRep: Report - other report
  • 5
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    The Gulf Stream, the North Atlantic Current, and the Origin of the Azores Current (1996)
    Bornträger
    In:  In: The Warmwatersphere of the North Atlantic Ocean. , ed. by Krauß, W. Bornträger, Berlin, Germany, pp. 291-337. ISBN 3-443-01033-4
    Details
    Publication Date: 2020-04-09
    Type: Book chapter , NonPeerReviewed
    Format: text
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    OceanRep: Book chapter
  • 6
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    Mean circulation and transports in the South Atlantic: Combining model and drifter data (1998)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 103 (C13). 30,985-31,002.
    Details
    Publication Date: 2018-04-30
    Description: Numerical experiments with a medium‐resolution primitive equation model of the South Atlantic mean circulation are described. The results from the standard model realization indicate that the model succeeds in representing the large‐scale transport and circulation features. However, a comparison with a velocity field derived from surface drifter data reveals discrepancies of the modeled velocities from the observations in magnitude as well as direction of the flow field. In order to diminish the model deviations from the data, an attempt is made to couple the model to the observations through a simple data assimilation technique. The assimilated model succeeds in improving the subtropical gyre circulation. Only a minor effect on the basin‐scale integrated quantities is observed. However, the density field may be deformed as a response to the assimilation of velocity data without simultaneously adapting a corresponding density structure. The influence of the disturbance of the density structure is most prominent at the edges of the observed data set, which does not cover the entire model domain, and is confined to the upper ocean and balanced above the thermocline. We calculated a meridional heat transport that is generally in accordance with estimates from other sources. The analysis of heat and salt fluxes suggests that the model features both the so‐called “warm water path” and “cold water path” in closing the global thermohaline circulation. While heat is mainly imported in surface and thermocline waters with the Agulhas Current around South Africa, it is the Antarctic Intermediate Water that compensates for more than 50% of the salt loss by the outflowing North Atlantic Deep Water.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/98JC02065
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    On the slippage of deep-drogued drifters (1996)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 101 (C1). pp. 1309-1319.
    Details
    Publication Date: 2018-04-30
    Description: Deep‐drogued drifters are in use to measure the near‐surface geostrophic currents. An attempt is made to study the slippage of these drifters due to wind and Ekman currents. The results are based on a data set from the unstratified North Sea obtained in winter 1991–1992, where the currents were decomposed into Ekman currents and barotropic currents. The influence of these Ekman currents on the drift performance of drifters drogued below the mixed layer in the barotropic current is determined by using quadratic drag laws. In 90% of all cases (1540 data points) the combined effect of wind drag and Ekman currents on buoy and 100‐m tether produces a slippage of less than 2 cm/s. Drifters drogued within the mixed layer show less slippage due to the reduced drag on the tether, but they are primarily designed to measure the actual near‐surface currents, which are strongly dependent on the wind conditions. It is concluded that deep‐drogued drifters are a reliable device to study weakly baroclinic geostrophic currents.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/95JC02686
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Lagrangian measurements in the Malvinas current (1996)
    Springer
    In:  In: The South Atlantic: Present and Past Circulation. , ed. by Wefer, G., Berger, W. H., Siedler, G. and Webb, D. J. Springer, Berlin, Heidelberg, New York, pp. 239-247.
    Details
    Publication Date: 2020-04-03
    Description: Direct measurements of magnitude of the northward flow of the Malvinas (Falkland) Current have recently been made with two types of Lagrangian platforms: ALACE floats which cycled between 750-m depth and the sea surface, and 100-m drogued surface drifters. Each data set clearly delineates the path of the Malvinas Current, and the vertical shears inferred from them are commensurate with historical geostrophic shears. Velocities from the surface drifters are used here to adjust geostrophic shears from historical measurements, and the results confirm a large transport of the current, as previously implied by numerical models and a regional inverse calculation. At 42°S, the northward transport of the Malvinas Current in the upper 3000 m appears to be about 70 Sv, several times larger than estimates obtained by adjusting geostrophic shears to assumed levels of no motion. This large barotropie component may have significance in the cross-frontal transfer of intermediate and deep waters from the circumpolar current to the adjacent flow regimes in the South Atlantic, and thus on the inter-basin exchange of water masses.
    Type: Book chapter , NonPeerReviewed
    Format: text
    DOI: 10.1007/978-3-642-80353-6_12
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    OceanRep: Book chapter
  • 9
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    Comments on the development of our knowledge of the gerenal circulation of the North Atlantic Ocean (1996)
    Bornträger
    In:  In: The Warmwatersphere of the North Atlantic Ocean. , ed. by Krauß, W. Bornträger, Berlin, Stuttgart, pp. 1-31.
    Details
    Publication Date: 2020-04-03
    Type: Book chapter , NonPeerReviewed
    Format: text
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    OceanRep: Book chapter
  • 10
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    The Warmwatersphere of the North Atlantic Ocean (1996)
    Gebr. Bornträger
    In:  Gebr. Bornträger, Berlin, Stuttgart, 446 pp.
    Details
    Publication Date: 2020-03-24
    Type: Book , PeerReviewed
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    OceanRep: Book
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