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The Weddell Gyre, Southern Ocean: Present Knowledge and Future Challenges

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The Weddell Gyre, Southern Ocean: Present Knowledge and Future Challenges Vernet Maria (Orcid ID: 0000-0001-7534-5343) Hoppema Mario (Orcid ID: 0000-0002-2326-619X) Geibert Walter (Orcid ID: 0000-0001-8646-2334) Brown Peter (Orcid ID: 0000-0002-1152-1114) Haas Christian (Orcid ID: 0000-0002-7674-3500) Hellmer Hartmut, Heinz (Orcid ID: 0000-0002-9357-9853) Jokat Wilfried (Orcid ID: 0000-0002-7793-5854) Jullion Loïc (Orcid ID: 0000-0001-6269-6750) Mazloff Matthew, R. (Orcid ID: 0000-0002-1650-5850) Bakker Dorothee, C. E. (Orcid ID: 0000-0001-9234-5337) Brearley J., Alexander (Orcid ID: 0000-0003-3700-8017) Hattermann Tore (Orcid ID: 0000-0002-5538-2267) Hauck Judith (Orcid ID: 0000-0003-4723-9652) Hillenbrand Claus-Dieter (Orcid ID: 0000-0003-0240-7317) Huhn Oliver (Orcid ID: 0000-0003-3626-9135) Koch Boris, Peter (Orcid ID: 0000-0002-8453-731X) Lechtenfeld Oliver (Orcid ID: 0000-0001-5313-6014) Meredith Michael (Orcid ID: 0000-0002-7342-7756) Naveira Garabato Alberto, C. (Orcid ID: 0000-0001-6071-605X) Peeken Ilka (Orcid ID: 0000-0003-1531-1664) Rutgers van der Loeff Michiel (Orcid ID: 0000-0003-1393-3742) Schmidtko Sunke (Orcid ID: 0000-0003-3272-7055) Strass Volker, H. (Orcid ID: 0000-0002-7539-1400) Torres-Valdés Sinhué (Orcid ID: 0000-0003-2749-4170) Verdy Ariane (Orcid ID: 0000-0002-2774-2691) This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1029/2018RG000604 © 2019 American Geophysical Union. All rights reserved. The Weddell Gyre, Southern Ocean: present knowledge and future challenges M. Vernet1*, W. Geibert2*, M. Hoppema2*, P.J. Brown3, C. Haas2, H.H. Hellmer2, W. Jokat2, L. Jullion4, M. Mazloff1, D.C.E. Bakker5, J.A. Brearley6, P. Croot7, T. Hattermann2,8, J. Hauck2, C.-D. Hillenbrand6, C.J.M. Hoppe2, O. Huhn9, B.P. Koch2, O.J. Lechtenfeld10, M.P. Meredith6, A.C. Naveira Garabato11, E.-M. Nöthig2, I. Peeken2, M.M. Rutgers van der Loeff2, S. Schmidtko12, M. Schröder2, V.H. Strass2, S. Torres-Valdés3,2, A. Verdy1 1 Scripps Institution of Oceanography, La Jolla, California, USA 2 Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany 3 National Oceanography Centre, Southampton, United Kingdom 4 Institut Méditerranéen d’Oceanologie, Marseille, France 5 Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, UK 6 British Antarctic Survey, Cambridge, UK 7 Irish Centre for Research in Applied Geoscience (iCRAG@NUIG), Earth and Ocean Sciences, National University of Ireland, Galway, Ireland 8 Norwegian Polar Institute, Fram Centre, Tromsø, Norway 9 Institute of Environmental Physics, University of Bremen, Bremen, Germany 10 Helmholtz Centre for Environmental Research, Leipzig, Germany 11 National Oceanography Centre, University of Southampton, Southampton, United Kingdom 12 GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany (*) Contributed equally to planning, organizing and editing this review © 2019 American Geophysical Union. All rights reserved. Corresponding author: Maria Vernet ([email protected]) Keywords: Weddell Gyre, Weddell Sea, Antarctica, deep-water formation, air-sea-ice interactions, ice shelf, sedimentation, carbon cycle, phytoplankton, ocean acidification, nutrients, iron, radionuclides, long-term sampling, regional models Key Points 1. Major research priorities to advance understanding of the Weddell Gyre are identified and justified against current knowledge. 2. Interdisciplinary approaches are needed to support system science research of the Weddell Gyre and promote collaborative projects. 3. Winter conditions, connectivity to lower latitudes, intensification of the gyre are the main interdisciplinary priorities. © 2019 American Geophysical Union. All rights reserved. Abstract The Weddell Gyre (WG) is one of the main oceanographic features of the Southern Ocean south of the Antarctic Circumpolar Current which plays an influential role in global ocean circulation as well as gas exchange with the atmosphere. We review the state-of-the art knowledge concerning the WG from an interdisciplinary perspective, uncovering critical aspects needed to understand this system’s role in shaping the future evolution of oceanic heat and carbon uptake over the next decades. The main limitations in our knowledge are related to the conditions in this extreme and remote environment, where the polar night, very low air temperatures and presence of sea ice year-round hamper field and remotely sensed measurements. We highlight the importance of winter and under-ice conditions in the southern WG, the role that new technology will play to overcome present-day sampling limitations, the importance of the WG connectivity to the low-latitude oceans and atmosphere, and the expected intensification of the WG circulation as the westerly winds intensify. Greater international cooperation is needed to define key sampling locations that can be visited by any research vessel in the region. Existing transects sampled since the 