Publications

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2018
Fine, EC, MacKinnon JA, Alford MH, Mickett JB.  2018.  Microstructure observations of turbulent heat fluxes in a warm-core Canada Basin eddy. Journal of Physical Oceanography. 48:2397-2418.   10.1175/jpo-d-18-0028.1   AbstractWebsite

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 degrees C, significantly warmer than the surrounding -1 degrees 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 epsilon was quite low . Three different processes were associated with elevated epsilon. 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.

2017
Alberty, MS, Sprintall J, MacKinnon J, Ganachaud A, Cravatte S, Eldin G, Germineaud C, Melet A.  2017.  Spatial patterns of mixing in the Solomon Sea. Journal of Geophysical Research-Oceans. 122:4021-4039.   10.1002/2016jc012666   AbstractWebsite

The Solomon Sea is a marginal sea in the southwest Pacific that connects subtropical and equatorial circulation, constricting transport of South Pacific Subtropical Mode Water and Antarctic Intermediate Water through its deep, narrow channels. Marginal sea topography inhibits internal waves from propagating out and into the open ocean, making these regions hot spots for energy dissipation and mixing. Data from two hydrographic cruises and from Argo profiles are employed to indirectly infer mixing from observations for the first time in the Solomon Sea. Thorpe and finescale methods indirectly estimate the rate of dissipation of kinetic energy (E) and indicate that it is maximum in the surface and thermocline layers and decreases by 2-3 orders of magnitude by 2000 m depth. Estimates of diapycnal diffusivity from the observations and a simple diffusive model agree in magnitude but have different depth structures, likely reflecting the combined influence of both diapycnal mixing and isopycnal stirring. Spatial variability of E is large, spanning at least 2 orders of magnitude within isopycnal layers. Seasonal variability of E reflects regional monsoonal changes in large-scale oceanic and atmospheric conditions with E increased in July and decreased in March. Finally, tide power input and topographic roughness are well correlated with mean spatial patterns of mixing within intermediate and deep isopycnals but are not clearly correlated with thermocline mixing patterns. Plain Language Summary In the ocean, a number of physical processes move heat, salt, and nutrients around vertically by mixing neighboring layers of the ocean together. This study investigates the strength and spatial patterns of this mixing in the Solomon Sea, which is located in the tropical west Pacific Ocean. Estimates of the strength of mixing are made using measurements of temperature, salinity, and velocity taken during two scientific cruises in the Solomon Sea. Measurements of temperature and salinity from a network of floats that move up and down through the ocean and travel with ocean currents were also used to estimate the strength and patterns of mixing. This research finds three key results for mixing in the Solomon Sea: (1) Mixing is strongest near the surface of the Solomon Sea and less strong at deeper depths. (2) Mixing varies horizontally, with stronger mixing above underwater ridges and seamounts, and with weaker mixing above smooth and flat seafloor. (3) The strength of mixing changes with the seasons, possibly related to the monsoonal winds which also change in strength over the seasons.

2016
Musgrave, RC, Pinkel R, MacKinnon JA, Mazloff MR, Young WR.  2016.  Stratified tidal flow over a tall ridge above and below the turning latitude. Journal of Fluid Mechanics. 793:933-957.: Cambridge University Press   10.1017/jfm.2016.150   Abstract

The interaction of the barotropic tide with a tall, two-dimensional ridge is examined analytically and numerically at latitudes where the tide is subinertial, and contrasted to when the tide is superinertial. When the tide is subinertial, the energy density associated with the response grows with latitude as both the oscillatory along-ridge flow and near-ridge isopycnal displacement become large. Where $f\neq 0$ , nonlinear processes lead to the formation of along-ridge jets, which become faster at high latitudes. Dissipation and mixing is larger, and peaks later in the tidal cycle when the tide is subinertial compared with when the tide is superinertial. Mixing occurs mainly on the flanks of the topography in both cases, though a superinertial tide may additionally generate mixing above topography arising from convective breaking of radiating waves.

