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Global ocean circulation models do not usually take high-latitude processes into account in an adequate form due to a limited model domain or insufficient resolution. Without the processes in key areas contributing to the lower part of the global thermohaline circulation, the characteristics and flow of deep and bottom waters often remain unrealistic in these models. In this study, various sections of the Bremerhaven Regional Ice Ocean Simulation model results are combined with a global inverse model by using temperature, salinity, and velocity constraints for the Hamburg Large Scale Geostrophic ocean general circulation model. The differences between the global model with and without additional constraints from the regional model demonstrate that the Weddell Sea circulation exerts a significant influence on the course of the Antarctic Circumpolar Current with consequences for Southern Ocean water mass characteristics and the spreading of deep and bottom waters in the South Atlantic. The influence of the Ross Sea is found to be less important in terms of global influences. However, regional changes in the Pacific sector of the Southern Ocean are found to be of Ross Sea origin. The additional constraints change the hydrographic conditions of the global model in the vicinity of the Antarctic Circumpolar Current in such a way that transport values, e.g., in Drake Passage no longer need to be prescribed to obtain observed transports. These changes not only improve the path and transport of the Antarctic Circumpolar Current but affect the meso- and large-scale circulation. With a higher (lower) mean Drake Passage transport, the mean Weddell Gyre transport is lower (higher). Furthermore, an increase (decrease) in the Antarctic Circumpolar Current leads to a decrease (increase) of the circum-Australian flow, i.e., a decrease (increase) of the Indonesian Throughflow. [ABSTRACT FROM AUTHOR]

The deep Scotia Sea is filled with ventilated Weddell Sea Deep Water. This in turn is an essential contributor to the ventilation of the World Ocean abyss. Depending on the formation process and/or its location along the Weddell Sea periphery, deep and bottom water masses follow different routes to cross the South Scotia Ridge. A primitive equation, hydrostatic, terrain-following coordinate ocean general circulation model (BRIOS-1) is used to investigate the water mass export from the Weddell Sea. The model is circumpolar focusing on the Weddell Sea, with particularly high resolution (∼20 km) in the DOVETAIL area. Eastward Weddell Sea Deep Water transport of 24×106 m3 s−1 is found in the northern limb of the Weddell Gyre across 44°W. Export rates of Weddell Sea Deep Water through gaps in the South Scotia Ridge are estimated to be 6.4×106 m3 s−1 with a semi-annual cycle of ±0.6×106 m3 s−1, which can be correlated to atmospheric cyclone activity and Weddell Gyre strength. Sensitivity studies considering extreme sea-ice conditions in the Weddell Sea show higher (lower) exports in years of minimum (maximum) winter sea-ice extent. Lagrangian particle trajectories illustrate the pathways of water masses from the inner Weddell Sea into the Scotia Sea through various gaps in the South Scotia Ridge. They highlight the existing flow divergence on the northwestern continental shelf, with one branch entering Bransfield Strait and the other continuing eastwards subsequently filling the deep Weddell and Scotia seas. Water masses flowing through the major gaps originate from the southwestern and southeastern Weddell Sea continental shelves. However, water masses formed east of the Weddell Sea (e.g., Prydz Bay) also seem to feed the deep Scotia Sea, since a large portion of floats flowing northward through the gaps of the South Scotia Ridge have been in contact with the mixed-layer processes outside the inner Weddell Sea. [ABSTRACT FROM AUTHOR]

Estimating the future evolution of the Antarctic Ice Sheet (AIS) is critical for improving future sea level rise (SLR) projections. Numerical ice sheet models are invaluable tools for bounding Antarctic vulnerability; yet, few continental-scale projections of century-scale AIS SLR contribution exist, and those that do vary by up to an order of magnitude. This is partly because model projections of future sea level are inherently uncertain and depend largely on the model's boundary conditions and climate forcing, which themselves are unknown due to the uncertainty in the projections of future anthropogenic emissions and subsequent climate response. Here, we aim to improve the understanding of how uncertainties in model forcing and boundary conditions affect ice sheet model simulations. With use of sampling techniques embedded within the Ice Sheet System Model (ISSM) framework, we assess how uncertainties in snow accumulation, ocean-induced melting, ice viscosity, basal friction, bedrock elevation, and the presence of ice shelves impact continental-scale 100-year model simulations of AIS future sea level contribution. Overall, we find that AIS sea level contribution is strongly affected by grounding line retreat, which is driven by the magnitude of ice shelf basal melt rates and by variations in bedrock topography. In addition, we find that over 1.2 m of AIS global mean sea level contribution over the next century is achievable, but not likely, as it is tenable only in response to unrealistically large melt rates and continental ice shelf collapse. Regionally, we find that under our most extreme 100-year warming experiment generalized for the entire ice sheet, the Amundsen Sea sector is the most significant source of model uncertainty (1032 mm 6σ spread) and the region with the largest potential for future sea level contribution (297 mm). In contrast, under a more plausible forcing informed regionally by literature and model sensitivity studies, the Ronne basin has a greater potential for local increases in ice shelf basal melt rates. As a result, under this more likely realization, where warm waters reach the continental shelf under the Ronne ice shelf, it is the Ronne basin, particularly the Evans and Rutford ice streams, that are the greatest contributors to potential SLR (161 mm) and to simulation uncertainty (420 mm 6σ spread). [ABSTRACT FROM AUTHOR]

