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Alana M. Althaus, Eric Kunze, and Thomas B. Sanford

Abstract

Strong semidiurnal internal tides are observed near Mendocino Escarpment in full-depth profile time series of velocity, temperature, and salinity. Velocity and density profiles are combined to estimate the internal tide energy flux. Divergence of this flux demonstrates that its source is the barotropic tide interacting with the escarpment. A baroclinic energy flux of 7 kW m−1 radiates from the escarpment, corresponding to 3% of the 220 kW m−1 fluxing poleward in the surface tide. Energy and energy flux are concentrated in packets that emanate from the flanks of the ridge surmounting the escarpment and one site ∼90 km north of the escarpment. Coherent beamlike structure along semidiurnal ray paths remains identifiable until the first surface reflection. Beyond the first surface reflection north of the escarpment, the energy flux drops by 2 kW m−1 and beams are no longer discernible. Turbulence, as inferred from finescale parameterizations, is elevated by over two orders of magnitude relative to the open-ocean interior in localized 500-m-thick layers at the bottom over the ridge crest, near the surface at the station closest to the first surface reflection to the north, slightly north of the first bottom reflection to the north, and on the south flank of the escarpment. Despite its intensity, turbulent dissipation integrated over the ridge crest is only 1% of the energy flux in the internal tides. Thus, the bulk of surface tidal losses at the escarpment is radiating away as internal waves. High turbulent dissipation rates near the surface reflection suggest that loss of energy flux there may be turbulent. This turbulence may arise from (i) Wentzel–Kramers–Brillouin amplification of semidiurnal shear as the internal tide propagates into high near-surface stratification or (ii) superposition of incident and reflected waves enhancing nonlinear transfers to small scales and turbulence production. Localized mixing due to internal tide beams impinging on the base of the mixed layer may be an important unconsidered cause of nutrient and water-mass fluxes between the surface layer and the upper pycnocline.

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A. E. Gargett, P. J. Hendricks, T. B. Sanford, T. R. Osborn, and A. J. Williams

Abstract

Results from three separate velocity profilers operated nearly simultaneously in the northwest Atlantic in 1975 are used to form a composite shear spectrum over vertical wavelengths from 100 m down to a few centimeters. This exercise constitutes an intercomparison of the three different measurement techniques and reveals a shear spectrum which is approximately fiat at a WKB-scaled level from k = 0.01 cpm through k 0 ≈ 0.1 cpm, then falls as k −1 to a buoyancy wavenumber k 0 = (N 3/ε)1/2 determined by the local average Väisälä frequency N and the volume-averaged dissipation rate ε. Various consequences of the observed shear spectral shape are explored.

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Je-Yuan Hsu, Ren-Chieh Lien, Eric A. D’Asaro, and Thomas B. Sanford

Abstract

Estimates of drag coefficients beneath Typhoon Megi (2010) are calculated from roughly hourly velocity profiles of three EM-APEX floats, air launched ahead of the storm, and from air-deployed dropsondes measurements and microwave estimates of the 10-m wind field. The profiles are corrected to minimize contributions from tides and low-frequency motions and thus isolate the current induced by Typhoon Megi. Surface wind stress is computed from the linear momentum budget in the upper 150 m. Three-dimensional numerical simulations of the oceanic response to Typhoon Megi indicate that with small corrections, the linear momentum budget is accurate to 15% before the passage of the eye but cannot be applied reliably thereafter. Monte Carlo error estimates indicate that stress estimates can be made for wind speeds greater than 25 m s−1; the error decreases with greater wind speeds. Downwind and crosswind drag coefficients are computed from the computed stress and the mapped wind data. Downwind drag coefficients increase to 3.5 ± 0.7 × 10−3 at 31 m s−1, a value greater than most previous estimates, but decrease to 2.0 ± 0.4 × 10−3 for wind speeds > 45 m s−1, in agreement with previous estimates. The crosswind drag coefficient of 1.6 ± 0.5 × 10−3 at wind speeds 30–45 m s−1 implies that the wind stress is about 20° clockwise from the 10-m wind vector and thus not directly downwind, as is often assumed.

