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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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Ivana Cerovečki and Matthew R. Mazloff

and salt budgets have been archived as 5-day averages, allowing construction of closed and complete three-dimensional and time-varying heat, freshwater, potential density, and hence volume budgets for individual isopycnal layers. The volume budget balances completely so that there is no need for assumptions about the estimated rates of SAMW formation and destruction. We focus on quantifying the effects of the physical processes important in formation and destruction of water in the SAMW density

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Louis-Philippe Nadeau and Raffaele Ferrari

to the upper half of the water column. This led a number of recent studies to argue that the circumpolar transport can be understood as a balance between the wind stress and geostrophic eddies, the two forces that dominate in the upper ocean ( Johnson and Bryden 1989 ). This balance has been invoked to explain the numerical evidence that, in eddy-resolving models of wind-driven reentrant channels, the transport does not increase much with increasing surface winds (e.g., Hallberg and Gnanadesikan

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

to calculate the diffusivity parameters ( K e , κ T ) in (6) from measured data. Notably, this study shows that replacing κ ρ from (1) with κ T is not an acceptable choice in an intrusive water mass where double-diffusive processes are active. Alternatively, if independent estimates of K e are available, they could be used in (6) to estimate κ T from estimates of 〈 χ 〉. Lacking such additional information the problem is underdetermined, so we study the limits of the balance by

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Alberto C. Naveira Garabato, Kurt L. Polzin, Raffaele Ferrari, Jan D. Zika, and Alexander Forryan

variance budgets in eddy-resolving numerical models (e.g., Wilson and Williams 2004 ; Abernathey and Marshall 2013 ) have routinely shown that the advection and triple correlation terms on the left-hand side of (4) , which we have neglected in deriving (5) – (6) , are significant over horizontal scales smaller than O (100–500) km. Thus, we expect the balance in (6) to hold only when integrated over larger horizontal scales. In section 6 , we will show that the assumption of (6) produces

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

by a balance among diapycnal advection, diapycnal diffusion, and, for radioisotopes and oxygen, decay or biological uptake ( Munk 1966 ; Munk and Wunsch 1998 ). More recent measurements of turbulent dissipation (e.g., Gregg et al. 1973 ; Osborn 1980 ; Gregg 1989 ) and passive tracer spreading rates ( Ledwell et al. 1998 ) have suggested that the diapycnal diffusivity in the middepth ocean interior is on the order of 10 −5 m 2 s −1 , that is, a factor of 10 smaller than predicted by large

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

open to alternative interpretations, as the unambiguous observation of lee waves in the Southern Ocean has remained elusive. It has been appreciated in the atmospheric literature that lee waves, or mountain waves, play an important role in the momentum budget and influence aspects of the general circulation (e.g., Fritts 2003 ) and that the results of general circulation models are improved when their effects are accounted for ( McFarlane 1987 ). The dominant momentum balance in the Antarctic

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L. St. Laurent, A. C. Naveira Garabato, J. R. Ledwell, A. M. Thurnherr, J. M. Toole, and A. J. Watson

(SAF) to the north. High rates of diapycnal mixing for the Southern Ocean as a whole have been suggested by box inverse studies of the circulation ( Heywood et al. 2002 ; Lumpkin and Speer 2007 ; Zika et al. 2009 ). However, these estimates are indirect, being based mainly on mass balance derived from geostrophic flow estimates, and are subject to large uncertainty. Other studies have utilized finescale parameterizations for estimation of the mixing rates as part of larger-scale Southern Ocean

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Ru Chen, Sarah T. Gille, Julie L. McClean, Glenn R. Flierl, and Alexa Griesel

) Comparison of vertical structures To assess how well the two theories capture the vertical structures of eddy mixing, we examined the correlation between float-based diffusivities/mixing lengths and their theoretical-based counterparts throughout the water column ( Fig. 11 ). In the regions where correlation coefficients are not significantly positive, the theory is considered not to have skill in representing diffusivities. Larger positive correlation coefficients imply better skill in representing

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