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Luc Rainville and Robert Pinkel

Abstract

An inexpensive vertically profiling float that draws its energy from the ocean surface wavefield is described. Termed the “Wirewalker,” it is a generalized platform capable of supporting a variety of self-contained instruments. The motion of the waves drives the positively buoyant profiler downward. It then free floats upward, decoupled from the surface motion field. The design focuses on mechanical simplicity and low cost. In moderate sea states, a prototype Wirewalker has completed profiles to depths of 60 m every 15 min. Profiles from the surface to 50–100 m can be obtained rapidly enough that diel and higher-frequency variability can be resolved.

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Luc Rainville and Robert Pinkel

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Intense and spatially coherent shear layers were detected in and under the Kuroshio in an April 2000 survey in the East China Sea. The sloping layers, revealed by shipboard Doppler sonars on the R/V Roger Revelle, appeared to cross isopycnal surfaces. Except in a small region near the Kuroshio shelfbreak front, the rms finescale shear associated with the layers significantly exceeded the geostrophic shear. An April 2002 follow-on cruise was organized to establish whether these motions were propagating internal waves. Shipboard and lowered ADCPs were both operated from the R/V Melville. In addition to CTD-sonar transects, a 30-h time series of currents and shear was obtained in the core of the Kuroshio near the island of Kyushu, Japan. The shear structures were indeed found to be propagating, with both up- and down-going internal wave motions present. In comparison with the nearby open ocean, the finescale (<160 m) vertical shear variance is increased by a factor of 3 in the Kuroshio and by a factor of 6 in the region between the shelf break and the Kuroshio. If energy dissipation indeed scales as shear variance squared, very high values of dissipation (over 30 times the open-ocean value) can be anticipated in this region. It is conjectured that the geostrophic vorticity associated with the Kuroshio acts as a barrier, impeding the seaward propagation of internal waves generated at the shelf break onshore of the Kuroshio front.

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Luc Rainville and Robert Pinkel

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Estimates of baroclinic energy flux are made in the immediate “Nearfield” (September–October 2002) and 450 km offshore (“Farfield”; October–November 2001) of the Kaena Ridge, an active barotropic-to-baroclinic conversion site. The flux estimates are based on repeated profiles of velocity and density obtained from the Research Platform Floating Instrument Platform (FLIP) as an aspect of the Hawaii Ocean Mixing Experiment. Energetic beams associated with both semidiurnal and diurnal internal waves are observed in the Kauai Channel. Beam depths and orientations are consistent with generation along the upper flanks of the ridge. At the far-field site, the baroclinic energy flux is borne primarily by first-mode semidiurnal waves. The energy flux associated with the entire spectrum of internal waves is computed by cross-spectral analysis. Significant energy fluxes are found in the inertial, diurnal, semidiurnal, and twice-semidiurnal frequency bands. The semidiurnal energy flux strongly dominates the spectrum at both sites. The flux magnitude follows the spring–neap cycle of the semidiurnal barotropic tide. The averaged depth-integrated mode-1 semidiurnal energy flux (over the entire water column) in the Farfield is found to be 1.7 ± 0.3 kW m−1 away from the ridge, with peak values up to 4 kW m−1. Small fluxes toward the ridge are occasionally seen at neap tide. At both sites, energy fluxes in the diurnal frequency band represent 15%–20% of the semidiurnal energy flux. In the Farfield, the magnitude of the diurnal energy flux varies in accord with the fortnightly cycle of the barotropic semidiurnal tide, rather than with the diurnal forcing, suggesting that energy for those waves is supplied by a cross-frequency transfer from the low-vertical-mode M 2 internal tide to higher-mode internal waves at frequencies ½M 2. In the Nearfield, the diurnal flux varies with fluctuations in both diurnal and semidiurnal forcing.

