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Ariaan Purich, Matthew H. England, Wenju Cai, Arnold Sullivan, and Paul J. Durack

1. Introduction Observed Southern Ocean changes over recent decades include a surface freshening ( Durack and Wijffels 2010 ; Durack et al. 2012 ; de Lavergne et al. 2014 ), surface cooling ( Fan et al. 2014 ; Marshall et al. 2014 ; Armour et al. 2016 ; Purich et al. 2016a ), and circumpolar increase in Antarctic sea ice ( Cavalieri and Parkinson 2008 ; Comiso and Nishio 2008 ; Parkinson and Cavalieri 2012 ). Various explanations for the increase in Antarctic sea ice extent (SIE) seen

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R. J. Matear and A. Lenton

1. Introduction The Southern Ocean (SO, oceans south of 40°S) has the highest annual-averaged winds over the oceans. These strong westerly winds help drive the Antarctic Circumpolar Current (ACC) eastward around Antarctica. These winds through Ekman transport also produce a northward flow, creating a divergent driven deep upwelling south of the ACC. In this region, the upwelling of middepth (2–2.5 km) water to the surface provides a unique connection between the deep ocean and the atmosphere

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Daniele Iudicone, Gurvan Madec, Bruno Blanke, and Sabrina Speich

1. Introduction The global thermohaline circulation (THC), sometimes referred to as the ocean’s conveyor belt ( Gordon 1986 ; Broecker 1987 ), is responsible for a large portion of the global redistribution of heat, freshwater, and biogeochemical tracers in the present climate. The Southern Ocean plays a major role in the THC as the main crossroad for the thermohaline circulation ( Macdonald and Wunsch 1996 ; Ganachaud and Wunsch 2000 ; Rintoul et al. 2001 ; Sloyan and Rintoul 2001a , b

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Wilbert Weijer, Bernadette M. Sloyan, Mathew E. Maltrud, Nicole Jeffery, Matthew W. Hecht, Corinne A. Hartin, Erik van Sebille, Ilana Wainer, and Laura Landrum

1. Introduction The Southern Ocean is a region of extremes: it is exposed to the most severe winds on the earth ( Wunsch 1998 ), the largest ice shelves ( Scambos et al. 2007 ), and the most extensive seasonal sea ice cover ( Thomas and Dieckmann 2003 ). These interactions among the atmosphere, ocean, and cryosphere greatly influence the dynamics of the entire climate system through the formation of water masses and the sequestration of heat, freshwater, carbon, and other properties ( Rintoul

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Christopher C. Chapman

1. Introduction Observations dating back to the Discovery expedition have revealed that the Antarctic Circumpolar Current (ACC) is composed of large-scale hydrographic fronts ( Deacon 1937 ). Although it is difficult to give a precise definition of a front ( Langlais et al. 2011 ; Chapman 2014 ), they are generally considered to be regions where water mass properties change rapidly ( Sokolov and Rintoul 2002 ). In the Southern Ocean, fronts are aligned more or less zonally. Water mass

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William J. M. Seviour, Anand Gnanadesikan, and Darryn W. Waugh

1. Introduction In recent decades significant trends in the summertime atmospheric circulation over the Southern Ocean (SO) have been observed. The extratropical jet has shifted poleward and intensified ( Thompson et al. 2011 ; Swart and Fyfe 2012 ; Hande et al. 2012 ), consistent with a more positive southern annular mode (SAM). These trends are outside the range of natural variability found in coupled climate models ( Thomas et al. 2015 ) and have been largely attributed to the impact of

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Björn Lund, Christopher J. Zappa, Hans C. Graber, and Alejandro Cifuentes-Lorenzen

1. Introduction The Southern Ocean is particularly sensitive to climate change, as evidenced by its rapidly rising heat content (e.g., Gille 2002 ). This and other changes to the Southern Ocean climate are dependent on the exchange of energy, mass, and momentum across the interface between ocean and atmosphere (and ice, if present) ( Sprintall et al. 2012 ). Yet, air–sea flux magnitudes and variations in the Southern Ocean are still poorly known ( Sahlée et al. 2012 ). The wave climate of the

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Earle A. Wilson, Stephen C. Riser, Ethan C. Campbell, and Annie P. S. Wong

1. Introduction In the sea ice–covered Southern Ocean, a relatively thin halocline separates the cold winter mixed layer (ML) from the significantly warmer ocean interior ( Gordon and Huber 1984 ). Over the course of winter, this halocline is gradually eroded by convective instabilities, triggered by the brine released from sea ice growth ( Gordon et al. 1984 ). In some regions, such as the weakly stratified Weddell Sea, the halocline is typically eroded to the point where a relatively small

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Stephanie M. Downes, Anand Gnanadesikan, Stephen M. Griffies, and Jorge L. Sarmiento

1. Introduction The zonal-mean view of Southern Ocean water mass circulation can be summarized as a balance between the southward-flowing thermocline waters and North Atlantic Deep Water (NADW) and the northward-flowing Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Antarctic Bottom Water (AABW) (e.g., Speer et al. 2000 ; Kuhlbrodt et al. 2007 ; Talley 2008 ). These water masses circulate throughout most of the global ocean and ventilate the ocean interior. South of

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Anthony E. Morrison, Steven T. Siems, and Michael J. Manton

1. Introduction The Southern Ocean region, together with the southern reaches of the Atlantic, Indian, and Pacific Oceans, represent approximately 15% of the earth’s surface. Home to the Antarctic Circumpolar Current, the largest movement of mass on the earth’s surface, the latitudinal band between 50° and 70°S contains a large amount of thermal inertia ( Barker and Thomas 2004 ). The poor representation of energy fluxes over this region has the potential to drastically affect the simulated

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