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C. S. Jones and Paola Cessi

1. Introduction In the current climate system, deep water is formed in the North Atlantic, but not in the North Pacific, resulting in a global meridional overturning circulation (MOC) that transports heat northward in the Atlantic and contributes to a more marked southward heat flux in the South Indo-Pacific. The MOC is a global cell, driven by the wind-induced upwelling in the circumpolar region ( Toggweiler and Samuels 1993 ; Wolfe and Cessi 2010 ), as well as by diffusive upwelling at the

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Paola Cessi

convergence zone ( Kang et al. 2009 ). The oceanic heat transport in the Atlantic sector, everywhere northward, causes this asymmetry: it arises from the interhemispheric meridional overturning circulation that occupies the middepths of the Atlantic basin. The conceptual framework for the maintenance of the middepth overturning circulation (MOC) and the associated stratification has changed in the last decades. The idea of a diffusive balance between advection of buoyancy by the global overturning and

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A. M. de Boer, J. R. Toggweiler, and D. M. Sigman

which it is released through convection is not yet understood ( Nilsson et al. 2003 ; Gnanadesikan et al. 2005 ; Kuhlbrodt et al. 2007 ). However, because the input of mechanical energy is needed to produce a deep meridional overturning circulation, it stands to reason that there is both an upper and lower limit of overturning associated with a given energy input and that these limits depend on the internal stratification of the ocean. For lack of a more descriptive definition, we refer here to

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Michael A. Spall and David Nieves

1. Introduction The Atlantic meridional overturning circulation (AMOC) transports significant quantities of heat and freshwater and, as such, represents an important component of the global climate system ( Ganachaud and Wunsch 2003 ; Lumpkin and Speer 2007 ). Variability in the AMOC is correlated with variability in sea surface temperature, air–sea fluxes, and heat storage in the ocean ( Williams et al. 2014 ; Häkkinen et al. 2015 ; Evans et al. 2017 ). The mean AMOC at 26.5°N is

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David P. Marshall and Helen R. Pillar

7 , we diagnose the rotational forces driving the meridional overturning circulation (MOC) in a closed interhemispheric basin. Finally, in section 8 , we briefly summarize our main results before discussing potential applications of the new approach. 2. Theoretical background a. Helmholtz decomposition of a force Consider an arbitrary force F acting on an incompressible (or Boussinesq) fluid. The momentum equation is where u is the fluid velocity, p is pressure, ρ 0 is a reference

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Kristofer Döös and Johan Nilsson

1. Introduction Meridional overturning mass transport streamfunctions can be useful for describing the zonally averaged meridional atmospheric circulation. These streamfunctions delineate aspects of the three-dimensional circulation in two dimensions, which is advantageous for the purpose of visualizing. A possible mistake in interpreting overturning mass transport streamfunctions is to assume that their “streamlines” correspond directly to Lagrangian particle trajectories of the underlying

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Fabian Schloesser

1. Introduction The Atlantic meridional overturning circulation (AMOC) is a key component of the Earth climate system. Dynamical considerations and model studies indicate that the AMOC strength is closely related to the meridional density gradient and thermocline depth in the North Atlantic ( Johnson et al. 2019 ). Through meridional advection of heat and freshwater, the AMOC does not only affect climate patterns, but also influences ocean density and stratification, providing the opportunity

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Matthew R. Mazloff, Raffaele Ferrari, and Tapio Schneider

1. Introduction The Southern Ocean (SO) plays a crucial role in transforming and transporting ocean water masses. The Atlantic, Pacific, and Indian Oceans are connected through the SO, and no description of the global ocean circulation is complete without a full understanding of this region. One wishes to understand the Antarctic Circumpolar Current (ACC) system, the polar gyres, and the meridional overturning circulation (MOC), which are linked as they represent branches of the three

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Andrew McC. Hogg, Henk A. Dijkstra, and Juan A. Saenz

1. Introduction The processes that drive ocean circulation, and particularly the ocean's meridional overturning circulation (MOC), remain a topic of debate. A key aspect of this debate is the source of the global ocean mechanical energy budget and can be illustrated using the “horizontal convection” paradigm for the ocean overturning circulation ( Hughes and Griffiths 2008 ). In this paradigm, one delineates the energetic contributions from surface buoyancy forcing (which generates regions of

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Jan Viebahn and Carsten Eden

1. Introduction The meridional overturning circulation (MOC) of the Southern Ocean (SO) appears to be an important part of the global overturning circulation system ( Kuhlbrodt et al. 2007 ). On one hand, the interbasin connection featured by the SO is a necessary condition for a global circulation to be possible. On the other hand, the water mass transformations, driven by wind and buoyancy forcing in the SO, establish connections between deep and surface waters in the SO and hence close the

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