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Luc Rainville and Robert Pinkel

Abstract

The baroclinic tides play a significant role in the energy budget of the abyssal ocean. Although the basic principles of generation and propagation are known, a clear understanding of these phenomena in the complex ocean environment is only now emerging. To advance this effort, a ray model is developed that quantifies the effects of spatially variable topography, stratification, and planetary vorticity on the horizontal propagation of internal gravity modes. The objective is to identify “baroclinic shoals” where wave energy is spatially concentrated and enhanced dissipation might be expected. The model is then extended to investigate the propagation of internal waves through a barotropic mesoscale current field. The refraction of tidally generated internal waves at the Hawaiian Ridge is examined using an ensemble of mesoscale background realizations derived from weekly Ocean Topography Experiment (TOPEX)/Poseidon altimetric measurements. The path of mode 1 is only slightly affected by typical currents, although its phase becomes increasingly random as the propagation distance from the source increases. The effect of the currents becomes more dramatic as mode number increases. For modes 3 and higher, wave phase can vary between realizations by ±π only a few wavelengths from the source. This phase variability reduces the magnitude of the baroclinic signal seen in altimetric data, creating a fictitious energy loss along the propagation path. In the TOPEX/Poseidon observations, the mode-1 M 2 internal tide does appear to lose significant energy as it propagates southwestward from the Hawaiian Ridge. The simulations suggest that phase modulation by mesoscale flows could be responsible for a large fraction of this apparent loss. In contrast, northeast-propagating internal tides encounter a less energetic mesoscale and should experience limited refraction. The apparent energy loss seen in the altimetric data on the north side of the ridge might indeed be real.

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Hayley V. Dosser and Luc Rainville

ABSTRACT

The dynamics of the wind-generated near-inertial internal wave field in the Canada Basin of the Arctic Ocean are investigated using the drifting Ice-Tethered Profiler dataset for the years 2005 to 2014, during a decade when sea ice extent and thickness decreased dramatically. This time series, with nearly 10 years of measurements and broad spatial coverage, is used to quantify a seasonal cycle and interannual trend for internal waves in the Arctic, using estimates of the amplitude of near-inertial waves derived from isopycnal displacements. The internal wave field is found to be most energetic in summer when sea ice is at a minimum, with a second maximum in early winter during the period of maximum wind speed. Amplitude distributions for the near-inertial waves are quantifiably different during summer and winter, due primarily to seasonal changes in sea ice properties that affect how the ice responds to the wind, which can be expressed through the “wind factor”—the ratio of sea ice drift speed to wind speed. A small positive interannual trend in near-inertial wave energy is linked to pronounced sea ice decline during the last decade. Overall variability in the internal wave field increases significantly over the second half of the record, with an increased probability of larger-than-average waves in both summer and winter. This change is linked to an overall increase in variability in the wind factor and sea ice drift speeds, and reflects a shift in year-round sea ice characteristics in the Arctic, with potential implications for dissipation and mixing associated with internal waves.

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Robert Pinkel, Luc Rainville, and Jody Klymak

Abstract

Kaena Ridge, Hawaii, is a site of energetic conversion of the semidiurnal barotropic tide. Diffuse baroclinic wave beams emanate from the critical-slope regions near the ridge crest, directed upward and southward from the north flank of the ridge and upward and northward from the south flank. Here, the momentum fluxes associated with generation at the ridge are estimated. Continuous vertical profiles of density and velocity from 80 to 800 m were obtained from the Research Platform Floating Instrument Platform (FLIP) over the southern edge of the ridge, as an aspect of the Hawaii Ocean Mixing Experiment. Data are used to estimate the Reynolds stress, Eulerian buoyancy flux, and the combined Eliassen–Palm flux in the semidiurnal band. An upward–southward stress maximum of ~0.5 × 10−4 m2 s−2 appears at depths of 300–500 m, generally consistent with beam-like behavior. A strong off-ridge buoyancy flux (~0.3 × 10−4 m2 s−3) combines with large along-ridge Reynolds stresses to form an Eliassen–Palm flux whose along-ridge and across-ridge magnitudes are comparable. The stress azimuth rotates clockwise with increasing altitude above the ridge crest. The principal upward–southward beam is found to be at depths 100–300 m shallower than are predicted by an analytic two-dimensional (2D) model and a 3D numerical simulation. This discrepancy is consistent with previous observations of the baroclinic energy flux. If these observed tidal momentum fluxes were to diverge in a 100-m-thick near-surface layer, the forcing would be comparable to a moderate wind stress. Pronounced lateral gradients of baroclinic tidal stresses can be expected offshore of Hawaiian topography.