Antarctica's floating ice shelves modulate discharge of grounded ice into the ocean by providing a backstress. Ice shelf thinning and grounding line retreat have reduced this backstress, driving rapid drawdown of key unstable areas of the Antarctic Ice Sheet, leading to sea-level rise. If ice shelf loss continues, it may initiate irreversible glacier retreat through the marine ice sheet instability. Identification of areas undergoing significant change requires knowledge of spatial and temporal patterns in recent ice shelf loss. We used 26 years (1992–2017) of satellite-derived Antarctic ice shelf thickness, flow, and basal melt rates to construct a time-dependent dataset of ice shelf thickness and basal melt on a 3 km grid every 3 months. We used a novel data fusion approach, state-of-the-art satellite-derived velocities, and a new surface mass balance model. Our data revealed an overall pattern of thinning all around Antarctica, with a thinning slowdown starting around 2008 widespread across the Amundsen, Bellingshausen, and Wilkes sectors. We attribute this slowdown partly to modulation in external ocean forcing, altered in West Antarctica by negative feedbacks between ice shelf thinning rates and grounded ice flow, and sub-ice-shelf cavity geometry and basal melting. In agreement with earlier studies, the highest rates of ice shelf thinning are found for those ice shelves located in the Amundsen and Bellingshausen sectors. Our study reveals that over the 1992–2017 observational period the Amundsen and Bellingshausen ice shelves experienced a slight reduction in rates of basal melting, suggesting that high rates of thinning are largely a response to changes in ocean conditions that predate our satellite altimetry record, with shorter-term variability only resulting in small deviations from the long-term trend. Our work demonstrates that causal inference drawn from ice shelf thinning and basal melt rates must take into account complex feedbacks between thinning and ice advection and between ice shelf draft and basal melt rates. [ABSTRACT FROM AUTHOR]

Meltwater content and pathways determine the impact of Antarctica's melting ice shelves on ocean circulation and climate. Using ocean glider observations, we quantify meltwater distribution and transport within the Bellingshausen Sea's Belgica Trough. Meltwater is present at different densities and with different turbidities: both are indicative of a layer's ice shelf of origin. To investigate how ice‐shelf origin separates meltwater into different export pathways, we compare these observations with high‐resolution tracer‐release model simulations. Meltwater filaments branch off the Antarctic Coastal Current into the southwestern trough. Meltwater also enters the Belgica Trough in the northwest via an extended western pathway, hence the greater observed southward (0.50 mSv) than northward (0.17 mSv) meltwater transport. Together, the observations and simulations reveal meltwater retention within a cyclonic in‐trough gyre, which has the potential to promote climactically important feedbacks on circulation and future melting. Plain Language Summary: Recent research has advanced our understanding of interactions between warm ocean waters and the underside of Antarctica's floating ice shelves. But what happens to meltwater that these ice shelves release? Meltwater is fresh: it reduces the salinity of sea water and upsets the delicate balance between salty and fresh water that drives many polar ocean processes ‐ for example, the sinking of cold, salty water to the deepest regions of the ocean, which is of global climatic importance. We use high‐resolution observations and state‐of‐the‐art model simulations to determine the pathways that meltwater takes as it flows in and around the Belgica Trough in the central Bellingshausen Sea, Antarctica. Meltwater is principally confined to the edges of the trough's clockwise gyre circulation. Meltwater from eastern ice shelves is generally found at shallower depths than meltwater from southern ice shelves; this explains evidence of two meltwater layers found in our observations. In addition, meltwater from eastern ice shelves is turbid, while meltwater from southern ice shelves is clear. These results will help us to better predict how Antarctica's coastal seas will respond to a warming climate as ever more meltwater is released into the ocean. Key Points: Meltwater pathways in the Bellingshausen Sea, Antarctica, are explored using observations from gliders and a high‐resolution regional modelMeltwater observed at different densities originates from different ice shelves: lighter meltwater layers originate from eastern ice shelvesMeltwater from different ice shelves is distinguished by different turbidity signatures, suggestive of different biogeochemical properties [ABSTRACT FROM AUTHOR]

