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Kristin L. Zeiden
,
Daniel L. Rudnick
, and
Jennifer A. MacKinnon

Abstract

In this study, a 2-yr time series of velocity profiles to 1000 m from meridional glider surveys is used to characterize the wake in the lee of a large island in the western tropical North Pacific Ocean, Palau. Surveys were completed along sections to the east and west of the island to capture both upstream and downstream conditions. Objectively mapped in time and space, mean sections of velocity show the incident westward North Equatorial Current accelerating around the island of Palau, increasing from 0.1 to 0.2 m s−1 at the surface. Downstream of the island, elevated velocity variability and return flow in the lee are indicative of boundary layer separation. Isolating for periods of depth-average westward flow reveals a length scale in the wake that reflects local details of the topography. Eastward flow is shown to produce an asymmetric wake. Depth-average velocity time series indicate that energetic events (on time scales from weeks to months) are prevalent. These events are associated with mean vorticity values in the wake up to 0.3f near the surface and with instantaneous values that can exceed f (the local Coriolis frequency) during periods of sustained, anomalously strong westward flow. Thus, ageostrophic effects become important to first order.

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André Palóczy
,
Jennifer A. MacKinnon
, and
Amy F. Waterhouse

Abstract

We describe the spatiotemporal variability and vertical structure of turbulent Reynolds stresses (RSs) in a stratified inner shelf with an energetic internal wave climate. The RSs are estimated from direct measurements of velocity variance derived from bottom-mounted acoustic Doppler current profilers. We link the RSs to different physical processes, namely, internal bores, midwater shear instabilities within vertical shear events related to wind-driven subtidal along-shelf currents, and nonturbulent stresses related to incoming nonlinear internal wave (NLIW) trains. The typical RS magnitudes are O(0.01) Pa for background conditions, with diurnal pulses of O(0.1–1) Pa, and O(1) Pa for the NLIW stresses. A NLIW train is observed to produce a depth-averaged vertical stress divergence sufficient to accelerate water 20 cm s−1 in 1 h, suggesting NLIWs may also be important contributors to the depth-averaged momentum budget. The subtidal stresses show significant periodic variability and are O(0.1) Pa. Conditionally averaged velocity and RS profiles for northward/southward flow provide evidence for downgradient turbulent momentum fluxes, but also indicate departures from this expected regime. Estimates of the terms in the depth-averaged momentum equation suggest that the vertical divergence of the RSs are important terms in both the cross-shelf and along-shelf directions, with geostrophy also present at leading-order in the cross-shelf momentum balance. Among other conclusions, the results highlight that internal bores and shoaling NLIWs may also be important dynamical players in other inner shelves with energetic internal waves.

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Zhongxiang Zhao
,
Matthew H. Alford
,
Jennifer A. MacKinnon
, and
Rob Pinkel

Abstract

The northeastward progression of the semidiurnal internal tide from French Frigate Shoals (FFS), Hawaii, is studied with an array of six simultaneous profiling moorings spanning 25.5°–37.1°N (≈1400 km) and 13-yr-long Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter data processed by a new technique. The moorings have excellent temporal and vertical resolutions, while the altimeter offers broad spatial coverage of the surface manifestation of the internal tide’s coherent portion. Together these two approaches provide a unique view of the internal tide’s long-range propagation in a complex ocean environment. The moored observations reveal a rich, time-variable, and multimodal internal tide field, with higher-mode motions contributing significantly to velocity, displacement, and energy. In spite of these contributions, the coherent mode-1 internal tide dominates the northeastward energy flux, and is detectable in both moored and altimetric data over the entire array. Phase and group propagation measured independently from moorings and altimetry agree well with theoretical values. Sea surface height anomalies (SSHAs) measured from moorings and altimetry agree well in amplitude and phase until the northern end of the array, where phase differences arise presumably from refraction by mesoscale flows. Observed variations in SSHA, energy flux, and kinetic-to-potential energy ratio indicate an interference pattern resulting from superposed northeastward radiation from Hawaii and southeastward from the Aleutian Ridge. A simple model of two plane waves explains most of these features.

