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Ramachandran, S, Tandon A, MacKinnon J, Lucas AJ, Pinkel R, Waterhouse AF, Nash J, Shroyer E, Mahadevan A, Weller RA, Farrar JT.  2018.  Submesoscale processes at shallow salinity fronts in the Bay of Bengal: Observations during the winter monsoon. Journal of Physical Oceanography. 48:479-509.   10.1175/jpo-d-16-0283.1   AbstractWebsite

Lateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a salinity-controlled density front, embedded in a zone of moderate mesoscale strain (0.15 times the Coriolis parameter) and forced by winds with a downfront orientation. Below a thin mixed layer, often <= 10 m, the analysis shows several dynamical signatures indicative of submesoscale processes: (i) negative Ertel potential vorticity (PV); (ii) low-PV anomalies with O(1-10) km lateral extent, where the vorticity estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to yield low PV; (iii) flow conditions susceptible to forced symmetric instability (FSI) or bearing the imprint of earlier FSI events; (iv) negative lateral gradients in the absolute momentum field (inertial instability); and (v) strong contribution from differential sheared advection at O(1) km scales to the growth rate of the depth-averaged stratification. The findings here show one-dimensional vertical processes alone cannot explain the vertical stratification and its lateral variability over O(1-10) km scales at the radiator survey.

MacKinnon, JA, Nash JD, Alford MH, Lucas AJ, Mickett JB, Shroyer EL, Waterhouse AF, Tandon A, Sengupta D, Mahadevan A, Ravichandran M, Pinkel R, Rudnick DL, Whalen CB, Alberty MS, Lekha JS, Fine EC, Chaudhuri D, Wagner GL.  2016.  A tale of two spicy seas. Oceanography. 29:50-61.   10.5670/oceanog.2016.38   AbstractWebsite

Upper-ocean turbulent heat fluxes in the Bay of Bengal and the Arctic Ocean drive regional monsoons and sea ice melt, respectively, important issues of societal interest. In both cases, accurate prediction of these heat transports depends on proper representation of the small-scale structure of vertical stratification, which in turn is created by a host of complex submesoscale processes. Though half a world apart and having dramatically different temperatures, there are surprising similarities between the two: both have (1) very fresh surface layers that are largely decoupled from the ocean below by a sharp halocline barrier, (2) evidence of interleaving lateral and vertical gradients that set upper-ocean stratification, and (3) vertical turbulent heat fluxes within the upper ocean that respond sensitively to these structures. However, there are clear differences in each ocean's horizontal scales of variability, suggesting that despite similar background states, the sharpening and evolution of mesoscale gradients at convergence zones plays out quite differently. Here, we conduct a qualitative and statistical comparison of these two seas, with the goal of bringing to light fundamental underlying dynamics that will hopefully improve the accuracy of forecast models in both parts of the world.

Alford, MH, MacKinnon JA, Simmons HL, Nash JD.  2016.  Near-inertial internal gravity waves in the ocean. Annual Review of Marine Science, Vol 8. 8( Carlson CA, Giovannoni SJ, Eds.).:95-123., Palo Alto: Annual Reviews   10.1146/annurev-marine-010814-015746   Abstract

We review the physics of near-inertial waves (NIWs) in the ocean and the observations, theory, and models that have provided our present knowledge. NIWs appear nearly everywhere in the ocean as a spectral peak at and just above the local inertial period f, and the longest vertical wavelengths can propagate at least hundreds of kilometers toward the equator from their source regions; shorter vertical wavelengths do not travel as far and do not contain as much energy, but lead to turbulent mixing owing to their high shear. NIWs are generated by a variety of mechanisms, including the wind, nonlinear interactions with waves of other frequencies, lee waves over bottom topography, and geostrophic adjustment; the partition among these is not known, although the wind is likely the most important. NIWs likely interact strongly with mesoscale and submesoscale motions, in ways that are just beginning to be understood.

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.

Alford, MH, MacKinnon JA, Nash JD, Simmons H, Pickering A, Klymak JM, Pinkel R, Sun O, Rainville L, Musgrave R, Beitzel T, Fu KH, Lu CW.  2011.  Energy Flux and Dissipation in Luzon Strait: Two Tales of Two Ridges. Journal of Physical Oceanography. 41:2211-2222.   10.1175/jpo-d-11-073.1   AbstractWebsite

Internal tide generation, propagation, and dissipation are investigated in Luzon Strait, a system of two quasi-parallel ridges situated between Taiwan and the Philippines. Two profiling moorings deployed for about 20 days and a set of nineteen 36-h lowered ADCP-CTD time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals. Measurements were concentrated on a northern line, where the ridge spacing was approximately equal to the mode-1 wavelength for semidiurnal motions, and a southern line, where the spacing was approximately two-thirds that. The authors contrast the two sites to emphasize the potential importance of resonance between generation sites. Throughout Luzon Strait, baroclinic energy, energy fluxes, and turbulent dissipation were some of the strongest ever measured. Peak-to-peak baroclinic velocity and vertical displacements often exceeded 2 m s(-1) and 300 m, respectively. Energy fluxes exceeding 60 kW m(-1) were measured at spring tide at the western end of the southern line. On the northern line, where the western ridge generates appreciable eastward-moving signals, net energy flux between the ridges was much smaller, exhibiting a nearly standing wave pattern. Overturns tens to hundreds of meters high were observed at almost all stations. Associated dissipation was elevated in the bottom 500-1000 m but was strongest by far atop the western ridge on the northern line, where >500-m overturns resulted in dissipation exceeding 2 x 10(-6) W kg(-1) (implying diapycnal diffusivity K(rho) > 0.2 m(2) s(-1)). Integrated dissipation at this location is comparable to conversion and flux divergence terms in the energy budget. The authors speculate that resonance between the two ridges may partly explain the energetic motions and heightened dissipation.

Zhao, ZX, Alford MH, MacKinnon JA, Pinkel R.  2010.  Long-Range Propagation of the Semidiurnal Internal Tide from the Hawaiian Ridge. Journal of Physical Oceanography. 40:713-736.   10.1175/2009jpo4207.1   AbstractWebsite

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 degrees-37.1 degrees N (approximate to 1400 km) and 13-yr-long Ocean Topography Experiment (TOPEX)/Poseidon (TIP) 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.

Polton, JA, Smith JA, MacKinnon JA, Tejada-Martinez AE.  2008.  Rapid generation of high-frequency internal waves beneath a wind and wave forced oceanic surface mixed layer. Geophysical Research Letters. 35   10.1029/2008gl033856   AbstractWebsite

High-frequency internal waves generated by Langmuir motions over stratified water may be an important source of turbulent mixing below the surface mixed layer. Large eddy simulations of a developing mixed layer and inertial current are employed to investigate this phenomena. Uniform surface wind stress and parallel Stokes drift wave forcing rapidly establishes a turbulent mixed-layer flow, which ( as the inertial motion veers off the wind) generates high-frequency internal waves in the stratified fluid below. The internal waves evolve such that their vector phase velocity matches the depth-averaged mixed-layer velocity that rotates as an inertial oscillation. The internal waves drain energy and momentum from the mixed layer on decay time-scales that are comparable to those of near-inertial oscillations. The high-frequency waves, which are likely to be trapped in the transition layer, may significantly contribute to mixing there and thus provide a potentially important energy sink for mixed-layer inertial motions.