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Jonathan D. Nash, Eric Kunze, Craig M. Lee, and Thomas B. Sanford

Abstract

Repeat transects of full-depth density and velocity are used to quantify generation and radiation of the semidiurnal internal tide from Kaena Ridge, Hawaii. A 20-km-long transect was sampled every 3 h using expendable current profilers and the absolute velocity profiler. Phase and amplitude of the baroclinic velocity, pressure, and vertical displacement were computed, as was the energy flux. Large barotropically induced isopycnal heaving and strong baroclinic energy-flux divergence are observed on the steep flanks of the ridge where upward and downward beams radiate off ridge. Directly above Kaena Ridge, strong kinetic energy density and weak net energy flux are argued to be a horizontally standing wave. The phasing of velocity and vertical displacements is consistent with this interpretation. Results compare favorably with the Merrifield and Holloway model.

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Thomas B. Sanford, James A. Carlson, John H. Dunlap, Mark D. Prater, and Ren-Chieh Lien

Abstract

An instrument has been developed that measures finescale velocity and vorticity in seawater based on the principles of motional induction. This instrument, the electromagnetic vorticity meter (EMVM), measures components of the gradient and Laplacian of the electrostatic potential field induced by the motion of seawater through an applied magnetic field. The principal innovation described here is the development of a sensor for measuring small-scale vorticity. The sensor head consists of a strong NdFeB magnet, a five-electrode array, low-noise preamplifiers, and 20-Hz digitizers. The main electronics includes attitude sensors, batteries, a microprocessor, and a hard disk. The vorticity sensors are usually carried on a heavy towed vehicle capable of vertically profiling to 200 m and at tow speeds of several knots.

The theoretical response functions of the EMVM are evaluated for velocity and vorticity. Extensive measurements were obtained in Pickering Passage, Washington, as the sensor vertically profiled in an unstratified tidal channel. During periods of strong flow, the vertical structure of all properties confirmed expectations for a fully developed turbulent bottom boundary layer. EMVM observations of velocity and vorticity are shown to be in agreement with the theoretical response function for isotropic turbulence. A principal result is that the vertical flux of spanwise vorticity (i.e., wωy) is positive (i.e., flux is away from seabed) and vertically uniform. The vertical eddy diffusivity for vorticity is about 5 × 10−2 m2 s−1, which is about the same value as for momentum.

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Je-Yuan Hsu, Ren-Chieh Lien, Eric A. D’Asaro, and Thomas B. Sanford

Abstract

Seven subsurface Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats measured the voltage induced by the motional induction of seawater under Typhoon Fanapi in 2010. Measurements were processed to estimate high-frequency oceanic velocity variance associated with surface waves. Surface wave peak frequency f p and significant wave height H s are estimated by a nonlinear least squares fitting to , assuming a broadband JONSWAP surface wave spectrum. The H s is further corrected for the effects of float rotation, Earth’s geomagnetic field inclination, and surface wave propagation direction. The f p is 0.08–0.10 Hz, with the maximum f p of 0.10 Hz in the rear-left quadrant of Fanapi, which is ~0.02 Hz higher than in the rear-right quadrant. The H s is 6–12 m, with the maximum in the rear sector of Fanapi. Comparing the estimated f p and H s with those assuming a single dominant surface wave yields differences of more than 0.02 Hz and 4 m, respectively. The surface waves under Fanapi simulated in the WAVEWATCH III (ww3) model are used to assess and compare to float estimates. Differences in the surface wave spectra of JONSWAP and ww3 yield uncertainties of <5% outside Fanapi’s eyewall and >10% within the eyewall. The estimated f p is 10% less than the simulated before the passage of Fanapi’s eye and 20% less after eye passage. Most differences between H s and simulated are <2 m except those in the rear-left quadrant of Fanapi, which are ~5 m. Surface wave estimates are important for guiding future model studies of tropical cyclone wave–ocean interactions.

