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  • 1
    Publication Date: 2018-05-01
    Description: High-frequency (HF) radars remotely measure ocean near-surface currents based on the Doppler shift of electromagnetic waves backscattered by surface gravity waves with half the electromagnetic wavelength, called Bragg waves. Since their phase velocity is affected not only by wave–current interactions with vertically sheared mean Eulerian currents but also by wave–wave interactions with all the other waves present at the sea surface, HF radars should measure a quantity related to the Stokes drift in addition to mean Eulerian currents. However, the literature is inconsistent—both theoretically and experimentally—on the specific expression and even on the existence of the Stokes drift contribution to the HF radar measurements. Three different expressions that have been proposed in the literature are reviewed and discussed in light of the relevant published experimental results: 1) the weighted depth-averaged Stokes drift, 2) the filtered surface Stokes drift, and 3) half of the surface Stokes drift. Effective measurement depths for these three expressions are derived for the Phillips wave spectrum. Recent experimental results tend to discard the second expression but are not inconsistent with the first and third expressions. The latter is physically appealing, since it is a quasi-Eulerian quantity that would be measured by a current meter at a fixed horizontal position but allowed to follow the free surface moving vertically up and down with the passage of the waves. A definitive answer will require further experimental investigations.
    Print ISSN: 0739-0572
    Electronic ISSN: 1520-0426
    Topics: Geography , Geosciences , Physics
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  • 2
    Publication Date: 2017-02-01
    Description: A 1.6-yr time series of radial current velocity from a 25-MHz high-frequency radar system located near a coastal river plume is analyzed to determine how the working range varies in response to changing near-surface conductivity, sea state, and tides. Working range is defined as the distance to the farthest radial velocity solution along a fixed bearing. A comparison to spatially resolved near-surface conductivity measurements from an instrumented ferry shows that fluctuations in conductivity had the largest impact of the three factors considered. The working range increases nearly linearly with increasing conductivity, almost doubling from 19.4 km at 0.9 S m−1 to 37.4 km at 3.5 S m−1, which yields a slope of 7.0 km per S m−1. The next largest factor was sea state, which was investigated using measured winds. The working range increases linearly at a rate of 1 km per m s−1 of wind speed over the range of 0.5–6.5 m s−1, but it decreases weakly for wind speeds higher than 7.5 m s−1. Finally, a power spectrum of the working range time series reveals variability at tidal frequencies. Tides cause about 3 km of range variation; however, the mechanism(s) underlying this are not known explicitly. Evidence for both sea level height and the interaction of tidal currents with sea state are presented.
    Print ISSN: 0739-0572
    Electronic ISSN: 1520-0426
    Topics: Geography , Geosciences , Physics
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  • 3
    Publication Date: 2016-03-01
    Description: High-frequency radars (HFRs) measure ocean surface currents remotely through the Bragg scattering of radio waves by surface gravity waves with wavelengths shorter than 50 m. HFR range is affected by sea ice, which dampens surface gravity waves and limits wind fetch for adjacent open waters. HFR range sensitivity to sea ice concentration was empirically determined for two types of HFR—Coastal Ocean Dynamics Applications Radar (CODAR) and Wellen Radar (WERA)—installed on the shores of the lower St. Lawrence estuary, Canada, during winter 2013. One CODAR was operating at 13.5 MHz on the southern shore, and one WERA was operating at 16.15 MHz on the northern shore. Ranges were determined using a signal-to-noise ratio threshold of 6 dB for first-order Bragg scattering measured by the receive antenna elements. Ranges were normalized for expected ranges in ice-free conditions, using empirical relationships determined during summer 2013 between the range and surface gravity wave energy at the Bragg frequencies. Normalized ranges Γ decrease approximately linearly with increasing sea ice concentration C (averaged over the ice-free observational domain) with a slope close to −1 for both HFR types, that is, Γ = 1 − C. However, for a given sea ice concentration, range can vary significantly depending on the sea ice spatial distribution.
    Print ISSN: 0739-0572
    Electronic ISSN: 1520-0426
    Topics: Geography , Geosciences , Physics
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  • 4
    Publication Date: 2016-01-01
    Description: A quasigeostrophic model is developed to diagnose the three-dimensional circulation, including the vertical velocity, in the upper ocean from high-resolution observations of sea surface height and buoyancy. The formulation for the adiabatic component departs from the classical surface quasigeostrophic framework considered before since it takes into account the stratification within the surface mixed layer that is usually much weaker than that in the ocean interior. To achieve this, the model approximates the ocean with two constant stratification layers: a finite-thickness surface layer (or the mixed layer) and an infinitely deep interior layer. It is shown that the leading-order adiabatic circulation is entirely determined if both the surface streamfunction and buoyancy anomalies are considered. The surface layer further includes a diabatic dynamical contribution. Parameterization of diabatic vertical velocities is based on their restoring impacts of the thermal wind balance that is perturbed by turbulent vertical mixing of momentum and buoyancy. The model skill in reproducing the three-dimensional circulation in the upper ocean from surface data is checked against the output of a high-resolution primitive equation numerical simulation.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
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  • 5
    Publication Date: 2019-03-01
