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  • The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) x
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Matthew R. Mazloff, Raffaele Ferrari, and Tapio Schneider

-dimensional pathways of ocean water masses. Here we use a synthesis of observations, a numerical model, and theory to investigate the force balance of the SO limb of the MOC. Standard scaling analysis for the large-scale ocean circulation assumes a small Rossby number, leading to the thermocline equations based on the linearized planetary geostrophic equations ( Robinson and Stommel 1959 ; Welander 1959 ; Phillips 1963 ; Pedlosky 1987 ). But in the Drake Passage latitude band of the SO, at depths where there

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Louis-Philippe Nadeau and Raffaele Ferrari

instabilities, while in the presence of uneven topography the geostrophic eddy field is dominated by large-scale meanders in the lee of the major ridges. Importantly, the meanders result in a substantial topographic form drag over the ridges. In this view, the ACC transport’s insensitivity to increase in wind forcing was interpreted as the localized increase in topographic form drag. Nadeau and Straub (2009 , 2012) and Nadeau et al. (2013) (hereinafter NS09 , NS12 , and NSH13 ) proposed that the

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J. R. Ledwell, L. C. St. Laurent, J. B. Girton, and J. M. Toole

Abstract

The vertical dispersion of a tracer released on a density surface near 1500-m depth in the Antarctic Circumpolar Current west of Drake Passage indicates that the diapycnal diffusivity, averaged over 1 yr and over tens of thousands of square kilometers, is (1.3 ± 0.2) × 10−5 m2 s−1. Diapycnal diffusivity estimated from turbulent kinetic energy dissipation measurements about the area occupied by the tracer in austral summer 2010 was somewhat less, but still within a factor of 2, at (0.75 ± 0.07) × 10−5 m2 s−1. Turbulent diapycnal mixing of this intensity is characteristic of the midlatitude ocean interior, where the energy for mixing is believed to derive from internal wave breaking. Indeed, despite the frequent and intense atmospheric forcing experienced by the Southern Ocean, the amplitude of finescale velocity shear sampled about the tracer was similar to background amplitudes in the midlatitude ocean, with levels elevated to only 20%–50% above the Garrett–Munk reference spectrum. These results add to a long line of evidence that diapycnal mixing in the interior middepth ocean is weak and is likely too small to dictate the middepth meridional overturning circulation of the ocean.

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Byron F. Kilbourne and James B. Girton

) found that the ocean surface mixed layer responds like a damped harmonic oscillator to impulsive wind forcing if momentum is assumed to diffuse instantaneously throughout the surface mixed layer (i.e., the layer acts like a solid, rather than a liquid, at time scales long relative to turbulence but short relative to the general circulation and mesoscale eddies). D’Asaro (1985) used this “slab” model to estimate the average wind energy flux from several long-term wind recording moorings surrounding

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Ivana Cerovečki and Matthew R. Mazloff

respect to density. In a seminal paper, only Iudicone et al. (2008a , hereinafter I8a) have carried out a similarly detailed diagnosis of water mass transformation in the SO. They, however, analyzed output from an ocean–sea ice model forced by monthly mean climatological atmospheric forcing and a horizontal resolution of 2° × 2°, with necessarily reduced spatiotemporal variability. A number of processes important for the formation and destruction of water in SAMW density range are strongly impacted

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Jesse M. Cusack, Alberto C. Naveira Garabato, David A. Smeed, and James B. Girton

. Lee waves extract energy and horizontal momentum from the forcing flow and can transport them both vertically and horizontally, redistributing them throughout the water column via nonlinear interactions with other waves, the large-scale flow, or instabilities that result in wave breaking (e.g., Munk 1980 ). Lee waves have garnered growing interest in recent years, as efforts have been made to understand the origins of small-scale turbulence and its role in returning dense waters to the upper

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Michael Bates, Ross Tulloch, John Marshall, and Raffaele Ferrari

to vary in space, systemic drifts in climate models can be reduced. Moreover, the response of models to changes in external forcing (such as trends in Southern Ocean winds due to anthropogenic forcing) is found to depend on the form of the eddy closure employed. A further complication arises because the along-isopycnal diffusivity for tracers ( Redi 1982 ) and the diffusivity used to close for the eddy-induced circulation may not be the same, a point emphasized by Smith and Marshall (2009

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Ross Tulloch, Raffaele Ferrari, Oliver Jahn, Andreas Klocker, Joseph LaCasce, James R. Ledwell, John Marshall, Marie-Jose Messias, Kevin Speer, and Andrew Watson

span the top 1900 m, are all less than 35 m thick. 1 The Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim; Simmons et al. 2006 ) 6-h winds and buoyancy fluxes force the model’s surface, and the Ocean Comprehensive Atlas (OCCA; Forget 2010 ) provides monthly transports, heat and salt fluxes, as well as sea ice area and thickness at the lateral boundaries. Initial model conditions are an interpolation of the 1° × 1° resolution OCCA state on 1 January 2005

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Sophia T. Merrifield, Louis St. Laurent, Breck Owens, Andreas M. Thurnherr, and John M. Toole

1. Introduction A number of processes in the Southern Ocean are thought to support high levels of mixing relative to other regions of the global ocean. At the surface, strong winds and storms force the ocean at near-inertial frequencies, generating internal waves that can propagate downward ( Price 1981 ). Upper-ocean and middepth values of diapycnal diffusivity are believed to be set in part by the breaking of these near-inertial waves (e.g., Wu et al. 2011 ). Deep-reaching currents

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J. Alexander Brearley, Katy L. Sheen, Alberto C. Naveira Garabato, David A. Smeed, and Stephanie Waterman

downstream of Drake Passage ( Gille 1994 ; Morrow et al. 1994 ). The strength of this Southern Ocean eddy field is believed to be sensitive to climatic changes in forcing on a range of time scales ( Thompson and Solomon 2002 ; Meredith and Hogg 2006 ; Meredith et al. 2012 ). While the upscale transfer of eddy energy from high- to low-baroclinic modes (and ultimately to the barotropic mode) is relatively well quantified ( Scott and Wang 2005 ), the processes by which eddy energy is dissipated are

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