ALBERT

Description: We present a benthic foraminiferal multi-proxy record of eastern equatorial Atlantic (EEA) mid depth water (1295 m) covering the last deglacial. We show that EEA mid depth water temperatures were elevated by 3.9 ± 0.5 °C and 5.2 ± 1.2 °C during Heinrich event 1 (H1) and Younger Dryas (YD), respectively. The radiocarbon content of the EEA mid depth during H1 and YD is relatively low and comparable to the values of the pre-H1 episode and Bølling-Allerød, respectively. A transient earth system model simulation, which mimics the observed deglacial AMOC history, qualitatively reproduces the major features of the EEA proxy records. The simulation results suggest that fresh water-induced weakening of the AMOC leads to a vertical shift of the horizon of Southern Ocean-sourced water and a stronger influence of EEA sea surface temperatures via mixing. Our findings reaffirm the lack of a distinctive signature of radiocarbon depletion and therefore do not support the notion of interhemispheric exchanges of strongly radiocarbon-depleted mid-depth water across the tropical Atlantic during H1 and YD. Our temperature reconstruction presents a critical zonal and water depth extension of existing tropical Atlantic data and documents a large-scale and basin-wide warming across the thermocline and mid-depth of the tropical Atlantic during H1 and YD. Significant difference in the timing and pace of H1 mid-depth warming between tropical Atlantic and North Atlantic likely points to a limited role of the tropical Atlantic mid-depth warming in the rapid heat build up in the North Atlantic mid depth.

Description: The Regional Antarctic ice and Global Ocean (RAnGO) model has been developed to study the interaction between the world ocean and the Antarctic ice sheet. The coupled model is based on a global implementation of the Finite Element Sea-ice Ocean Model (FESOM) with a mesh refinement in the Southern Ocean, particularly in its marginal seas and in the sub-ice-shelf cavities. The cryosphere is represented by a regional setup of the ice flow model RIMBAY comprising the Filchner–Ronne Ice Shelf and the grounded ice in its catchment area up to the ice divides. At the base of the RIMBAY ice shelf, melt rates from FESOM's ice-shelf component are supplied. RIMBAY returns ice thickness and the position of the grounding line. The ocean model uses a pre-computed mesh to allow for an easy adjustment of the model domain to a varying cavity geometry. RAnGO simulations with a 20th-century climate forcing yield realistic basal melt rates and a quasi-stable grounding line position close to the presently observed state. In a centennial-scale warm-water-inflow scenario, the model suggests a substantial thinning of the ice shelf and a local retreat of the grounding line. The potentially negative feedback from ice-shelf thinning through a rising in situ freezing temperature is more than outweighed by the increasing water column thickness in the deepest parts of the cavity. Compared to a control simulation with fixed ice-shelf geometry, the coupled model thus yields a slightly stronger increase in ice-shelf basal melt rates.

Description: The development of coastal polynyas, areas of enhanced heat flux and sea ice production strongly depend on atmospheric conditions. In Antarctica, measurements are scarce and models are essential for the investigation of polynyas. A robust quantification of polynya exchange processes in simulations relies on a realistic representation of atmospheric conditions in the forcing dataset. The sensitivity of simulated coastal polynyas in the south-western Weddell Sea to the atmospheric forcing is investigated with the Finite-Element Sea ice-Ocean Model (FESOM) using daily NCEP/NCAR reanalysis data (NCEP), 6 hourly Global Model Europe (GME) data and two different hourly datasets from the high-resolution Consortium for Small-Scale Modelling (COSMO) model. Results are compared for April to August in 2007–09. The two coarse-scale datasets often produce the extremes of the data range, while the finer-scale forcings yield results closer to the median. The GME experiment features the strongest winds and, therefore, the greatest polynya activity, especially over the eastern continental shelf. This results in higher volume and export of High Salinity Shelf Water than in the NCEP and COSMO runs. The largest discrepancies between simulations occur for 2008, probably due to differing representations of the ENSO pattern at high southern latitudes. The results suggest that the large-scale wind field is of primary importance for polynya development.

Description: The ocean plays an important role in modulating the mass balance of the polar ice sheets by interacting with the ice shelves in Antarctica and with the marine-terminating outlet glaciers in Greenland. Given that the flux of warm water onto the continental shelf and into the sub-ice cavities is steered by complex bathymetry, a detailed topography data set is an essential ingredient for models that address ice–ocean interaction. We followed the spirit of the global RTopo-1 data set and compiled consistent maps of global ocean bathymetry, upper and lower ice surface topographies, and global surface height on a spherical grid with now 30 arcsec grid spacing. For this new data set, called RTopo-2, we used the General Bathymetric Chart of the Oceans (GEBCO_2014) as the backbone and added the International Bathymetric Chart of the Arctic Ocean version 3 (IBCAOv3) and the International Bathymetric Chart of the Southern Ocean (IBCSO) version 1. While RTopo-1 primarily aimed at a good and consistent representation of the Antarctic ice sheet, ice shelves, and sub-ice cavities, RTopo-2 now also contains ice topographies of the Greenland ice sheet and outlet glaciers. In particular, we aimed at a good representation of the fjord and shelf bathymetry surrounding the Greenland continent. We modified data from earlier gridded products in the areas of Petermann Glacier, Hagen Bræ, and Sermilik Fjord, assuming that sub-ice and fjord bathymetries roughly follow plausible Last Glacial Maximum ice flow patterns. For the continental shelf off Northeast Greenland and the floating ice tongue of Nioghalvfjerdsfjorden Glacier at about 79° N, we incorporated a high-resolution digital bathymetry model considering original multibeam survey data for the region. Radar data for surface topographies of the floating ice tongues of Nioghalvfjerdsfjorden Glacier and Zachariæ Isstrøm have been obtained from the data centres of Technical University of Denmark (DTU), Operation Icebridge (NASA/NSF), and Alfred Wegener Institute (AWI). For the Antarctic ice sheet/ice shelves, RTopo-2 largely relies on the Bedmap-2 product but applies corrections for the geometry of Getz, Abbot, and Fimbul ice shelf cavities. The data set is available in full and in regional subsets in NetCDF format from the PANGAEA database at doi:10.1594/PANGAEA.856844.

