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Chaojiao Sun, Ming Feng, Richard J. Matear, Matthew A. Chamberlain, Peter Craig, Ken R. Ridgway, and Andreas Schiller

1. Introduction Ocean boundary currents are poorly represented in the current climate models that contribute to the Coupled Model Intercomparison Project phase 3 (CMIP3), an initiative by the World Climate Research Programme (WCRP). This representation is partly due to an insufficient horizontal resolution of about 1°–2° (about 100–200km) in the ocean component of climate models, too large to realistically simulate these narrow jets. As a result there is limited confidence in the structural

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Kathryn A. Kelly, R. Justin Small, R. M. Samelson, Bo Qiu, Terrence M. Joyce, Young-Oh Kwon, and Meghan F. Cronin

1. Introduction In the strong Northern Hemisphere midlatitude western boundary current (WBC) systems—the Gulf Stream (GS) in the North Atlantic and the Kuroshio Extension (KE) in the North Pacific—there is a complex interaction between dynamics and thermodynamics and between the atmosphere and ocean ( Fig. 1 ). A precipitous drop in the meridional transport of heat in the Northern Hemisphere ocean occurs where these warm WBCs separate from the coast and flow into the ocean interior ( Trenberth

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Bunmei Taguchi, Niklas Schneider, Masami Nonaka, and Hideharu Sasaki

anomalous advection of spiciness. Questions to be addressed are the following: What are the relative contribution of Rossby waves and spiciness to total OHC variability and what are their regional difference in the North Pacific, particularly in relation to western boundary current variability? What are the spatiotemporal structure, the propagation feature, and the origin of each process contributing to the OHC variability? The rest of the manuscript is organized as follows. Section 2 describes the

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Yue Sun, Jing-Wu Liu, and Shang-Ping Xie

1. Introduction Satellites have revealed a salient precipitation band residing just over the Gulf Stream ( Hobbs 1987 ; Minobe et al. 2008 ), which is the strongest oceanic western boundary current in the Northern Hemisphere ( Tomczak and Godfrey 2003 ). In winter, the precipitation over the Gulf Stream releases a huge amount of latent heat into the atmosphere ( Bane and Osgood 1989 ) and strongly influences regional climate and weather ( Hamilton 1981 ; Forbes et al. 1997 ; Pfahl et al

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Ryusuke Masunaga, Hisashi Nakamura, Bunmei Taguchi, and Takafumi Miyasaka

1. Introduction Satellite and in situ observations have captured local augmentation in time-mean surface wind convergence along the warm midlatitude western boundary currents (WBCs), including the Kuroshio Extension (KE), Gulf Stream (GS), and Agulhas Return Current (ARC), and divergence slightly poleward (e.g., Tokinaga et al. 2005 ; Minobe et al. 2008 , 2010 ; O’Neill et al. 2003 , 2005 ; Nkwinkwa Njouodo et al. 2018 ). The surface wind convergence accompanies local enhancement in

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Ryusuke Masunaga, Hisashi Nakamura, Bunmei Taguchi, and Takafumi Miyasaka

1. Introduction The midlatitude western boundary currents, which flow poleward along the western flank of each of the ocean basins, transport an enormous amount of heat from the tropics, releasing it into the midlatitude atmosphere in the form of turbulent sensible heat flux (SHF) and latent heat flux (LHF) while maintaining relatively warm sea surface temperature (SST) along their axes. The midlatitude oceanic frontal zones, which are characterized by steep gradients in SST along the poleward

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Woo Geun Cheon and Jong-Seong Kug

open-ocean polynyas are closely linked to near-boundary convection and open-ocean deep convection, respectively; therefore, they play a major role in the Antarctic Bottom Water (AABW) formation. Moreover, it has been proposed that the SH westerly winds controlling northward Ekman transport in the Southern Ocean (SO) affect the surface western boundary currents (WBCs) in the North Pacific and Atlantic ( McDermott 1996 ; Klinger and Cruz 2009 ). Herein, impacts of oscillating SH westerly winds on

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Samantha M. Wills and David W. J. Thompson

.g., Chelton et al. 2001 ) indicate that surface air blowing across an ocean front (or a region of large horizontal SST gradients) accelerates the surface flow, leading to patterns of convergence and divergence at the surface (e.g., O’Neill et al. 2003 ; Nonaka and Xie 2003 ; Chelton et al. 2004 ; Chelton and Xie 2010 ). Observational and numerical studies have shown that the climatological-mean SST gradients associated with the major western boundary currents are capable of forcing vertical motion

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Igor A. Dmitrenko, Sergey A. Kirillov, Vladimir V. Ivanov, Bert Rudels, Nuno Serra, and Nikolay V. Koldunov

1963–88 when the AW boundary current demonstrated a negative temperature and salinity anomaly ( Polyakov et al. 2004 ). In contrast, the majority of the summer data represent two periods of warmer and saltier AW from the 1950s and from 1990 to 2010 ( Polyakov et al. 2003 , 2004 ; Dmitrenko et al. 2008 ). For the upper AW (150–250 m) in the central Laptev Sea, this results in artificial bias between the long-term mean summer and winter temperatures and salinities by 0.65°C and 0.1 psu

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Sidonie Brachet, Francis Codron, Yizhak Feliks, Michael Ghil, Hervé Le Treut, and Eric Simonnet

1. Introduction and motivation Western boundary currents, including the Gulf Stream and Kuroshio, play a unique role in midlatitude ocean–atmosphere interactions because of the magnitude of local surface heat fluxes, and because local SST variability is mostly driven by ocean dynamics ( Kwon et al. 2010 ). Recent satellite observations show that small-scale SST features—such as meanders and fronts—have a strong impact on air–sea heat and momentum fluxes ( Kelly et al. 2010 ). In particular

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