Can Drake Passage Observations Match Ekman's Classic Theory?

Abstract Ekman's theory of the wind-driven ocean surface boundary layer assumes a constant eddy viscosity and predicts that the current rotates with depth at the same rate as it decays in amplitude. Despite its wide acceptance, Ekman current spirals are difficult to observe. This is primarily because the spirals are small signals that are easily masked by ocean variability and cannot readily be separated from the geostrophic component. This study presents a method for estimating ageostrophic currents from shipboard acoustic Doppler current profiler data in Drake Passage and finds that observations are consistent with Ekman's theory. By taking into account the sampling distributions of wind stress and ageostrophic velocity, the authors find eddy viscosity values in the range of 0.08–0.12 m2 s−1 that reconcile observations with the classic theory in Drake Passage. The eddy viscosity value that most frequently reconciles observations with the classic theory is 0.094 m2 s−1, corresponding to an Ekman depth scale of 39 m.

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Observing Finescale Oceanic Velocity Structure with an Autonomous Nortek Acoustic Doppler Current Profiler

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-17-0108.1 ◽

2018 ◽

Vol 35 (2) ◽

pp. 411-427 ◽

Cited By ~ 8

Author(s):

Andrey Y. Shcherbina ◽

Eric A. D’Asaro ◽

Sven Nylund

Keyword(s):

Velocity Structure ◽

Acoustic Doppler Current Profiler ◽

Doppler Velocity ◽

Surface Boundary ◽

Ocean Turbulence ◽

Surface Boundary Layer ◽

Oceanic Surface ◽

Current Profiler ◽

Recent Developments ◽

Targeted Observation

AbstractThis paper describes the instrumentation and techniques for long-term targeted observation of the centimeter-scale velocity structure within the oceanic surface boundary layer, made possible by the recent developments in capabilities of autonomous platforms and self-contained pulse-coherent acoustic Doppler current profilers (ADCPs). Particular attention is paid to the algorithms of ambiguity resolution (“unwrapping”) of pulse-coherent Doppler velocity measurements. The techniques are demonstrated using the new Nortek Signature1000 ADCP mounted on a Lagrangian float, a combination shown to be capable of observing ocean turbulence in a number of recent studies. Statistical uncertainty of the measured velocities in relation to the ADCP setup is also evaluated. Described techniques and analyses should be broadly applicable to other autonomous and towed applications of pulse-coherent ADCPs.

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Improving Estimates of the Antarctic Circumpolar Current Streamlines in Drake Passage

Journal of Physical Oceanography ◽

10.1175/2007jpo3834.1 ◽

2008 ◽

Vol 38 (5) ◽

pp. 1000-1010 ◽

Cited By ~ 15

Author(s):

Yueng-Djern Lenn ◽

Teresa K. Chereskin ◽

Janet Sprintall

Keyword(s):

Southern Ocean ◽

Antarctic Circumpolar Current ◽

Acoustic Doppler Current Profiler ◽

Drake Passage ◽

Meridional Overturning Circulation ◽

Dynamic Height ◽

Current Profiler ◽

Meridional Overturning ◽

The Mean ◽

Circumpolar Current

Abstract Accurately resolving the mean Antarctic Circumpolar Current (ACC) is essential for determining Southern Ocean eddy fluxes that are important to the global meridional overturning circulation. Previous estimates of the mean ACC have been limited by the paucity of Southern Ocean observations. A new estimate of the mean surface ACC in Drake Passage is presented that combines sea surface height anomalies measured by satellite altimetry with a recent dataset of repeat high-resolution acoustic Doppler current profiler observations. A mean streamfunction (surface height field), objectively mapped from the mean currents, is used to validate two recent dynamic height climatologies. The new streamfunction has narrower and stronger ACC fronts separated by quiescent zones of much weaker flow, thereby improving on the resolution of ACC fronts observed in the other climatologies. Distinct streamlines can be associated with particular ACC fronts and tracked in time-dependent maps of dynamic height. This analysis shows that varying degrees of topographic control are evident in the preferred paths of the ACC fronts through Drake Passage.