    Description: High-frequency Doppler radar (HFDR) and acoustic Doppler current profiler (ADCP) time-series observations during the Philippine Straits Dynamics Experiment (PhilEx) were analyzed to describe the mesoscale currents in Panay Strait, Philippines. Low-frequency surface currents inferred from three HFDR (July 2008–July 2009), reveal a clear seasonal signal concurrent with the reversal of the Asian monsoon. A mesoscale cyclonic eddy west of Panay Island is generated during the winter northeast (NE) monsoon. This causes changes in the strength, depth, and width of the intraseasonal Panay coastal (PC) jet as its eastern limb. Winds from QuikSCAT and from a nearby airport indicate that these flow structures correlate with the strength and direction of the prevailing local wind. An intensive survey in 8–9 February 2009 using 24 h of successive cross-shore conductivity–temperature–depth (CTD) sections, which in conjunction with shipboard ADCP measurements, show a well-developed cyclonic eddy characterized by near-surface velocities of 50 cm s−1. This eddy coincides with the intensification of the wind in between Mindoro and Panay Islands, generating a positive wind stress curl in the lee of Panay, which in turn induces divergent surface currents. Water column response from the mean transects show a pronounced signal of upwelling, indicated by the doming of isotherms and isopycnals. A pressure gradient then is set up, resulting in the spin up of a cyclonic eddy in geostrophic balance. Evolution of the vorticity within the vortex core confirms wind stress curl as the dominant forcing.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
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  • 6
    Publication Date: 2012-05-01
    Description: The short answer to the question posed in the title is that it depends on the frame of reference chosen to describe the motions. In the inertial limit, the frequency in a rotating frame of reference corresponds to the rotation rate of the inertial current vectors relative to that frame. When described in a reference frame rotating with a geostrophic flow having a relative vertical vorticity ζ, inertial oscillations have a frequency f + ζ, equal to twice the fluid’s rotation rate around the local vertical axis. From a nonrotating frame of reference, one would measure only half this frequency; the other half arises from describing inertial motions in a reference frame rotating with the background flow. However, when described in a reference frame rotating with Earth, hence rotating at −ζ/2 relative to the geostrophic frame, inertial oscillations have a frequency reduced to f + ζ/2.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
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  • 7
    Publication Date: 2010-08-01
    Description: The evolution of a submesoscale anticyclonic vortex was observed by high-frequency Doppler radio current meters and satellite radiometers. The vortex formed between two large cyclones to the southwest of Oahu, Hawaii. The radius of the core was ∼15 km; the azimuthal velocity reached 35 cm s−1; and the surface vorticity remained below −f for 9 days, reaching an extremum of −1.7f. The flow was ageostrophic near the center and around the periphery of the vortex. The initial growth may have been driven by negative wind stress curl in the lee of Oahu. The vortex was prone to inertial, symmetric, and anticyclonic ageostrophic instabilities, but the temporal evolution of radial profiles of vorticity was inconsistent with angular momentum redistribution by inertial instability. A tongue of surface water 0.7°C warmer became entrained northward between the vortex and the colder cyclone to the west. As the vortex strengthened, a 0.14°C km−1 front formed along the eastern flank of the tongue. The sea surface temperature gradient remained weaker on the western flank. The flow was anticyclonic (−0.4f ) and divergent (0.1f ) on the warm side of the front but cyclonic (0.6f ) and convergent (−0.2f ) on the cold side. This suggests ageostrophic cross-frontal circulations maintaining alongfront thermal wind balance in the presence of large-scale strain σ. Surface divergence δ was proportional to vorticity ζ during the 3-day frontogenesis: δ ∼ −(σ/f )ζ. This is consistent with a semigeostrophic model of a front confined to a surface layer of zero potential vorticity.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
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  • 8
    Publication Date: 2016-12-01
    Electronic ISSN: 2041-1723
    Topics: Biology , Chemistry and Pharmacology , Natural Sciences in General , Physics
    Published by Springer Nature
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  • 9
    Publication Date: 2010-10-01
    Description: Short-wavelength (L ∼ 100 km) Rossby waves with an eastward zonal phase velocity were observed by high-frequency radio Doppler current meters and moored ADCPs west of Oahu, Hawaii, during spring 2003. They had Rossby numbers Ro = |ζ/f| = O(1), periods of 12–15 days, and phase speeds of 8–9 cm s−1, and they were surface trapped with vertical e-folding scales of 30–170 m. They transferred horizontal kinetic energy to the background flow of a mesoscale cyclone lying 160–190 km west of Oahu, revealed by altimetry. The waves approximately satisfied the dispersion relation of vortex Rossby waves propagating through the radial gradient of potential vorticity associated with the cyclone. Vertical shear of the background currents may also affect wave propagation. Theoretical studies have shown that vortex Rossby waves provide a mechanism by which perturbed vortices axisymmetrize and strengthen and may be important to the dynamics of oceanic vortices.
    Print ISSN: 0022-3670
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  • 10
    Publication Date: 2019-11-01
    Description: The ocean’s inverse cascade of energy from small to large scales has been confirmed from satellite altimetry for scales larger than 100 km. However, measurements of the direct energy cascade to smaller scales have remained difficult to obtain. Here, the possibility of estimating these energy transfers to smaller scales from observations by high-frequency radars is investigated using numerical simulations. Synthetic measurements are first extracted from a quasigeostrophic simulation of freely decaying turbulence for which the reference energy flux is characterized by the transition from positive to negative values. Fluxes obtained from synthetic data are compared to this reference flux in order to assess the robustness to various measurement limitations. The geometry of the observational domain (nonperiodicity, domain size, and aspect ratio) affects mostly large scales, while the spatial resolution of the instruments affects mostly small scales. In contrast, measurement noise and missing data affect both large and small scales. Despite resulting in significant biases in the amplitude of the fluxes, the transition scale between the positive and negative fluxes is relatively robust to measurement limitations. These results are also confirmed using a simulation from a primitive-equations model in a realistic coastal geometry.
    Print ISSN: 0739-0572
    Electronic ISSN: 1520-0426
    Topics: Geography , Geosciences , Physics
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