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Lisa M. Beal, Shane Elipot, Adam Houk, and Greta M. Leber

1. Introduction The Agulhas Current is the western boundary current of the south Indian Ocean subtropical gyre. Its variability has been linked upstream to the Indonesian Throughflow and Pacific El Niño–Southern Oscillation ( De Ruijter et al. 2005 ; Le Bars et al. 2013 ; Putrasahan et al. 2014, manuscript submitted to J. Climate ), while downstream it feeds an interocean transport—or “leakage”—of warm and saline waters into the South Atlantic ( Gordon 1986 ). This Agulhas leakage is thought

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Bo Qiu, Shuiming Chen, Daniel L. Rudnick, and Yuji Kashino

]. After impinging upon the Philippine coast, the NEC bifurcates into the northward-flowing Kuroshio and the southward-flowing Mindanao Current (MC), the two prominent western boundary currents in the North Pacific Ocean (e.g., Toole et al. 1988 ; Qiu and Lukas 1996 ). Heading equatorward and at the southern tip of the Mindanao Island near 5°N, a significant portion of the MC veers eastward to form the North Equatorial Countercurrent (NECC), with the remaining portion intruding westward into the

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Sara Broomé and Johan Nilsson

1. Introduction The continents of the Northern Hemisphere provide boundaries for the oceans that allow the oceanic meridional heat transport to largely be brought about by time-mean geostrophic currents. This can be compared to the Southern Ocean where the lack of boundaries yields a situation where meridional heat transport instead is mainly mediated by eddies ( Döös and Webb 1994 ; Marshall and Speer 2012 ). Furthermore, the relatively weak stratification of northern high-latitude ocean

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L. Chérubin, X. Carton, and D. G. Dritschel

between the predictions of the weakly nonlinear theory and the results of fully nonlinear numerical simulations. In particular, nonlinear amplification of the perturbation predicted by the weakly nonlinear theory corresponds to regions of parameter space where the simulations show vortex or dipole formation. In the simulations, the ejection of dipoles from the boundary current occurs for ρ < −1 and ρ b < 0, conditions under which long waves dominate and where linear baroclinic instability is

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Arjun Jagannathan, Kaushik Srinivasan, James C. McWilliams, M. Jeroen Molemaker, and Andrew L. Stewart

output so as to exclude the spinup time. 3. Theoretical formulation a. An integrated vorticity balance We develop a vertically integrated vorticity formulation to analyze the vorticity balances in our solutions. The central question is, what causes vorticity generation when a current encounters sloping bathymetry. The hitherto overlooked role of the bottom stress divergence torque (BSDT), which appears as one of the boundary terms in this formulation, will be demonstrated in section 4 . The starting

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Marlos Goes, Gustavo Goni, Shenfu Dong, Timothy Boyer, and Molly Baringer

variability of ocean currents (i.e., boundary currents and surface/subsurface currents) and basinwide integrated heat and volume transports across fixed high-density sections. XBT transect observations also have the advantage of being relatively low cost, with deployments generally coming from commercial vessels. Boundary currents carry a significant part of the transport of mass and heat in the ocean ( Sutton and Allen 1997 ; Visbeck et al. 2003 ; Todd et al. 2019 ) and, therefore, are a key component

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Andrew E. Kiss and Leela M. Frankcombe

1. Introduction Western boundary currents (WBCs) are regions of large heat transport and atmosphere–ocean heat fluxes, with very high variability on time scales ranging from seasonal to multidecadal, making them potentially important in climate variability ( Kelly et al. 2010 ; Kwon et al. 2010 ; Frankignoul et al. 2011 ; Hu et al. 2015 ). Here we address the causes of temporal variability in wind-driven gyres, in particular whether a time scale observed in a WBC can be attributed (even in

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Janna Köhler, Christian Mertens, Maren Walter, Uwe Stöber, Monika Rhein, and Torsten Kanzow

et al. 2000 ; Mauritzen et al. 2002 ; Shcherbina et al. 2003 ; Garabato et al. 2004 ), and the strength of geostrophic flow impinging on topography ( Nikurashin and Ferrari 2010a , b ; Waterman et al. 2013 ; Sheen et al. 2013 ). Diapycnal diffusivities in the deep western boundary current (DWBC) in the tropical Atlantic obtained from a parameterization based on the internal wave spectrum strongly increase with increasing background velocities ( Stöber et al. 2008 ). Elevated diapycnal

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Paola Cessi and Christopher L. Wolfe

upwelling (or downwelling). Because of the important implications to fisheries, much of the focus on coastal circulation has been on near-surface upwelling and the associated equatorward currents, which we believe arise because of the local wind stress effect. However, all of the eastern boundary current (EBC) systems also have poleward undercurrents—usually faster than the surface equatorward flow—overlying a deeper equatorward current ( Barton 1998 ; Pierce et al. 2000 ; Penven et al. 2005 ). In

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Bernadette M. Sloyan, Ken R. Ridgway, and Rebecca Cowley

1. Introduction The East Australian Current (EAC) is the complex and highly energetic poleward western boundary current system of the South Pacific Subtropical Gyre. It is the dominant mechanism for the redistribution of heat between the ocean and atmosphere in the Australian region by transporting heat from the tropical Pacific Ocean to the midlatitude ocean and atmosphere. Between 10° and 15°S the South Equatorial Current (SEC) meets the Australian continental margin and bifurcates into the

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