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Jörn Callies and Raffaele Ferrari

1. Introduction Submesoscale baroclinic instabilities are thought to be an important source of stratification in the surface ocean (e.g., Haine and Marshall 1998 ; Boccaletti et al. 2007 ). Mixed layer baroclinic instabilities at density fronts slide dense under light water, a process that tends to flatten isopycnal surfaces. A positive tendency in stratification is induced by a positive eddy buoyancy flux , where and are the vertical velocity and buoyancy anomalies associated with the

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Leif N. Thomas and Callum J. Shakespeare

. 2005 ). Mode waters are identified as a local maximum in a volumetric census in a temperature–salinity diagram ( Hanawa and Talley 2001 ). They are distinguished from other water masses by their defining characteristic—weak stratification and anomalously low potential vorticity. Mode water formation requires a convergent diapycnal mass flux that can fill isopycnal layers and reduce the stratification. The mechanism that selects the density class where a diapycnal mass flux converges and mode water

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Callum J. Shakespeare and Leif N. Thomas

). However, because of the large thermal inertia of mode waters, their temperature is insensitive to wintertime heat loss, implying that their temperature–salinity relation is not set by air–sea fluxes ( Warren 1972 ). Instead, processes in the ocean interior likely play a key role in the selection of their water mass properties. Mode waters are found on the equatorward side of major ocean fronts, suggesting a role for frontal processes in their generation and maintenance ( Marshall et al. 2009

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Gualtiero Badin, Amit Tandon, and Amala Mahadevan

1. Introduction In the oceanic mixed layer (ML), atmospheric forcing, ocean dynamics, and their interplay act to leave the surface waters well mixed. While the ML waters are mixed in the vertical, lateral gradients in temperature and salinity are a common feature. Processes responsible for the creation of lateral gradients in temperature and salinity in the open ocean include nonhomogeneous heat and freshwater fluxes, wind mixing associated with the passage of a storm, and ocean convection

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Jörn Callies and Raffaele Ferrari

becoming increasingly clear, however, that lateral exchanges contribute crucially to the dynamical balances of the mixed layer. Baroclinic instability in the mixed layer, one such agent of lateral exchange, can achieve large vertical buoyancy fluxes by laterally sliding light over dense water, tending to restratify the mixed layer (e.g., Spall 1995 ; Haine and Marshall 1998 ; Boccaletti et al. 2007 ; Fox-Kemper et al. 2008 ). This restratification modifies the surface properties and thereby feeds

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

2005 ; Thomas et al. 2013 ). Under steady, unidirectional winds, theory and large-eddy simulations (LES) predict that this sink of KE for the circulation scales with the so-called Ekman buoyancy flux, defined as the dot product of the Ekman transport and the surface buoyancy gradient ( Thomas and Taylor 2010 ). Observations of upper-ocean turbulence made in the wind-forced Kuroshio when it was symmetrically unstable revealed enhanced turbulent dissipation at levels consistent with this theoretical

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Daniel B. Whitt and Leif N. Thomas

and Müller 1989 ; Müller et al. 2005 ; Polzin 2010 ; Thomas 2012 ; Vanneste 2013 ; Alford et al. 2013 ). Here, we investigate the wind-driven generation of inertial oscillations in laterally sheared rectilinear geostrophic flows. The focus in this paper is largely on the process by which the background flow modifies the local flux of near-inertial energy from the winds to the boundary layer, although energy exchanges between the oscillations and the mean flow are also discussed in this

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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

made between isopycnal processes, which act along surfaces of constant potential density (or, more strictly, neutral surfaces; Montgomery 1940 ; McDougall 1984 ), and diapycnal processes, which act across these surfaces ( Gregg 1987 ; MacKinnon et al. 2013 ). Interpretation of lateral dispersion of tracers in the ocean in terms of mixing is fraught with ambiguity. The unresolved flux J T of a tracer T ascribed to lateral mixing is commonly parameterized with Fickian diffusion law, where K h

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

. Lett. , 35 , L06809 , doi: 10.1029/2007GL032906 . Callies , J. , and R. Ferrari , 2013 : Interpreting energy and tracer spectra of upper-ocean turbulence in the submesoscale range (1–200 km) . J. Phys. Oceanogr. , 43 , 2456 – 2474 , doi: 10.1175/JPO-D-13-063.1 . Capet , X. , J. C. McWilliams , M. J. Molemaker , and A. F. Shchepetkin , 2008a : Mesoscale to submesoscale transition in the California Current System. Part I: Flow structure, eddy flux, and observational tests . J

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Daniel Mukiibi, Gualtiero Badin, and Nuno Serra

the observation from numerical simulations that MLIs produce stronger fluxes in the middle of the ML ( Fox-Kemper et al. 2008 ) and have a fast decrease below the ML base, showing nonzero values at all depths. Analysis of the vertical structure of horizontally averaged forward FTLEs from the different approximations ( Fig. 1d ) shows that 3D (black line), approx1 (black dotted line), and approx4 (gray dot dashed line) FTLEs are indistinguishable at all depths. The same result holds for approx2

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