1980s along the Prime Meridian and along an East-West section at ~62°S should be maintained with regularity to provide answers to the relevant questions. This approach will provide long-term data to determine trends and will improve representation of processes for regional, Antarctic-wide and global modeling efforts – thereby enhancing predictions of the WG in global ocean circulation and climate. © 2019 American Geophysical Union. All rights reserved. Plain Language Summary The Weddell Gyre is one of the main oceanographic features in the ocean surrounding Antarctica, the Southern Ocean. Although located far from other continents, this polar region affects the planet through the exchange of gases between frigid ocean waters and the atmosphere, regulating oxygen and carbon dioxide farther north. Studying the Weddell Gyre is challenging, as sea ice covers the ocean surface year around, restricting access by research ships and sensing of ocean surface from satellites. New technology is now available to avoid past limitations, autonomous underwater vehicles, instruments flown by planes, and floats instrumented with sea ice detection. Only through international collaboration can we obtain adequate data to populate environmental models and study key areas in the gyre, or hot spots. In this review we identify the missing links in our knowledge of the gyre, propose research to address those questions. Three aspects are critical to understanding the processes that drive the gyre’s oceanography, ice, geology, chemistry and biology: winter and under-ice conditions that set the stage for the evolution of physics, ice and biogeochemistry; exchange of water, material and energy (or heat) with lower latitudes; and intensification of the clockwise circulation of the gyre with changes in winds. © 2019 American Geophysical Union. All rights reserved. 1 INTRODUCTION The Weddell Gyre (WG) is in many respects an unusual and extreme region, directly controlling the properties of substantial parts of the global deep (200 – 3000 m) and abyssal (>3000 m) ocean, and indirectly shaping global climate. South of the Antarctic Circumpolar Current (ACC) – the strongest current system on Earth – the WG is the largest of several coherent oceanic regions (Figure 1), spanning about 5.6 million km2. Together with the Ross Gyre, the WG comprises the southernmost open ocean reaches on the planet (Figure 1). Extremely low sea surface temperatures and extensive sea-ice formation are characteristics of this region. Consequently, it is a foremost location of deep and bottom water formation on the southern hemisphere. This region also plays an important role in regulating air-sea exchanges and heat flow as a result of a delicate balance between upwelling of relatively warm sub-surface waters and via its sea-ice cover (Fahrbach et al., 1994; Gordon & Huber, 1990). As outlined in more detail below, this strong connection between the atmosphere, surface ocean and deep waters make the WG arguably a most influential oceanic region with the capability of influencing global climate on timescales of hundreds to thousands of years. The WG is situated in the Atlantic sector of the Southern Ocean, south of 55-60°S and roughly between 60°W and 30°E (Deacon, 1979). It stretches over the Weddell abyssal plain, where the Weddell Sea is situated, and extends east into the Enderby abyssal plain (10-20°E; Figure 2). The gyre is, however, a dynamic feature, with its eastern boundary not fixed to a location; it has been suggested that it might extend as far as 70°E (Mackintosh, 1972; Y.H. Park et al., 2001). The WG is principally driven by wind forcing (see Section 4.1) and steered by topography, including continental boundaries and seabed structures. In the south and west (50°W), the gyre is bordered by the Antarctic mainland and its northward extension, the Antarctic Peninsula. In the north, the boundary is formed by the frontal structure of the southern ACC, which in turn is topographically guided by the South Scotia Ridge in the west and the North Weddell Ridge in the © 2019 American Geophysical Union. All rights reserved. east. In this way, the northern boundary of the gyre in the west is at about 60°S, whereas further east it moves to about 55°S, locally extending further north into the South Sandwich Trench at about 30°W (Figure 2). East of about 20°E, the gyre boundary takes a southward direction following the course of the ACC, steered east of the Gunnerus Ridge. Along its southern and western boundaries, the WG is flanked by floating ice shelves, the seaward extension of the Antarctic ice sheet (depicted by the red contour in Figure 2; see Section 5). They form as a result of the gravity-driven flow of inland ice towards the coast. Some of these ice shelves extend more than a hundred kilometers offshore, with the Filchner-Ronne Ice Shelf, which covers a portion of the southern Weddell Sea continental shelf over an area extent roughly the same size of Sweden, being the largest body of floating ice on earth.
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