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Olivier Marchal and William B. Curry

glacial data are incompatible with the modern circulation. Finally, locations in the abyssal Atlantic are determined, where sediment data would provide additional information about the circulation. This paper is organized as follows: section 2 is a brief description of the inverse method used to combine noisy observations with imperfect theory. Some notation is introduced and a formal expression of the two problems addressed in this paper is provided. In section 3 , observations and dynamical

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Paul Spence, Erik van Sebille, Oleg A. Saenko, and Matthew H. England

1. Introduction The abyssal circulation of the Southern Ocean is often considered, at least in a zonally integrated sense, to consist of two compensating flows. One is associated with the poleward flow of Lower Circumpolar Deep Water (LCDW), and the other is associated with the equatorward flow of Antarctic Bottom Water (AABW) (e.g., Sloyan and Rintoul 2001 ). LCDW is transformed into AABW by surface buoyancy fluxes at several locations near the Antarctic continental shelf and also through

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Chiung-Yin Chang and Malte F. Jansen

1. Introduction The abyssal ocean is a large reservoir of heat and carbon for the climate system, and the abyssal overturning circulation has often been hypothesized to be a key player that has modulated climate feedbacks during past climate changes (e.g., Adkins 2013 , and references therein). Despite its importance, the mechanisms that govern these changes in the abyssal overturning circulation remain poorly understood. The classic theoretical picture for the abyssal overturning circulation

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Jörn Callies and Raffaele Ferrari

1. Motivation The large-scale circulation of the abyssal ocean—below about 2000-m depth—is enabled by diabatic water-mass transformation. Antarctic Bottom Water sinks to the ocean bottom at the Antarctic margin, and it must cross density surfaces to come back up toward the surface (e.g., Lumpkin and Speer 2007 ; Talley 2013 ; Ferrari 2014 ). In steady state, this net upwelling across density surfaces must be balanced by diabatic transformation. This transformation is achieved by turbulence

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Qi Quan and Huijie Xue

1. Introduction Mixing in the ocean occurs at multiscales and is closely associated with various dynamical processes ranging from eddy stirring, internal wave breaking, turbulence, and down to molecular diffusion. In the abyssal ocean, diapycnal mixing is of vital importance in maintaining the stratification and meridional overturning circulation (MOC) and hence is able to significantly affect both local and global climate through heat transport (e.g., Rahmstorf 2003 ; Jayne 2009 ; Talley

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Steven R. Jayne

modeling studies. This formulation allows the vertical mixing in the model to be fully interactive with the evolving ocean state, varying as a function of three-dimensional space and time. In general, they found that using this form for the vertical mixing did not result in major changes to the modeled circulation compared to a control run with a small globally constant diffusivity. Rather, more modest changes were seen: an increase in abyssal temperatures and salinities, a strengthening of the MOC

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Ryan M. Holmes, Casimir de Lavergne, and Trevor J. McDougall

importance of seafloor geometry for the abyssal overturning circulation. De Lavergne et al. (2016b , a) combine parameterizations for abyssal ocean sources of mixing with a climatological neutral density distribution to show that the transformation of AABW into lighter waters is strongly sensitive to the vertical structure of the buoyancy flux. This conclusion is supported by the global, energetically consistent numerical simulations of Melet et al. (2016) . For a bottom-intensified buoyancy flux, de

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Henri F. Drake, Raffaele Ferrari, and Jörn Callies

-scale climate change, whether natural (e.g., Sarmiento and Toggweiler 1984 ) or anthropogenic in origin ( Archer et al. 1998 ). It is thus vital to have a firm phenomenological and dynamical understanding of the abyssal ocean’s mean state. The general structure of the abyssal ocean circulation is easily inferred from surface buoyancy fluxes and large-scale tracer properties ( Sverdrup et al. 1942 ). Antarctic Bottom Waters, the densest oceanic waters, form in the Southern Ocean and fill the global abyssal

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A. Mashayek, R. Ferrari, M. Nikurashin, and W. R. Peltier

of a few hundred meters ( Lumpkin and Speer 2007 ; Ganachaud 2003 ; Melet et al. 2013 ; Waterhouse et al. 2014 ). In this work, we explore the implications of the downward increase of κ on the abyssal ocean stratification and MOC, leaving the implications of horizontal variations for future work. To illustrate the importance of the vertical structure in κ on the MOC, in Fig. 1 we compare two numerical solutions of the ocean circulation in an idealized geometry with a basin and a

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Jörn Callies

, become nonlinear, and break, typically within a few hundred meters of the ocean bottom (e.g., Legg and Huijts 2006 ; Garrett and Kunze 2007 ; Muller and Bühler 2009 ; Nikurashin and Ferrari 2010 ; Nikurashin and Legg 2011 ). This topographically induced turbulence can achieve substantial water mass transformation and thus help close the large-scale overturning circulation of the abyssal ocean, producing a few tens of Sverdrups (1 Sv ≡ 10 6 m 3 s −1 ) of both diapycnal up- and downwelling (e

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