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Kristin L. Zeiden
,
Daniel L. Rudnick
,
Jennifer A. MacKinnon
,
Verena Hormann
, and
Luca Centurioni

Abstract

Wake eddies are important to physical oceanographers because they tend to dominate current variability in the lee of islands. However, their generation and evolution has been difficult to study due to their intermittency. In this study, 2 years of observations from Surface Velocity Program (SVP) drifters are used to calculate relative vorticity (ζ) and diffusivity (κ) in the wake generated by westward flow past the archipelago of Palau. Over 2 years, 19 clusters of five SVP drifters ∼5 km in scale were released from the north end of the archipelago. Out of these, 15 were entrained in the wake. We compare estimates of ζ from both velocity spatial gradients (least squares fitting) and velocity time series (wavelet analysis). Drifters in the wake were entrained in either energetic submesoscale eddies with initial ζ up to 6f, or island-scale recirculation and large-scale lateral shear with ζ ∼ 0.1f. Here f is the local Coriolis frequency. Mean wake vorticity is initially 1.5f but decreases inversely with time (t), while mean cluster scale (L) increases as Lt. Kinetic energy measured by the drifters is comparatively constant. This suggests ζ is predominantly a function of scale, confirmed by binning enstrophy (ζ 2) by inverse scale. We find κL 4/3 and upper and lower bounds for L(t) are given by t 3/2 and t 1/2, respectively. These trends are predicted by a model of dispersion due to lateral shear. We argue the observed time dependence of cluster scale and vorticity suggest island-scale shear controls eddy growth in the wake of Palau.

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Gunnar Voet
,
Matthew H. Alford
,
Jennifer A. MacKinnon
, and
Jonathan D. Nash

Abstract

Towed shipboard and moored observations show internal gravity waves over a tall, supercritical submarine ridge that reaches to 1000 m below the ocean surface in the tropical western Pacific north of Palau. The lee-wave or topographic Froude number, Nh 0/U 0 (where N is the buoyancy frequency, h 0 the ridge height, and U 0 the farfield velocity), ranged between 25 and 140. The waves were generated by a superposition of tidal and low-frequency flows and thus had two distinct energy sources with combined amplitudes of up to 0.2 m s−1. Local breaking of the waves led to enhanced rates of dissipation of turbulent kinetic energy reaching above 10−6 W kg−1 in the lee of the ridge near topography. Turbulence observations showed a stark contrast between conditions at spring and neap tide. During spring tide, when the tidal flow dominated, turbulence was approximately equally distributed around both sides of the ridge. During neap tide, when the mean flow dominated over tidal oscillations, turbulence was mostly observed on the downstream side of the ridge relative to the mean flow. The drag exerted by the ridge on the flow, estimated to O ( 10 4 ) N m 1 for individual ridge crossings, and the associated power loss, thus provide an energy sink both for the low-frequency ocean circulation and the tidal flow.

Free access
Elizabeth C. Fine
,
Matthew H. Alford
,
Jennifer A. MacKinnon
, and
John B. Mickett

Abstract

In the Beaufort Sea in September of 2015, concurrent mooring and microstructure observations were used to assess dissipation rates in the vicinity of 72°35′N, 145°1′W. Microstructure measurements from a free-falling profiler survey showed very low [ O (10 10) W kg−1] turbulent kinetic energy dissipation rates ε. A finescale parameterization based on both shear and strain measurements was applied to estimate the ratio of shear to strain R ω and ε at the mooring location, and a strain-based parameterization was applied to the microstructure survey (which occurred approximately 100 km away from the mooring site) for direct comparison with microstructure results. The finescale parameterization worked well, with discrepancies ranging from a factor of 1–2.5 depending on depth. The largest discrepancies occurred at depths with high shear. Mean R ω was 17, and R ω showed high variability with values ranging from 3 to 50 over 8 days. Observed ε was slightly elevated (factor of 2–3 compared with a later survey of 11 profiles taken over 3 h) from 25 to 125 m following a wind event which occurred at the beginning of the mooring deployment, reaching a maximum of ε= 6 × 10−10 W kg−1 at 30-m depth. Velocity signals associated with near-inertial waves (NIWs) were observed at depths greater than 200 m, where the Atlantic Water mass represents a reservoir of oceanic heat. However, no evidence of elevated ε or heat fluxes was observed in association with NIWs at these depths in either the microstructure survey or the finescale parameterization estimates.