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Craig M. Lee, Thomas B. Sanford, Eric Kunze, Jonathan D. Nash, Mark A. Merrifield, and Peter E. Holloway

Abstract

Full-depth velocity and density profiles taken along the 3000-m isobath characterize the semidiurnal internal tide and bottom-intensified turbulence along the Hawaiian Ridge. Observations reveal baroclinic energy fluxes of 21 ± 5 kW m−1 radiating from French Frigate Shoals, 17 ± 2.5 kW m−1 from Kauai Channel west of Oahu, and 13 ± 3.5 kW m−1 from west of Nihoa Island. Weaker fluxes of 1–4 ± 2 kW m−1 radiate from the region near Necker Island and east of Nihoa Island. Observed off-ridge energy fluxes generally agree to within a factor of 2 with those produced by a tidally forced numerical model. Average turbulent diapycnal diffusivity K is (0.5–1) × 10−4 m2 s–1 above 2000 m, increasing exponentially to 20 × 10−4 m2 s–1 near the bottom. Microstructure values agree well with those inferred from a finescale internal wave-based parameterization. A linear relationship between the vertically integrated energy flux and vertically integrated turbulent dissipation rate implies that dissipative length scales for the radiating internal tide exceed 1000 km.

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Thomas B. Sanford, Peter G. Black, James R. Haustein, James W. Feeney, George Z. Forristall, and James F. Price

Abstract

The response of the ocean to hurricanes was investigated using aircraft-deployable expendable current profilers (AXCP). The goals were to observe and separate the surface wave and surface mixed layer (SML) velocities under the storms and to map the across-track and along-track velocity and temperature response in the mixed layer and thermocline. Custom instrumentation was prepared, including slower failing AXCPs, and the AXCP equipment was installed on NOAA WP-3D aircraft. Research flights were made into two 1984 hurricanes: Norbert, in the eastern Pacific off Baja California (19°N, 109°W), and Josephine, off the east coast of the United States (29°N, 72°W). Thirty-one probes were deployed in each hurricane, and about half the AXCPs provided temperature and velocity profiles. Most velocity profiles exhibited strong surface wave contributions, slablike velocities in the SML, strong shears beneath the SML, and only weak flows in the upper thermocline. Separation of the surface gravity wave velocities from the steady and inertial motions was obtained by fitting the profiles to steady flows and shears in three layers and to a single surface wave at all levels. The velocity profiles displayed large divergences to the horizontal SML velocities in the wake of the hurricanes. The observations show a strong enhancement of SML velocities to the right of the storm as expected from numerical simulations. The largest SML velocities were 1.1 m s−1 in Norbert and 0.73 m s−1in Josephine. Numerical simulations will be compared with the observations in Part II.

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Jody M. Klymak, James N. Moum, Jonathan D. Nash, Eric Kunze, James B. Girton, Glenn S. Carter, Craig M. Lee, Thomas B. Sanford, and Michael C. Gregg

Abstract

An integrated analysis of turbulence observations from four unique instrument platforms obtained over the Hawaiian Ridge leads to an assessment of the vertical, cross-ridge, and along-ridge structure of turbulence dissipation rate and diffusivity. The diffusivity near the seafloor was, on average, 15 times that in the midwater column. At 1000-m depth, the diffusivity atop the ridge was 30 times that 10 km off the ridge, decreasing to background oceanic values by 60 km. A weak (factor of 2) spring–neap variation in dissipation was observed. The observations also suggest a kinematic relationship between the energy in the semidiurnal internal tide (E) and the depth-integrated dissipation (D), such that DE 1±0.5 at sites along the ridge. This kinematic relationship is supported by combining a simple knife-edge model to estimate internal tide generation, with wave–wave interaction time scales to estimate dissipation. The along-ridge kinematic relationship and the observed vertical and cross-ridge structures are used to extrapolate the relatively sparse observations along the length of the ridge, giving an estimate of 3 ± 1.5 GW of tidal energy lost to turbulence dissipation within 60 km of the ridge. This is roughly 15% of the energy estimated to be lost from the barotropic tide.

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Peter G. Black, Eric A. D'Asaro, William M. Drennan, Jeffrey R. French, Pearn P. Niiler, Thomas B. Sanford, Eric J. Terrill, Edward J. Walsh, and Jun A. Zhang

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (C D) values derived from CBLAST momentum flux measurements show C D becoming invariant with wind speed near a 23 m s−1 threshold rather than a hurricane-force threshold near 33 m s−1 . Values above 23 m s−1 are lower than previous open-ocean measurements.

The Dalton number estimates (C E) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s −1 which is in approximate agreement with previous measurements at lower winds. These observations imply a C E/C D ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be “roll vortex” or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery.

As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and subsurface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s−1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.

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