Description: Antarctic ice sheet mass loss and thus part of global sea-level rise is related to enhanced ice stream discharge to the fringing ice shelves. The transfer of ice into the ocean occurs via iceberg calving and ice shelf basal melting. For decades the balance of both terms was assumed to be in favor of the calving, but recent results, based on remote sensing, revealed that basal melting seems to be at least of similar importance. A recent model study indicates that future atmospheric conditions in the southern Weddell Sea may switch the continental shelf, formerly dominated by the formation of cold saline waters, to one influenced by warm open ocean waters with consequences for the basal mass flux and ice shelf/ice sheet dynamics. Here, we continue the simulations showing a warming of the Filchner-Ronne Ice Shelf cavity, applying 20th-century atmospheric and basal mass flux forcing at different future points in time. Our numerical study indicates that once the system reaches the 'warm phase', a positive meltwater feedback stabilizes the shelf circulation such that only a reduction to 20th century basal mass flux can stop warm water from penetrating onto the continental shelf and into the sub-ice cavity. This has implications for the future of the Antarctic Ice Sheet, since a major decrease of basal melting only can be achieved by a significant disintegration of the floating portion of the ice sheet.

Description: A general ocean circulation model is coupled with a 3D-thermodynamical ice-sheet/shelf model to simulate the response of the Filchner–Ronne Ice Shelf (FRIS, Antarctica) and coastal parts of its catchment basin to a postulated inflow of Warm Deep Water into the ice-shelf cavity on a 1000-yr timescale. Prescribed ocean warming (based on climate projections) enters the ice-shelf cavity in the up to 1500 m deep Filchner Trough and penetrates deep into the sub-ice cavity. Increasing basal melt rates induce geometry changes of the cavity, which in turn have an impact on the ocean circulation and therefore the modelled melt rates. Highest melt rates of about 20 m yr−1 follow the (up to 180 km) retreating grounding line. Basal mass loss reaches about 250 km3 yr−1, doubling the present-day value. The most vulnerable areas below the FRIS are the Bailey Ice Stream and the area between the Institute and Moeller Ice Streams, where the increased melting accounts for about 80 km of the modelled grounding line retreat on the backward sloping bedrock. The potential additional contribution to the eustatic sea level rise due to the grounded-ice loss, simulated in an ensemble approach against a transient control experiment, is about 0.05 mm yr−1 during the first 500 yr and about 0.17 mm yr−1 thereafter.

Description: Der Westantarktische Eisschild hat aufgrund seiner unter dem Meeresspiegel gegründeten Basis einen besonderen Einfluss auf den globalen Meereisspiegelanstieg. Änderungen in der Ozeantemperatur oder im Zustrom warmer Wassermassen auf den Kontinentalschelf und in die Schelfeiskavernen hinein führen zu einem vermehrten Ausdünnen der Schelfeisgebiete, die eine rückstauende Wirkung auf den Abfluss von Eismassen aus dem Inland ausüben. Für viele Regionen der Westantarktis kommt hinzu, dass sich der Meeresboden landeinwärts neigt. Dies stellt eine instabile Position für die Gründungszone der Schelfeise dar und einmal in Bewegung gebracht, führt es zu einem fortschreitenden Rückzug und einem vermehrten Eisabfluss. Simulationen mit eisdynamischen Modellen unter der Annahme einer zukünftigen Klimaerwärmung prognostizieren einen verstärkten Beitrag insbesondere der Westantarktis zum globalen Meeresspiegel. Für die nächsten hundert bis tausend Jahre kann dieser bis zu einem halben Meter betragen. Es ist sogar mit einem teilweisen Kollaps des Westantarktischen Eisschild zu rechnen, der den globalen Meeresspiegel um mehrere Meter ansteigen lassen würde. The West Antarctic Ice Sheet has a particular influence on global sea-level rise owing to its base being below sea level. Changes in ocean temperature or in the inflow of warm water masses onto the continental shelf and into the ice-shelf cavities lead to increased thinning of the ice shelves which exert a buttressing effect on the outflow of ice from the ice sheet. For widespread regions of West Antarctica, in addition the seabed slopes downward inland. This causes possible instabilities of the grounding-line position of the ice shelves and once set in motion, might cause an irreversible retreat of the grounding line and an increased ice loss. Simulations with thermomechanical ice-sheet models, based on future climate-change scenarios, predict an increasing contribution in particular of West Antarctica to global sea-level rise. For the next few hundred to a thousand years, this contribution can be up to half a meter. Even a partial collapse of the West Antarctic Ice Sheet is possible, which would raise the global sea level by several meters.

Description: Germany intends to present the Working Group on Ecosystem Monitoring and Management (WG EMM) the background document that provides the scientific basis for the evaluation of a marine protected area (MPA) in the Weddell Sea planning area. The contents and structure of the whole document reflect its main objectives, i.e. to set out the general context of the establishment of MPAs and to provide the background information on the Weddell Sea MPA (WSMPA) planning area (Part A); to inform on the data retrieval process (Part B) and to describe the results of the scientific analyses and the MPA scenario development with the directly science-based aspects of the WSMPA proposal, i.e. the objectives and the boundaries and zones of the MPA (Part C). Here, the authors intend to update WG EMM on the current state of Part A of the document that has been presented at the meeting of the CCAMLR Scientific Committee in 2014. The Scientific Committee had welcomed and endorsed the scientific background document (SC-CAMLR-XXXIII/BG/02) as a foundation reference for the Weddell Sea MPA planning (SC-CAMLR-XXXIII, § 5.21). Part A contains (i) a synopsis in terms of the establishment of MPAs (chapter 1); (ii) a description of the boundaries of the WSMPA planning area (chapter 2); (iii) a comprehensive, yet succinct, general description of the Weddell Sea ecosystem (chapter 3); (iv) and finally a guidance regarding the future work beyond the development of the scientific basis for the evaluation of a WSMPA (chapter 4). Please note that the current state of Part A of the document presents a comprehensive yet incomplete version concerning chapters that have to be (further) developed or revised.

Description: The dense water flowing out from the Weddell Sea (WS) significantly contributes to Antarctic Bottom Water (AABW) and plays an important role in the Meridional Overturning Circulation. The larger amount of this dense water consists of Weddell Sea Deep Water (WSDW) formed in the WS, mainly in front of the Filchner-Ronne Ice Shelves and the Larsen Ice Shelf (LIS). We performed model simulations and analysis of hydrographic data that highlight the importance of the second source. Model simulations indicate that dense waters placed on the continental shelf off LIS flow down the slope and contribute to the WSDW that renews the AABW further downstream. Measurements made during the Polarstern cruise ANT XXIX-3 (2013) add evidence to the importance of the source in the western Weddell Sea. Using Optimum Multiparameter Analysis we show that the dense water found on the continental shelf in front of the former Larsen A and B together with water originating from Larsen C increases the thickness of the WSDW layer in 50%, and changes the temperature and salinity of this water mass. These modifications occur close to the WSDW outflow paths and therefore have high influence on the AABW properties.

Description: Ocean simulations performed with the Finite Element Ocean Model (FEOM) were used to show the relevance of the location of the dense water plume source on the western Weddell Sea continental shelf. When the plume starts close to the tip of the Antarctic Peninsula it flows into Bransfield Strait, but if it is found further south it can flow down the slope and contribute to Weddell Sea Deep Water (WSDW). The influence of density on the spreading was also tested indicating that a denser plume reaches greater depths while lighter plumes do not interact with the WSDW.