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Jody M. Klymak, Robert Pinkel, and Luc Rainville

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Barotropic to baroclinic conversion and attendant phenomena were recently examined at the Kaena Ridge as an aspect of the Hawaii Ocean Mixing Experiment. Two distinct mixing processes appear to be at work in the waters above the 1100-m-deep ridge crest. At middepths, above 400 m, mixing events resemble their open-ocean counterparts. There is no apparent modulation of mixing rates with the fortnightly cycle, and they are well modeled by standard open-ocean parameterizations. Nearer to the topography, there is quasi-deterministic breaking associated with each baroclinic crest passage. Large-amplitude, small-scale internal waves are triggered by tidal forcing, consistent with lee-wave formation at the ridge break. These waves have vertical wavelengths on the order of 400 m. During spring tides, the waves are nonlinear and exhibit convective instabilities on their leading edge. Dissipation rates exceed those predicted by the open-ocean parameterizations by up to a factor of 100, with the disparity increasing as the seafloor is approached. These observations are based on a set of repeated CTD and microconductivity profiles obtained from the research platform (R/P) Floating Instrument Platform (FLIP), which was trimoored over the southern edge of the ridge crest. Ocean velocity and shear were resolved to a 4-m vertical scale by a suspended Doppler sonar. Dissipation was estimated both by measuring overturn displacements and from microconductivity wavenumber spectra. The methods agreed in water deeper than 200 m, where sensor resolution limitations do not limit the turbulence estimates. At intense mixing sites new phenomena await discovery, and existing parameterizations cannot be expected to apply.

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Suneil Iyer, Kyla Drushka, and Luc Rainville

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As part of the second Salinity Processes in the Upper Ocean Regional Study (SPURS-2), the ship-towed Surface Salinity Profiler (SSP) was used to measure near-surface turbulence and stratification on horizontal spatial scales of tens of kilometers over time scales of hours, resolving structures outside the observational capabilities of autonomous or Lagrangian platforms. Observations of microstructure variability of temperature were made at approximately 37 cm depth from the SSP. The platform can be used to measure turbulent kinetic energy dissipation rate when the upper ocean is sufficiently stratified by calculating temperature gradient spectra from the microstructure data and fitting to low-wavenumber theoretical Batchelor spectra. Observations of dissipation rate made across a range of wind speeds under 12 m s−1 were consistent with the results of previous studies of near-surface turbulence and with existing turbulence scalings. Microstructure sensors mounted on the SSP can be used to investigate the spatial structure of near-surface turbulence. This provides a new means to study the connections between near-surface turbulence and the larger-scale distributions of heat and salt in the near-surface layer of the ocean.

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Olivier Asselin, Leif N. Thomas, William R. Young, and Luc Rainville

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Fast-moving synoptic-scale atmospheric disturbances produce large-scale near-inertial waves in the ocean mixed layer. In this paper, we analyze the distortion of such waves by smaller-scale barotropic eddies, with a focus on the evolution of the horizontal wavevector k under the effects of straining and refraction. The model is initialized with a horizontally uniform (k = 0) surface-confined near-inertial wave, which then evolves according to the phase-averaged model of Young and Ben Jelloul. A steady barotropic vortex dipole is first considered. Shear bands appear in the jet region as wave energy propagates downward and toward the anticyclone. When measured at a fixed location, both horizontal and vertical wavenumbers grow linearly with the time t elapsed since generation such that their ratio, the slope of wave bands, is time independent. Analogy with passive scalar dynamics suggests that straining should result in the exponential growth of |k|. Here instead, straining is ineffective, not only at the jet center, but also in its confluent and diffluent regions. Low modes rapidly escape below the anticyclonic core such that weakly dispersive high modes dominate in the surface layer. In the weakly dispersive limit, k = −tζ(x, y, t)/2 provided that (i) the eddy vertical vorticity ζ evolves according to the barotropic quasigeostrophic equation and (ii) k = 0 initially. In steady flows, straining is ineffective because k is always perpendicular to the flow. In unsteady flows, straining modifies the vorticity gradient and hence k, and may account for significant wave–eddy energy transfers.

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Hayley V. Dosser, Luc Rainville, and John M. Toole

Abstract

Salinity and temperature profiles from drifting ice-tethered profilers in the Beaufort gyre region of the Canada Basin are used to characterize and quantify the regional near-inertial internal wave field over one year. Vertical displacements of potential density surfaces from the surface to 750-m depth are tracked from fall 2006 to fall 2007. Because of the time resolution and irregular sampling of the ice-tethered profilers, near-inertial frequency signals are marginally resolved. Complex demodulation is used to determine variations with a time scale of several days in the amplitude and phase of waves at a specified near-inertial frequency. Characteristics and variability of the wave field over the course of the year are investigated quantitatively and related to changes in surface wind forcing and sea ice cover.

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