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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
    Location Call Number Limitation Availability
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    The response of drifting buoys to currents and wind (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 94 (C3). pp. 3201-3210.
    Details
    Publication Date: 2020-10-20
    Description: Two buoy types have been tested with respect to their drift performance under drogued and undrogued conditions. Additionally, forces acting on the buoys were measured directly. Quadratic drag laws have been confirmed for the drag in water and the combined drag of wind and waves. Stokes drift contributes about one half to the wind factor of 0.023, which is obtained for undrogued buoys in the Atlantic. The forces on a windowshade drogue are given by a linear relation between force and water velocity for speeds exceeding 10 cm/s. They have been extrapolated to speeds of less than 10 cm/s by both a linear and a quadratic relationship. Correlations between drift and wind speed in the Atlantic suggest that the linear law is a better approximation under realistic conditions. According to these measurements in the Atlantic the described buoy-drogue system with a windowshade drogue in 100-m depth is a good current-measuring device. Slippage is negligible for wind speeds of less than 15 m/s and is less than 2 cm/s under gale conditions. Undrogued buoys are strongly affected by wind and cannot be used for the analysis of currents without correction, even under light winds.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/JC094iC03p03201
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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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
  • 4
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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
  • 5
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    A note on overflow eddies (1996)
    Elsevier
    In:  Deep Sea Research Part I: Oceanographic Research Papers, 43 . pp. 1661-1667.
    Details
    Publication Date: 2016-10-20
    Description: Satellite-tracked drifters drogued at 100 m depth show very intensive cyclonic eddies over the Denmark Strait Overflow Water (DSOW) along the continental slope of East Greenland between the Denmark Strait and Cape Farewell. Their core of 20 km diameter is probably in near solid-body rotation; azimuthal speed increases with depth, which is consistent with downward velocities in the core, and supports the conclusions of Bruce (1995). At 100m depth, velocities in the eddies reach 50 cm s−1 south of the Denmark Strait and 15 cm s−1 near Cape Farewell. Maximum speed in one of the eddies is found at the top of the DSOW, σθ = 27.80. The eddies may play an important role in water mass conversion in the Irminger Sea.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/S0967-0637(96)00073-8
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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