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Caitlin B. Whalen
,
Jennifer A. MacKinnon
,
Lynne D. Talley
, and
Amy F. Waterhouse

Abstract

Finescale methods are currently being applied to estimate the mean turbulent dissipation rate and diffusivity on regional and global scales. This study evaluates finescale estimates derived from isopycnal strain by comparing them with average microstructure profiles from six diverse environments including the equator, above ridges, near seamounts, and in strong currents. The finescale strain estimates are derived from at least 10 nearby Argo profiles (generally <60 km distant) with no temporal restrictions, including measurements separated by seasons or decades. The absence of temporal limits is reasonable in these cases, since the authors find the dissipation rate is steady over seasonal time scales at the latitudes being considered (0°–30° and 40°–50°). In contrast, a seasonal cycle of a factor of 2–5 in the upper 1000 m is found under storm tracks (30°–40°) in both hemispheres. Agreement between the mean dissipation rate calculated using Argo profiles and mean from microstructure profiles is within a factor of 2–3 for 96% of the comparisons. This is both congruous with the physical scaling underlying the finescale parameterization and indicates that the method is effective for estimating the regional mean dissipation rates in the open ocean.

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Matthew H. Alford
,
Jennifer A. MacKinnon
,
Robert Pinkel
, and
Jody M. Klymak

Abstract

The spatial, temporal, and directional characteristics of shear are examined in the upper 1400 m of the North Pacific during late spring with an array of five profiling moorings deployed from 25° to 37°N (1330 km) and simultaneous shipboard transects past them. The array extended from a regime of moderate wind generation at the north to south of the critical latitude 28.8°N, where parametric subharmonic instability (PSI) can transfer energy from semidiurnal tides to near-inertial motions. Analyses are done in an isopycnal-following frame to minimize contamination by Doppler shifting. Approximately 60% of RMS shear at vertical scales >20m (and 80% for vertical scales >80 m) is contained in near-inertial motions. An inertial back-rotation technique is used to index shipboard observations to a common time and to compute integral time scales of the shear layers. Persistence times are O(7) days at most moorings but O(25) days at the critical latitude. Simultaneous shipboard transects show that these shear layers can have lateral scales ≥100 km. Layers tend to slope downward toward the equator north of the critical latitude and are more flat to its south. Phase between shear and strain is used to infer lateral propagation direction. Upgoing waves are everywhere laterally isotropic. Downgoing waves propagate predominantly equatorward north and south of the critical latitude but are isotropic near it. Broadly, results are consistent with wind generation north of the critical latitude and PSI near it—and suggest a more persistent and laterally coherent near-inertial wave field than previously thought.

Open access
Elizabeth C. Fine
,
Jennifer A. MacKinnon
,
Matthew H. Alford
, and
John B. Mickett

Abstract

An intrahalocline eddy was observed on the Chukchi slope in September of 2015 using both towed CTD and microstructure temperature and shear sections. The core of the eddy was 6°C, significantly warmer than the surrounding −1°C water and far exceeding typical temperatures of warm-core Arctic eddies. Microstructure sections indicated that outside of the eddy the rate of dissipation of turbulent kinetic energy ε was quite low . However, at the edges of the eddy core, ε was elevated to . Three different processes were associated with elevated ε. Double-diffusive steps were found at the eddy’s top edge and were associated with an upward heat flux of 5 W m−2. At the bottom edge of the eddy, shear-driven mixing played a modest role, generating a heat flux of approximately 0.5 W m−2 downward. Along the sides of the eddy, density-compensated thermohaline intrusions transported heat laterally out of the eddy, with a horizontal heat flux of 2000 W m−2. Integrating these fluxes over an idealized approximation of the eddy’s shape, we estimate that the net heat transport due to thermohaline intrusions along the eddy flanks was 2 GW, while the double-diffusive flux above the eddy was 0.4 GW. Shear-driven mixing at the bottom of the eddy accounted for only 0.04 GW. If these processes continued indefinitely at the same rate, the estimated life-span would be 1–2 years. Such eddies may be an important mechanism for the transport of Pacific-origin heat, freshwater, and nutrients into the Canada Basin.

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Amy F. Waterhouse
,
Jennifer A. Mackinnon
,
Ruth C. Musgrave
,
Samuel M. Kelly
,
Andy Pickering
, and
Jonathan Nash

Abstract

Observations from Eel Canyon, located on the north coast of California, show that elevated turbulence in the full water column arises from the convergence of remotely generated internal wave energy. The incoming semidiurnal and bottom-trapped diurnal internal tides generate complex interference patterns. The semidiurnal internal tide sets up a partly standing wave within the canyon due to reflection at the canyon head, dissipating all of its energy within the canyon. Dissipation in the near bottom is associated with the diurnal trapped tide, while midwater isopycnal shear and strain is associated with the semidiurnal tide. Dissipation is elevated up to 600 m off the bottom, in contrast to observations over the flat continental shelf where dissipation occurs closer to the topography. Slope canyons are sinks for internal wave energy and may have important influences on the global distribution of tidally driven mixing.

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