Description: The ocean plays an important role in modulating the mass balance of the polar ice sheets by interacting with the ice shelves in Antarctica and with the marine-terminating outlet glaciers in Greenland. Given that the flux of warm water onto the continental shelf and into the sub-ice cavities is steered by complex bathymetry, a detailed topography data set is an essential ingredient for models that address ice-ocean interaction. We followed the spirit of the global RTopo-1 data set and compiled consistent maps of global ocean bathymetry, upper and lower ice surface topographies and global surface height on a spherical grid with now 30-arc seconds resolution. We used the General Bathymetric Chart of the Oceans (GEBCO_2014) as the backbone and added the International Bathymetric Chart of the Arctic Ocean version 3 (IBCAOv3) and the International Bathymetric Chart of the Southern Ocean (IBCSO) version 1. While RTopo-1 primarily aimed at a good and consistent representation of the Antarctic ice sheet, ice shelves and sub-ice cavities, RTopo-2 now also contains ice topographies of the Greenland ice sheet and outlet glaciers. In particular, we aimed at a good representation of the fjord and shelf bathymetry surrounding the Greenland continent. We corrected data from earlier gridded products in the areas of Petermann Glacier, Hagen Bræ and Sermilik Fjord assuming that sub-ice and fjord bathymetries roughly follow plausible Last Glacial Maximum ice flow patterns. For the continental shelf off northeast Greenland and the floating ice tongue of Nioghalvfjerdsfjorden Glacier at about 79° N, we incorporated a high-resolution digital bathymetry model considering original multibeam survey data for the region. Radar data for surface topographies of the floating ice tongues of Nioghalvfjerdsfjorden Glacier and Zachariæ Isstrøm have been obtained from the data centers of Technical University of Denmark (DTU), Operation Icebridge (NASA/NSF) and Alfred Wegener Institute (AWI). For the Antarctic ice sheet/ice shelves, RTopo-2 largely relies on the Bedmap-2 product but applies corrections for the geometry of Getz, Abbot and Fimbul ice shelf cavities. The data set is available in full and in regional subsets in NetCDF format from the PANGAEA database at https://doi.pangaea.de/10.1594/PANGAEA.856844.

Description: An increasing number of Southern Ocean models now include Antarctic ice shelf cavities, and simulate thermodynamics at the ice-shelf/ocean interface. This adds another level of complexity to Southern Ocean simulations, as ice shelves interact directly with the ocean and indirectly with sea ice. Here we present the first published model intercomparison and evaluation of present-day ocean/sea-ice/ice-shelf interactions, as simulated by two models: a circumpolar Antarctic configuration of MetROMS (ROMS: Regional Ocean Modelling System coupled to CICE: Community Ice CodE) and the global model FESOM (Finite Element Sea-ice/ice-shelf Ocean Model), where the latter is run at two different levels of horizontal resolution. From a circumpolar Antarctic perspective, we compare and evaluate simulated ice shelf basal melting and sub-ice shelf circulation, as well as sea ice properties and Southern Ocean water mass characteristics as they influence the sub-ice shelf processes. Despite their differing numerical methods, the two models produce broadly similar results, and share similar biases in many cases. Both models reproduce many key features of observations, but struggle to reproduce others, such as the high melt rates observed in the small warm-cavity ice shelves of the Amundsen and Bellingshausen Seas. Several differences in model design show a particular influence on the simulations. For example, FESOM's greater topographic smoothing can alter the geometry of some ice shelf cavities enough to affect their melt rates; this improves at higher resolution, since less smoothing is required. In the interior Southern Ocean, the vertical coordinate system affects the degree of water mass erosion due to spurious diapycnal mixing, with MetROMS' terrain-following coordinates leading to more erosion than FESOM's z-coordinates. Finally, increased horizontal resolution in FESOM leads to higher basal melt rates for small ice shelves, through a combination of stronger circulation and small-scale intrusions of warm water from offshore.

Description: A Regional Antarctic and Global Ocean (RAnGO) model has been developed to study the interaction between the world ocean and the Antarctic ice sheet. The coupled model is based on a global implementation of the Finite Element Sea-ice Ocean Model (FESOM) with a mesh refinement in the Southern Ocean, particularly in its marginal seas and in the sub-ice shelf cavities. The cryosphere is represented by a regional setup of the ice flow model RIMBAY comprising the Filchner-Ronne Ice Shelf and the grounded ice in its catchment area up to the ice divides. At the base of the RIMBAY ice shelf, melt rates from FESOM's ice-shelf component are supplied. RIMBAY returns ice thickness and the position of the grounding line. The ocean model uses a pre-computed mesh to allow for an easy adjustment of the model domain to a varying cavity geometry. RAnGO simulations with a 20th-century climate forcing yield realistic basal melt rates and a quasi-stable grounding line position close to the presently observed state. In a centennial-scale warm-water-inflow scenario, the model suggests a substantial thinning of the ice shelf and a local retreat of the grounding line. The potentially negative feedback from ice-shelf thinning through a rising in-situ freezing temperature is more than outweighed by the increasing water column thickness in the deepest parts of the cavity. Compared to a control simulation with fixed ice-shelf geometry, the coupled model thus yields a slightly stronger increase of ice-shelf basal melt rates.