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Understanding changes in Antarctic ice shelf basal melting is a major challenge for predicting future sea level. Currently, warm Circumpolar Deep Water surrounding Antarctica has limited access to the Weddell Sea continental shelf; consequently, melt rates at Filchner‐Ronne Ice Shelf are low. However, large‐scale model projections suggest that changes to the Antarctic Slope Front and the coastal circulation may enhance warm inflows within this century. We use a regional high‐resolution ice shelf cavity and ocean circulation model to explore forcing changes that may trigger this regime shift. Our results suggest two necessary conditions for supporting a sustained warm inflow into the Filchner Ice Shelf cavity: (i) an extreme relaxation of the Antarctic Slope Front density gradient and (ii) substantial freshening of the dense shelf water. We also find that the on‐shelf transport over the western Weddell Sea shelf is sensitive to the Filchner Trough overflow characteristics. Plain Language Summary: The Weddell Sea continental shelf is presently filled with water masses that are too cold to melt the Filchner‐Ronne Ice Shelf from below. If warmer offshore water masses gain access to the Weddell Sea continental shelf and flow into the ice shelf cavity, the ocean‐driven melting would increase rapidly and cause ice shelf thinning. Increased ice shelf melting would supply the continental shelf with freshwater that influence sea ice production and generation of dense water masses and cause an increased discharge of grounded ice, which would contribute to global sea level rise. Two main factors currently prevent warm water from accessing the Weddell Sea continental shelf. First, the warm water is located deeper than the continental shelf and does not have direct access. Second, the Weddell Sea continental shelf is filled with dense water masses that block an inflow of warmer and lighter water. We use a regional ocean model to investigate what would happen if the warm offshore water is lifted higher up or if the dense water masses become fresher and lighter. We find that the Weddell Sea system is robust, and we need to make extreme changes to both factors to allow warm water access to the continental shelf. Key Points: Thermocline heaving and freshening of the high‐density shelf water are both needed to cause a cold‐to‐warm regime shift in the Weddell SeaThe presence of dense shelf water in Filchner Trough limits inflow of warm water toward the Filchner Ice Shelf frontReduced Filchner Trough overflow may cause increased on‐shelf transport, and thus warming, in the western Weddell Sea [ABSTRACT FROM AUTHOR]

Melting at the base of floating ice shelves is a dominant term in the overall Antarctic mass budget. This study applies a high-resolution regional ice shelf/ocean model, constrained by observations, to (i) quantify present basal mass loss at the Fimbul Ice Shelf (FIS); and (ii) investigate the oceanic mechanisms that govern the heat supply to ice shelves in the Eastern Weddell Sea. The simulations confirm the low melt rates suggested by observations and show that melting is primarily determined by the depth of the coastal thermocline, regulating deep ocean heat fluxes towards the ice. Furthermore, the uneven distribution of ice shelf area at different depths modulates the melting response to oceanic forcing, causing the existence of two distinct states of melting at the FIS. In the simulated present-day state, only small amounts of Modified Warm Deep Water enter the continental shelf, and ocean temperatures beneath the ice are close to the surface freezing point. The basal mass loss in this so-called state of “shallow melting” is mainly controlled by the seasonal inflow of solar-heated surface water affecting large areas of shallow ice in the upper part of the cavity. This is in contrast to a state of “deep melting”, in which the thermocline rises above the shelf break depth, establishing a continuous inflow of Warm Deep Water towards the deep ice. The transition between the two states is found to be determined by a complex response of the Antarctic Slope Front overturning circulation to varying climate forcings. A proper representation of these frontal dynamics in climate models will therefore be crucial when assessing the evolution of ice shelf basal melting along this sector of Antarctica. [ABSTRACT FROM AUTHOR]

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. 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, proposing 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. Key Points: Major research priorities to advance understanding of the Weddell Gyre are identified and justified against current knowledgeInterdisciplinary approaches are needed to support system science research of the Weddell Gyre and promote collaborative projectsWinter conditions, connectivity to lower latitudes, and intensification of the gyre are the main interdisciplinary priorities [ABSTRACT FROM AUTHOR]