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

mixing by moving slantwise along sloping isopycnals ( Thomas and Taylor 2010 ; D’Asaro et al. 2011 ). Here, the time-dependent forcing and rapid boundary layer deepening near yearday 65.4 allows us to extend these concepts of SI turbulence to the unsteady regime. Fig . 3. Time series of stratification and shear from Knorr along the float trajectory. (a) Potential density (colors) interpolated to a uniform grid from all Triaxus profiles within 3 km of the float and float depth (blue line) underlaid

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

; Lelong and Sundermeyer 2005 ). In a forward cascade, energy moves from larger to smaller scales, where it is ultimately dissipated by molecular viscosity; this is typical for three-dimensional turbulence. Conversely, in an inverse cascade, energy continues to increase at the largest scales unless and until some external forcing or dissipation removes it. Mechanisms that may limit upscale energy transfer exist at different scales and include, at smaller scales, submesoscale surface frontogenesis ( D

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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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Daniel B. Whitt and Leif N. Thomas

homogeneous solutions to (1) – (2) in order to introduce some of the physical concepts without the added complications associated with damping and forcing. Then, in section 2b , we will consider (1) – (2) as an underdamped harmonic oscillator susceptible to resonance and use the linear response function to interpret the physics of the forced-dissipative equilibrium solutions. Finally, we will use the energy equation associated with (1) – (2) to discuss the full solution, which may be represented as

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

Abstract

Submesoscale stirring contributes to the cascade of tracer variance from large to small scales. Multiple nested surveys in the summer Sargasso Sea with tow-yo and autonomous platforms captured submesoscale water-mass variability in the seasonal pycnocline at 20–60-m depths. To filter out internal waves that dominate dynamic signals on these scales, spectra for salinity anomalies on isopycnals were formed. Salinity-gradient spectra are approximately flat with slopes of −0.2 ± 0.2 over horizontal wavelengths of 0.03–10 km. While the two to three realizations presented here might be biased, more representative measurements in the literature are consistent with a nearly flat submesoscale passive tracer gradient spectrum for horizontal wavelengths in excess of 1 km. A review of mechanisms that could be responsible for a flat passive tracer gradient spectrum rules out (i) quasigeostrophic eddy stirring, (ii) atmospheric forcing through a relict submesoscale winter mixed layer structure or nocturnal mixed layer deepening, (iii) a downscale vortical-mode cascade, and (iv) horizontal diffusion because of shear dispersion of diapycnal mixing. Internal-wave horizontal strain appears to be able to explain horizontal wavenumbers of 0.1–7 cycles per kilometer (cpkm) but not the highest resolved wavenumbers (7–30 cpkm). Submesoscale subduction cannot be ruled out at these depths, though previous observations observe a flat spectrum well below subduction depths, so this seems unlikely. Primitive equation numerical modeling suggests that nonquasigeostrophic subinertial horizontal stirring can produce a flat spectrum. The last need not be limited to mode-one interior or surface Rossby wavenumbers of quasigeostrophic theory but may have a broaderband spectrum extending to smaller horizontal scales associated with frontogenesis and frontal instabilities as well as internal waves.

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Angelique C. Haza, Tamay M. Özgökmen, Annalisa Griffa, Andrew C. Poje, and M.-Pascale Lelong

. The first set of synthetic trajectories are generated using flow fields from a ° horizontal-resolution Hybrid Coordinate Ocean Model (HYCOM; Chassignet et al. 2006 ) configured in the North Atlantic ( Fig. 1 ). The model is subject to realistic forcing and boundary conditions in the North Atlantic, focusing on the Gulf Stream extension and recirculation area. We have studied this flow field in order to develop Lagrangian parameterizations of submesoscale dispersion ( Haza et al. 2012 ) and to

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Jörn Callies and Raffaele Ferrari

considered the restratification by mixed layer baroclinic instabilities as an initial-value problem: they tested the scaling (1) in a suite of spindown experiments of mixed layer fronts. In these experiments, it was assumed that a mixed layer had been created in a lateral buoyancy front, which in turn had been generated by mesoscale straining. It was further assumed that the atmospheric forcing that had created the mixed layer had subsided and that the mesoscale straining that had created the front had

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

mixed patch has formed, the fluid spreads laterally outward under the pressure gradient and an anticyclone is spun up by the Coriolis force. Above and below the anticyclone, fluid converges, forming two cyclones (e.g., McWilliams 1988 ). Such a multipolar compound vortex has been referred to as an “S vortex” ( Morel and McWilliams 1997 ), as it is dominated by stretching; a “sandwich vortex,” as the anticyclone is sandwiched between two cyclones; and as pancake eddy, or blini, due to the flat

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Ren-Chieh Lien and Thomas B. Sanford

the right-hand side is smaller than the second term and can be ignored. The linear component of PV,   ζ z − f ∂ z η ,   is the difference between the vertical component of relative vorticity   ζ z   and vortex stretching f ∂ z η . The nonlinear component, − ζ x ∂ x η − ζ y ∂ y η − ζ z ∂ z η , is the sum of twisting and stretching of isopycnals by the relative vorticity vector. PV in the ocean can only be modified by external forcing or turbulent dissipation. In the absence of nonadiabatic

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

ships at points near the float path illustrate the temporal evolution of the banded shear and the atmospheric forcing during the drift ( Fig. 4 ). Individual time series of the shear averaged between −45 and −65 m and −65 and −85 m further highlight this evolution ( Fig. 5 ). The time series show that the shear anomalies evolve significantly with time/downstream position at essentially all depths/isopycnals during the drift. Yet, the shear anomalies exhibit a consistent dominant vertical wavelength

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