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Jiwei Tian, Qingxuan Yang, and Wei Zhao

1. Introduction Diapycnal mixing is an important but poorly understood dynamic process in the ocean. The global ocean circulation results from downwelling at a few selected regions in the North Atlantic and the Southern Ocean and upwelling elsewhere in the ocean ( Munk and Wunsch 1998 ). The upwelling occurs because of the decrease in density of the cold, deep water caused by a downward mixing of heat across the thermocline. After upwelling into the upper ocean, the water can flow back to the

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

1. Introduction As evidence has accumulated in the last 30 years from finestructure and microstructure measurement programs ( St. Laurent and Simmons 2006 ) in combination with tracer release experiments ( Ledwell et al. 1993 , 2000 ), a paradigm for ocean diapycnal mixing has developed in which mixing is weak through vast reaches of the ocean interior but greatly enhanced in the vicinity of rough bathymetry. Sites of particular interest are where ocean currents impinge on rough and/or abrupt

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

remain sparse and mostly limited to mid- and low latitudes ( Gregg et al. 1973 ; Toole et al. 1994 ; Gregg et al. 2003 ; Klymak et al. 2006 ). Measurements of vertical shear and strain at scales of tens of meters, from which mixing estimates can be inferred, are more widespread, but formulations relating these internal wave characteristics to dissipation rates and diapycnal diffusivity are subject to a number of added approximations ( Gregg 1989 ; Kunze et al. 2006 ). The Southern Ocean is a

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Zhao Jing and Lixin Wu

1. Introduction Turbulent diapycnal mixing in the ocean affects the transport of heat, freshwater, dissolved gases, nutrients, and pollutants. Understanding the spatial and temporal variation of diapycnal mixing is important for improving models’ representation and prediction of large-scale ocean circulation and climate ( Saenko and Merrifield 2005 ; Wunsch and Ferrari 2004 ). So far, general circulation models usually treat diapycnal mixing with simplified parameterization schemes ( Bryan and

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Zhao Jing, Ping Chang, Steven F. DiMarco, and Lixin Wu

1. Introduction Diapycnal mixing in the ocean plays an important role in transport of heat, freshwater, dissolved gases, nutrients, and pollutants, such as oil spills. Understanding its spatial and temporal variation is the key to improving model representation and prediction of large-scale ocean circulation and climate ( Richards et al. 2009 ; Saenko and Merrifield 2005 ; Wunsch and Ferrari 2004 ), as well as marine environment hazards, such as hypoxia (e.g., DiMarco et al. 2012 ) and oil

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Thomas Decloedt and Douglas S. Luther

1. Introduction and motivation Diapycnal mixing 1 occurs at scales of decameters to millimeters where turbulence generates property gradients that are irreversibly destroyed by molecular diffusion. The physical processes leading to turbulence in the abyssal ocean (depth > 1 km) are generally associated with instabilities of the internal wave field on vertical scales of O (10 m) (e.g., McComas and Müller 1981 ; Henyey et al. 1986 ; Toole 1998 ). These scales will remain unresolved, and most

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Yuan-Zheng Lu, Xian-Rong Cen, Shuang-Xi Guo, Ling Qu, Peng-Qi Huang, Xiao-Dong Shang, and Sheng-Qi Zhou

, where the intensive diapycnal mixing is driven by energetic internal waves near the Luzon Strait ( Alford et al. 2011 ; Liu and Lozovatsky 2012 ; St. Laurent 2008 ; Tian et al. 2009 ). The Luzon Strait is one of the most energetic internal wave–generating regions in the world’s oceans. This is attributed to the interaction of strong tides with the steep topography along its two north–south ridges ( St. Laurent et al. 2011 ; Alford et al. 2011 , 2015 ). The prominent pressure difference across

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David P. Marshall and Alberto C. Naveira Garabato

an impressive list of model improvements including a sharper and more realistic pycnocline and removal of spurious overturning cells in the Southern Ocean ( Danabasoglu et al. 1994 ). Most of these improvements can be attributed to the removal of horizontal diffusion across sloping isopycnals, thereby allowing the effective rate of diapycnal mixing in such models to be reduced from a magnitude of 10 −4 to 10 −5 m 2 s −1 , consistent with inferences from microstructure measurements (e

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Amy F. Waterhouse, Jennifer A. MacKinnon, Jonathan D. Nash, Matthew H. Alford, Eric Kunze, Harper L. Simmons, Kurt L. Polzin, Louis C. St. Laurent, Oliver M. Sun, Robert Pinkel, Lynne D. Talley, Caitlin B. Whalen, Tycho N. Huussen, Glenn S. Carter, Ilker Fer, Stephanie Waterman, Alberto C. Naveira Garabato, Thomas B. Sanford, and Craig M. Lee

1. Introduction Understanding diapycnal mixing in the global ocean and how it is distributed is important, because diapycnal diffusivity plays a primary role in the meridional overturning and heat budget of the ocean ( Munk and Wunsch 1998 ). Globally, 1–2 TW of diapycnal mixing is thought to be needed to maintain the observed stratification ( Munk and Wunsch 1998 ; Wunsch and Ferrari 2004 ). The distribution of diapycnal mixing is extremely patchy in space, varying with both depth and

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Olivier Arzel and Alain Colin de Verdière

of scales from circulation to molecular diffusion scales so that turbulent diapycnal diffusivity coefficients are imposed to represent this direct cascade of tracer variance. Idealized OGCM studies have shown that the circulation is indeed sensitive to the values of the diapycnal mixing coefficient [see review by Kuhlbrodt et al. (2007) ], as predicted by Welander (1971) . Large-scale dynamical studies by Vallis (2000) support the idea of weak (stronger) diffusive processes respectively above

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