Abstract: The mesozooplankton community, with special emphasis on calanoid copepods, was studied with respect to its species composition, abundance, vertical distribution and developmental structure during the “Ice Station POLarstern” (ISPOL) expedition to the ice-covered western Weddell Sea. Stratified zooplankton tows were carried out nine times between 1 December 2004 and 2 January 2005 with a multiple opening–closing net between 0 and 1000m depth. Copepods were by far the most abundant taxon, contributing more than 94% of the total mesozooplankton. Numerical dominants were cyclopoid copepods, mostly Oncaea spp. A total of 66 calanoid copepod species were identified, but the calanoid copepod community was characterised by the dominance of only a few species. The most numerous species was Microcalanus pygmaeus, which comprised on average 70% of all calanoids. Calanoides acutus and Metridia gerlachei represented other abundant calanoid species contributing an average of 8% and 7%, respectively. All other species comprised less than 3%. The temporal changes in the abundance and population structure of M. pygmaeus and M. gerlachei were small while a shift in the stage frequency distribution of C. acutus was observed during the study: copepodite stage IV (C IV) dominated the C. acutus population with 48–50% during the first week of December, while C V comprised 48% in late December. C I and C II of C. acutus were absent in the samples, and males occurred only in very low numbers in greater depths. In M. gerlachei, C I was not found, whereas all developmental stages of M. pygmaeus occurred throughout the study. All three species showed migratory behaviour, and they occurred in upper water layers towards the end of the investigation. This vertical ascent was most pronounced in C. acutus and relatively weak in the other two species. In M. pygmaeus and M. gerlachei, copepodids were responsible for the upward migration in late December, while the vertical distribution of adults did not change. In C. acutus, all abundant developmental stages (C IV, C V and females) ascended to upper water layers. Almost exclusively (93%) medium- and semi-ripe females of C. acutus and M. gerlachei were found, and only 3–4% of the ovaries were ripe. The absence of C I and the low number of ripe females indicate that the main reproductive period had not started in C. acutus and M. gerlachei until the end of our study in early January. In contrast, the high portion of C I and C II of M. pygmaeus suggests that reproduction of this species had started in October–November and hence before the onset of the phytoplankton bloom in the water. The community structure did not differ between stations with one exception on 26 December, when the station was strongly influenced by the continental shelf. [Copyright &y& Elsevier]

Abstract: During Ice Station POLarstern (ISPOL; R.V. Polarstern cruise ANT XXII/2, November 2004–January 2005), hydrographic and tracer observations were obtained in the western Weddell Sea while drifting closely in front of the Larsen Ice Shelf. These observations indicate recently formed Weddell Sea Bottom Water, which contains significant contributions of glacial melt water in its upper part, and High-Salinity Shelf Water in its lower layer. The formation of this bottom water cannot be related to the known sources in the south, the Filchner–Ronne Ice Shelf. We show that this bottom water is formed in the western Weddell Sea, most likely in interaction with the Larsen C Ice Shelf. By applying an Optimum Multiparameter Analysis (OMP) using temperature, salinity, and noble gas observations (helium isotopes and neon), we obtained mean glacial melt-water fractions of about 0.1% in the bottom water. On sections across the Weddell Gyre farther north, melt-water fractions are still on the order of 0.04%. Using chlorofluorocarbons (CFCs) as age tracers, we deduced a mean transit time between the western source and the bottom water found on the slope toward the north (9±3 years). This transit time is larger and the inferred transport rate is small in comparison to previous findings. But accounting for a loss of the initially formed bottom water volume due to mixing and renewal of Weddell Sea Deep Water, a formation rate of 1.1±0.5Sv in the western Weddell Sea is plausible. This implies a basal melt rate of 35±19Gt/year or 0.35±0.19m/year at the Larsen Ice Shelf. This bottom water is shallow enough that it could leave the Weddell Basin through the gaps in the South Scotia Ridge to supply Antarctic Bottom Water. These findings emphasize the role of the western Weddell Sea in deep- and bottom-water formation, particularly in view of changing environmental conditions due to climate variability, which might induce enhanced melting or even decay of ice shelves. [Copyright &y& Elsevier]