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Computation of Geostrophic Streamfunction, Its Derivatives, and Error Estimates from an Array of CPIES in Drake Passage

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-13-00142.1 ◽

2014 ◽

Vol 31 (3) ◽

pp. 656-680 ◽

Cited By ~ 13

Author(s):

Y. L. Firing ◽

T. K. Chereskin ◽

D. R. Watts ◽

K. L. Tracey ◽

C. Provost

Keyword(s):

Error Estimates ◽

Acoustic Doppler Current Profiler ◽

Drake Passage ◽

Height Anomaly ◽

Dimensional Error ◽

Current Profiler ◽

Low Pass ◽

Higher Derivatives ◽

Independent Observations ◽

Temperature And Velocity Fields

Abstract Current and pressure-recording inverted echo sounders (CPIES) were deployed in an eddy-resolving local dynamics array (LDA) in the eddy-rich polar frontal zone (PFZ) in Drake Passage as part of the cDrake experiment. Methods are described for calculating barotropic and baroclinic geostrophic streamfunction and its first, second, and third derivatives by objective mapping of current, pressure, or geopotential height anomaly data from a two-dimensional array of CPIES like the cDrake LDA. Modifications to previous methods result in improved dimensional error estimates on velocity and higher streamfunction derivatives. Simulations are used to test the reproduction of higher derivatives of streamfunction and to verify mapping error estimates. Three-day low-pass-filtered velocity in and around the cDrake LDA can be mapped with errors of 0.04 m s−1 at 4000 dbar, increasing to 0.13 m s−1 at the sea surface; these errors are small compared to typical speeds observed at these levels, 0.2 and 0.65 m s−1, respectively. Errors on vorticity are 9 × 10−6 s−1 near the surface, decreasing with depth to 3 × 10−6 s−1 at 4000 dbar, whereas vorticities in the PFZ eddy field are 4 × 10−5 s−1 (surface) to 1.3 × 10−5 s−1 (4000 dbar). Vorticity gradient errors range from 4 × 10−10 to 2 × 10−10 m −1 s−1, just under half the size of typical PFZ vorticity gradients. Comparisons between cDrake mapped temperature and velocity fields and independent observations (moored current and temperature, lowered acoustic Doppler current profiler velocity, and satellite-derived surface currents) help validate the cDrake method and results.

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Development of a Simple Loop Method for Correcting Acoustic Doppler Current Profiler Discharge Measurements Biased by Sediment Transport

World Environmental and Water Resource Congress 2006 ◽

10.1061/40856(200)161 ◽

2006 ◽

Cited By ~ 1

Author(s):

David S. Mueller ◽

Chad R. Wagner

Keyword(s):

Sediment Transport ◽

Acoustic Doppler Current Profiler ◽

Simple Loop ◽

Loop Method ◽

Current Profiler ◽

Discharge Measurements

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Prediction of Atmospheric Turbulence Characteristics for Surface Boundary Layer using Empirical Spectral Methods

Revista Brasileira de Meteorologia ◽

10.1590/0102-77863540083 ◽

2020 ◽

Author(s):

Yagya Dutta Dwivedi ◽

Vasishta Bhargava Nukala ◽

Satya Prasad Maddula ◽

Kiran Nair

Keyword(s):

Boundary Layer ◽

Time Series ◽

Surface Roughness ◽

Wind Speed ◽

Atmospheric Turbulence ◽

Surface Boundary ◽

Low Frequencies ◽

Surface Boundary Layer ◽

Power Spectral ◽

Mean Wind Speed

Abstract Atmospheric turbulence is an unsteady phenomenon found in nature and plays significance role in predicting natural events and life prediction of structures. In this work, turbulence in surface boundary layer has been studied through empirical methods. Computer simulation of Von Karman, Kaimal methods were evaluated for different surface roughness and for low (1%), medium (10%) and high (50%) turbulence intensities. Instantaneous values of one minute time series for longitudinal turbulent wind at mean wind speed of 12 m/s using both spectra showed strong correlation in validation trends. Influence of integral length scales on turbulence kinetic energy production at different heights is illustrated. Time series for mean wind speed of 12 m/s with surface roughness value of 0.05 m have shown that variance for longitudinal, lateral and vertical velocity components were different and found to be anisotropic. Wind speed power spectral density from Davenport and Simiu profiles have also been calculated at surface roughness of 0.05 m and compared with k−1 and k−3 slopes for Kolmogorov k−5/3 law in inertial sub-range and k−7 in viscous dissipation range. At high frequencies, logarithmic slope of Kolmogorov −5/3rd law agreed well with Davenport, Harris, Simiu and Solari spectra than at low frequencies.

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