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E. Kunze, J. M. Klymak, R.-C. Lien, R. Ferrari, C. M. Lee, M. A. Sundermeyer, and L. Goodman

means of teasing apart the dynamics controlling the passive tracer spectrum over 0.1 < k < 100 cpkm. Subinertial frontogenesis O (1) or higher Rossby number shears and the secondary adjustment circulations that set up to restore balance seem likely candidates at high k with statistical homogeneity, but nonlocal processes such as the subduction of mixed layer anomalies or formation of coherent eddies may also play a role. More, deeper measurements would help confirm that an almost flat tracer

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Jörn Callies and Raffaele Ferrari

1. Introduction Atmospheric cooling and surface winds frequently mix the surface layer of the ocean. The resulting mixed layer mediates the transfer of heat and momentum between the atmosphere and ocean and thereby affects both the atmospheric climate and the oceanic general circulation. The evolution of the ocean mixed layer has traditionally been understood column by column; atmospheric cooling and wind forcing leads to mixing and deepening of the mixed layer into the thermocline below. It is

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Jörn Callies and Raffaele Ferrari

slumping of isopycnals by mesoscale eddies in the thermocline is often parameterized ( Gent and McWilliams 1990 ). To set the rate of mixed layer restratification, FFH proposed a scaling for the buoyancy flux: where f is the Coriolis parameter, Λ is the geostrophic shear associated with the lateral buoyancy gradient of the mixed layer front, and H is the mixed layer depth. The geostrophic shear is defined as a vertical and along-front average of the instantaneous geostrophic shear. FFH

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Leif N. Thomas and Callum J. Shakespeare

1. Introduction In every ocean basin, on the equatorward side of major ocean fronts, layers of weakly stratified waters with nearly homogeneous properties are found. These so-called mode waters are thought to play an important role in the ocean–atmosphere climate system by sequestering and releasing heat and carbon dioxide on interannual time scales ( Dong et al. 2007 ; Bates et al. 2002 ) and by affecting the large-scale circulation through shaping the potential vorticity field ( Dewar et al

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Daniel B. Whitt, Leif N. Thomas, Jody M. Klymak, Craig M. Lee, and Eric A. D’Asaro

and Thomas (2013) observed banded shear in a ~200-km-wide section (with ~10-km resolution) across the winter Gulf Stream and noted that the properties of the shear were consistent with the spatial structure of a subinertial wave at a slantwise critical layer. Collocated shear microstructure profiles showed enhanced dissipation between 400 and 600 m in the upper thermocline in the same location as these shear structures ( Inoue et al. 2010 ), suggesting a link between the shear and enhanced

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Andrey Y. Shcherbina, Miles A. Sundermeyer, Eric Kunze, Eric D’Asaro, Gualtiero Badin, Daniel Birch, Anne-Marie E. G. Brunner-Suzuki, Jörn Callies, Brandy T. Kuebel Cervantes, Mariona Claret, Brian Concannon, Jeffrey Early, Raffaele Ferrari, Louis Goodman, Ramsey R. Harcourt, Jody M. Klymak, Craig M. Lee, M.-Pascale Lelong, Murray D. Levine, Ren-Chieh Lien, Amala Mahadevan, James C. McWilliams, M. Jeroen Molemaker, Sonaljit Mukherjee, Jonathan D. Nash, Tamay Özgökmen, Stephen D. Pierce, Sanjiv Ramachandran, Roger M. Samelson, Thomas B. Sanford, R. Kipp Shearman, Eric D. Skyllingstad, K. Shafer Smith, Amit Tandon, John R. Taylor, Eugene A. Terray, Leif N. Thomas, and James R. Ledwell

dynamical processes have been proposed to explain this mesoscale–submesoscale transition, including spontaneous instability of deep mixed layers, ageostrophic instability, frontogenesis, and direct wind forcing at mesoscale fronts ( Boccaletti et al. 2007 ; Thomas et al. 2008 ). Numerical simulations suggest that non-quasigeostrophic baroclinic mixed layer instabilities can penetrate into the thermocline, leading to lateral stirring of tracers below the mixed layer ( Badin et al. 2011 ). It appears

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Leif N. Thomas, John R. Taylor, Eric A. D’Asaro, Craig M. Lee, Jody M. Klymak, and Andrey Shcherbina

1. Introduction The ocean’s main frontal systems, the Gulf Stream, Kuroshio, and Antarctic Circumpolar Current, underlie the midlatitude westerlies. As a consequence, the strongest wind work on the ocean circulation is found in these regions ( Wunsch 1998 ). At the same time, the westerlies tend to lower the potential vorticity of the currents and make the fronts susceptible to symmetric instability (SI), an overturning instability that removes kinetic energy (KE) from the circulation ( Thomas

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Daniel B. Whitt and Leif N. Thomas

and strong fronts (e.g., D’Asaro et al. 2011 ; Whitt et al. 2014, manuscript submitted to J. Geophys. Res. ), there is ample opportunity for interactions between inertial oscillations and strong geostrophic vorticity (e.g., Mooers 1975 ; Kunze 1985 ; Young and Ben-Jelloul 1997 ; Whitt and Thomas 2013 ). These wave–mean flow interactions may result in regionally elevated internal wave energy and enhanced turbulent mixing in the boundary layer and upper thermocline of the western boundary

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Anne-Marie E. G. Brunner-Suzuki, Miles A. Sundermeyer, and M.-Pascale Lelong

: Observations of overturning in the thermocline: The context of ocean mixing . J. Phys. Oceanogr. , 30 , 805 – 832 , doi: 10.1175/1520-0485(2000)030<0805:OOOITT>2.0.CO;2 . Arbic , B. K. , R. B. Scott , G. R. Flierl , A. J. Morten , J. G. Richman , and J. F. Shriver , 2012 : Nonlinear cascades of surface oceanic geostrophic kinetic energy in the frequency domain . J. Phys. Oceanogr. , 42 , 1577–1600 , doi: 10.1175/JPO-D-11-0151.1 . Arneborg , L. , 2002 : Mixing efficiencies in

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Anne-Marie E. G. Brunner-Suzuki, Miles A. Sundermeyer, and M.-Pascale Lelong

influence on stratification; see, for instance, Flierl and McGillicuddy (2002) for the impact of submesoscale physics on the biological dynamics. These physical and biological processes are at subgrid scale for global circulation models, and thus the particular submesoscale dynamics discussed here are not included in such models. Parameterizations for this type of process do not yet exist. To better understand the problem, several numerical studies have examined the stability of single monopolar

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