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

typically greater than the threshold for Kelvin–Helmholtz instability (0.25), even for strong currents with cyclonic vorticity like the Gulf Stream ( Stone 1966 ). Figure 6 shows cross-stream sections of q g , q , and density. The geostrophic PV in the boundary layer grew progressively more negative through the period of strong mixing ( Figs. 6a–d ). The vertical vorticity averaged over the top 60 m was mostly cyclonic with the absolute vorticity ( f + ζ ) always positive ( Figs. 2a–e ). The

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Daniel B. Whitt and Leif N. Thomas

1. Introduction Midlatitude storms efficiently inject energy into boundary layer inertial oscillations (e.g., Pollard 1970 ; Pollard and Millard 1970 ; D’Asaro 1985 ; D’Asaro et al. 1995 ; Alford 2003b ), and therefore boundary layer near-inertial energy density exhibits spatial and temporal patterns similar to atmospheric storm tracks (e.g., Chaigneau et al. 2008 ; Elipot et al. 2010 ). As storm tracks overlie western boundary current regions, which contain energetic geostrophic flows

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Daniel Mukiibi, Gualtiero Badin, and Nuno Serra

obligatory. The forward and backward integration allows us to use the same flow; however, it relies on different initial conditions. This choice has been made in order to compare the statistics of the FTLEs; however, no comparison of snapshots of the field should be attempted. b. Computation of FTLEs In the current study, we consider the operator to be the 3 × 3 matrix whose entries are numerically obtained as finite differences. For a particle located away from the channel boundaries, there are six

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Jörn Callies and Raffaele Ferrari

when mixed-layer instabilities are able to energize submesoscales and thereby affect the bulk properties of the upper ocean. We employ an idealized setup that allows the development of both baroclinic instabilities and forced convection in a layer of Boussinesq fluid. The strength of the baroclinic instability is controlled by a prescribed lateral buoyancy gradient, and the strength of convection is controlled by an imposed buoyancy flux at the top and bottom boundaries. In the limit of weak

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Leif N. Thomas and Callum J. Shakespeare

section, and (bottom right) vertical vorticity and density cross section. The North Wall of the Gulf Stream is a region where the speed of the current drops off precipitously, resulting in a flow that tends to be confluent (e.g., sea surface height) and that is characterized by strong, cyclonic vorticity. The resultant collision of water masses generates a strong front in temperature. Below the surface boundary layer (i.e., z < −100 m), an extremely sharp, compensated salinity front is found that

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Gualtiero Badin, Amit Tandon, and Amala Mahadevan

polar vortices as barriers to mixing tracers have been studied through both observations ( Nakamura and Ma 1997 ; Haynes and Shuckburgh 2000a , b ) and modeled flows ( Shuckburgh and Haynes 2003 ). It has also been used for oceanic jets along the Antarctic Circumpolar Current that are shown to act as barriers to mixing ( Marshall et al. 2006 ; Shuckburgh et al. 2009 ). Under the approximation that the evolution of a certain tracer concentration class C takes place along isopycnals in the

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

), which could be associated with enhanced turbulence. As turbulent mixing may be important for driving water mass transformation at the North Wall of the Gulf Stream (e.g., Klymak et al. 2016 ), coherent downward-propagating NIWs may also play an important role in water mass transformation in the upper pycnocline here. In addition, similar behavior could also occur at other western boundary currents, such as the Kuroshio, where filaments of strong cyclonic vorticity and near-inertial waves are

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Ren-Chieh Lien and Thomas B. Sanford

and forcing processes, PV is conserved following the flow, that is, ( D / D t ) π ′ = 0. Briscoe (1977) and Müller et al. (1988) report the presence of current finestructure at vertical scales smaller than 10 m in the upper Sargasso Sea observed during the Internal Waves Experiment (IWEX). Müller (1984) hypothesizes that the current finestructure represents the potential-vorticity-carrying motion (i.e., vortical motion) at small scales. During the last three decades there have been extensive

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

al. (2008a) reported that tracer gradient spectra at fixed depths were flat at all depths, even though HKE and APE transitioned from k −2 at the surface to k −1 at depth consistent with quasigeostrophic theory. Tulloch et al.’s (2011) eddy-resolving numerical simulations revealed that, in weak baroclinic eddy fields without the deep PV gradient reversal that characterizes western boundary currents (i.e., conditions that typify the Sargasso Sea and North Pacific gyres), baroclinic

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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

Doppler current profiler (ADCP) velocity in the depth bin occupied by the dye. Details about the instrumentation and sampling are given in the appendix . F ig . 3. Illustration of the nested sampling from R/V Oceanus (Triaxus, Hammerhead, and T-REMUS), R/V Endeavor (MVP), and R/V Cape Hatteras (Acrobat). Triaxus measured along a 30-km radiator grid (red), MVP made repeated 15-km bowtie surveys (blue), Acrobat performed adaptive surveying of the dye with a 3-km radiator grid, and Hammerhead was

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