Musgrave, RC, MacKinnon JA, Pinkel R, Waterhouse AF, Nash J.  2016.  Tidally driven processes leading to near-field turbulence in a channel at the crest of the Mendocino Escarpment*. Journal of Physical Oceanography. 46:1137-1155.   10.1175/jpo-d-15-0021.1   AbstractWebsite

In situ observations of tidally driven turbulence were obtained in a small channel that transects the crest of the Mendocino Ridge, a site of mixed (diurnal and semidiurnal) tides. Diurnal tides are subinertial at this latitude, and once per day a trapped tide leads to large flows through the channel giving rise to tidal excursion lengths comparable to the width of the ridge crest. During these times, energetic turbulence is observed in the channel, with overturns spanning almost half of the full water depth. A high-resolution, nonhydrostatic, 2.5-dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave that extends from the ridge crest to the surface and the subsequent development of a hydraulic jump on the flanks of the ridge. Modeled dissipation rates show that turbulence is strongest on the flanks of the ridge and that local dissipation accounts for 28% of the energy converted from the barotropic tide into baroclinic motion.

2014
Buijsman, MC, Klymak JM, Legg S, Alford MH, Farmer D, MacKinnon JA, Nash JD, Park JH, Pickering A, Simmons H.  2014.  Three-dimensional double-ridge internal tide resonance in Luzon Strait. Journal of Physical Oceanography. 44:850-869.   10.1175/jpo-d-13-024.1   AbstractWebsite

The three-dimensional (3D) double-ridge internal tide interference in the Luzon Strait in the South China Sea is examined by comparing 3D and two-dimensional (2D) realistic simulations. Both the 3D simulations and observations indicate the presence of 3D first-mode (semi)diurnal standing waves in the 3.6-km-deep trench in the strait. As in an earlier 2D study, barotropic-to-baroclinic energy conversion, flux divergence, and dissipation are greatly enhanced when semidiurnal tides dominate relative to periods dominated by diurnal tides. The resonance in the 3D simulation is several times stronger than in the 2D simulations for the central strait. Idealized experiments indicate that, in addition to ridge height, the resonance is only a function of separation distance and not of the along-ridge length; that is, the enhanced resonance in 3D is not caused by 3D standing waves or basin modes. Instead, the difference in resonance between the 2D and 3D simulations is attributed to the topographic blocking of the barotropic flow by the 3D ridges, affecting wave generation, and a more constructive phasing between the remotely generated internal waves, arriving under oblique angles, and the barotropic tide. Most of the resonance occurs for the first mode. The contribution of the higher modes is reduced because of 3D radiation, multiple generation sites, scattering, and a rapid decay in amplitude away from the ridge.

2013
MacKinnon, J, St Laurent L, Naveira Garabato AC.  2013.  Diapycnal Mixing Processes in the Ocean Interior. Ocean Circulation and Climate: A 21st Century Perspective. 103( Siedler G, Griffies SM, Gould J, Church JA, Eds.).:159-183.: Academic Press   10.1016/B978-0-12-391851-2.00007-6   Abstract

Diapycnal mixing in the ocean interior is driven by a wide range of processes, each with distinct governing physics and unique global geography. Here we review the primary processes responsible for turbulent mixing in the ocean interior, with an emphasis on active work from the past decade. We conclude with a discussion of global patterns of mixing and their importance for regional and large-scale modeling accuracy.

2012
Whalen, CB, Talley LD, MacKinnon JA.  2012.  Spatial and temporal variability of global ocean mixing inferred from Argo profiles. Geophysical Research Letters. 39:n/a-n/a.   10.1029/2012GL053196   AbstractWebsite

The influence of turbulent ocean mixing transcends its inherently small scales to affect large scale ocean processes including water-mass transformation, stratification maintenance, and the overturning circulation. However, the distribution of ocean mixing is not well described by sparse ship-based observations since this mixing is both spatially patchy and temporally intermittent. We use strain information from Argo float profiles in the upper 2,000 m of the ocean to generate over 400,000 estimates of the energy dissipation rate, indicative of ocean mixing. These estimates rely on numerous assumptions, and do not take the place of direct measurement methods. Temporally averaged estimates reveal clear spatial patterns in the parameterized dissipation rate and diffusivity distribution across all the oceans. They corroborate previous observations linking elevated dissipation rates to regions of rough topography. We also observe heightened estimated dissipation rates in areas of high eddy kinetic energy, as well as heightened diffusivity in high latitudes where stratification is weak. The seasonal dependence of mixing is observed in the Northwest Pacific, suggesting a wind-forced response in the upper ocean.

2005
MacKinnon, JA, Gregg MC.  2005.  Near-inertial waves on the New England shelf: The role of evolving stratification, turbulent dissipation, and bottom drag. Journal of Physical Oceanography. 35:2408-2424.   10.1175/jpo2822.1   AbstractWebsite

Energetic variable near-inertial internal waves were observed on the springtime New England shelf as part of the Coastal Mixing and Optics (CMO) project. Surface warming and freshwater advection tripled the average stratification during a 3-week observational period in April/May 1997. The wave field was dominated by near-inertial internal waves generated by passing storms. Wave evolution was controlled by a balance among wind stress, bottom drag, and turbulent dissipation. As the stratification evolved, the vertical structure of these near-inertial waves switched from mode 1 to mode 2 with associated changes in the magnitude and location of wave shear. The growth of mode-2 waves was attributable to a combination of changing wind stress forcing and a nonlinear coupling between the first and second vertical modes through quadratic bottom stress. To explore both forcing mechanisms, an open-ocean mixed layer model is adapted to the continental shelf. In this model, surface wind stress and bottom stress are distributed over the surface and bottom mixed layers and then projected onto orthogonal vertical modes. The model replicates the correct magnitude and evolving modal distribution of the internal waves and confirms that bottom stress can act to transfer energy between internal wave modes.

2003
MacKinnon, JA, Gregg MC.  2003.  Mixing on the late-summer New England shelf - Solibores, shear, and stratification. Journal of Physical Oceanography. 33:1476-1492.   10.1175/1520-0485(2003)033<1476:motlne>2.0.co;2   AbstractWebsite

Observations are presented of microstructure and velocity measurements made on the outer New England shelf in the late summer of 1996 as part of the Coastal Mixing and Optics Experiment. The depth- and time-averaged turbulent dissipation rate was 5-50 (x 10(-9) W kg(-1)). The associated average diapycnal diffusivity in stratified water was 5-20 (x 10(26) m(2) s(-1)), comparable to observed open-ocean thermocline values and too low to explain the strong variability observed in local water properties. Dissipation rates and diffusivity were both highly episodic. Turbulent boundary layers grew down from the surface and up from the bottom. The dissipation rate within the bottom boundary layer had an average of 1.2 x 10(-7) W kg(-1) and varied in magnitude with the strength of near-bottom flow from the barotropic tide, an along-shelf flow, and low-frequency internal waves. The average dissipation rate in the peak thermocline was 5 x 10(-8) W kg(-1); one-half of the thermocline dissipation was due to the strong shear and strain within six solibores that cumulatively lasted less than a day but contained 100-fold elevated dissipation and diffusivity. Nonsolibore, midcolumn dissipation was strongly correlated with shear from low-frequency internal waves. Dissipation was not well parameterized by Gregg-Henyey-type scaling. An alternate scaling, modified to account for observed coastal internal wave properties, was in good agreement with measured dissipation rates. At the end of the observational period Hurricane Edouard passed by, producing strong dissipation rates (4 x 10(-6) W kg(-1)) and consequent mixing during and for several days following the peak winds.

MacKinnon, JA, Gregg MC.  2003.  Shear and baroclinic energy flux on the summer New England shelf. Journal of Physical Oceanography. 33:1462-1475.   10.1175/1520-0485(2003)033<1462:sabefo>2.0.co;2   AbstractWebsite

Observations are presented of internal wave properties and energy fluxes through a site near the 70-m isobath on the New England shelf in late summer. Data collected from a shipboard ADCP and microstructure profiler over a three-week period and projected onto dynamic vertical modes reveals large variations in the magnitude and vertical structure of internal waves. Baroclinic energy and shear were primarily associated with low-mode near-inertial and semidiurnal waves and, at times, high-frequency solibores. The energies in each mode varied by factors from 2 to 10 over several days and were not significantly correlated with one another. The associated shear variance was concentrated in the thermocline. However, the strength and vertical range of shear varied significantly throughout the research period and depended sensitively on both the magnitude and evolving vertical mode content of the wave field. Shear during the quasi-two-layer solibores was strong enough to temporarily lower the 4-m Richardson number below the threshold for shear instability. Energy flux through the site came primarily from the mode-1 internal tide, in both linear and nonlinear (solibore) forms. The average energy flux from the first five baroclinic modes was 130 W m(-1). A comparison of energy fluxes from each mode and locally measured average dissipation rates suggests that near-inertial and high-mode waves were generated near the experimental site.